Endocrine Pancreas & Insulin

Logic-Based Integrated Notes (Zero Omission)

1. Islets of Langerhans – What they secrete & why it matters

Polypeptides secreted (≥4)

Extra-islet secretion:

• Glucagon, somatostatin, and possibly PP are also secreted by GI mucosa.

2. Functional logic of insulin vs glucagon (core metabolic concept)

Insulin = Anabolic

• ↑ Storage of:
• Glucose
• Fatty acids
• Amino acids

Glucagon = Catabolic

• Mobilizes into blood:
• Glucose
• Fatty acids
• Amino acids

➡️ Reciprocal relationship

• Opposing actions
• Reciprocally secreted in most physiological situations

3. Pathophysiologic consequences (exam-critical)

➡️ Important note: Many other hormones also regulate carbohydrate metabolism.

4. Islet cell structure – macro organization

Islet characteristics

• Shape: Ovoid
• Size: 76 × 175 μm
• Distribution:
• Scattered throughout pancreas
• Most abundant in tail
• Volume contribution:
• Islets: ~2%
• Exocrine pancreas: ~80%
• Ducts & vessels: remainder
• Number in humans: 1–2 million

Blood supply (unique feature)

• Very rich capillary supply
• Venous drainage:
• Into hepatic portal vein
• Same as GI tract
• Unlike other endocrine organs

5. Islet cell types & secretions

⚠️ Greek letter naming can cause confusion with other systems (e.g., adrenergic receptors).

6. Spatial arrangement inside islets

Cell proportions

• B cells: 60–75% (most common)
• A cells: ~20%
• D & F cells: fewer

Typical layout

• B cells → central core
• A cells → surrounding B cells
• D & F cells → scattered, fewer

7. Regional differences within the pancreas

Tail, body, anterior & superior head

• Many A cells
• Few or no F cells in outer rim

Posterior head (rats and probably humans)

• Many F cells
• Few A cells

8. Embryologic origin (high-yield)

➡️ Dorsal and ventral buds arise separately from the duodenum.

9. Secretory granules – structural logic

B cell granules (insulin)

• Cytoplasmic packets of insulin
• Shape varies by species
• Humans: round or rectangular
• Insulin stored as:
• Polymers
• Zinc complexes
• Shape variation due to:
• Polymer size
• Zinc aggregation differences

A cell granules (glucagon)

• Uniform across species

D cell granules

• Large numbers
• Relatively homogeneous

10. Insulin structure & species specificity

Molecular structure

• Polypeptide with:
• Two amino acid chains
• Linked by disulfide bridges

Species variation

• Minor amino acid differences
• Biological activity usually preserved
• Antigenicity differs

Clinical relevance

• Prolonged heterologous insulin → anti-insulin antibodies
• Bovine insulin:
• Antibodies in most humans after >2 months
• Usually low titer
• Porcine insulin:
• Differs by 1 amino acid
• Low antigenicity
• Recombinant human insulin:
• Produced in bacteria
• Widely used
• Avoids antibody formation

11. Insulin biosynthesis – step-by-step logic

Cellular pathway

1. Rough ER (B cell): insulin synthesis
2. Golgi apparatus: packaging into membrane-bound granules
3. Granule transport: via microtubules
4. Exocytosis: insulin released
5. Diffusion path to blood:
• Crosses basal lamina of B cell
• Crosses adjacent capillary
• Passes through fenestrated endothelium
• Enters bloodstream

12. Preproinsulin → insulin processing (exam favorite)

Gene details

• Location: Short arm of chromosome 11
• Structure:
• 3 exons
• 2 introns

Processing sequence

1. Preproinsulin synthesized in ER
2. Signal peptide removed → Proinsulin
3. Folding + disulfide bond formation
4. C-peptide connects A & B chains during folding
5. In granules:
• C-peptide cleaved by two proteases
6. Final secretion:
• 90–97% insulin
• Equimolar C-peptide
• Small amount of proinsulin

13. Clinical importance of C-peptide

• Measured by radioimmunoassay
• Reflects endogenous B-cell function
• Especially useful in patients receiving exogenous insulin

🔑 Core logic lock (one-line exam synthesis)

Insulin and glucagon are reciprocally secreted pancreatic hormones from structurally specialized islets, where B cells centrally store zinc-complexed insulin synthesized as preproinsulin and released with equimolar C-peptide via regulated exocytosis into the portal circulation, enabling fine control of intermediary metabolism.

Endocrine Pancreas & Insulin — COMPLETE MASTER TABLE (Zero Omission)

A. Islets of Langerhans — Hormones, Roles & Origins

B. Functional Hormonal Logic & Pathophysiology (Exam-Critical)

⚠️ Note: Carbohydrate metabolism is also regulated by many non-pancreatic hormones.

C. Islet Morphology, Distribution & Vascular Logic

D. Islet Cell Types, Proportions & Spatial Arrangement

⚠️ Greek lettering may confuse with adrenergic receptors (α, β).

E. Regional Cellular Differences in Pancreas

F. Secretory Granules — Structural Logic

➡️ Insulin granule shape depends on polymer size + zinc aggregation.

G. Insulin Molecular Structure & Species Specificity

H. Insulin Biosynthesis, Processing & Secretion Pathway

I. Preproinsulin → Insulin (High-Yield Molecular Processing)

J. C-Peptide — Clinical Utility

ONE-LINE EXAM LOCK

Islets of Langerhans are portal-drained endocrine microorgans where centrally located β-cells synthesize zinc-complexed insulin from preproinsulin and secrete it with equimolar C-peptide, reciprocally balanced by α-cell glucagon to tightly regulate intermediary metabolism.

Fate of Secreted Insulin & Its Physiologic Actions

1. Insulin & insulin-like activity in blood

1.1 Insulin-like substances in plasma

• Plasma contains substances with insulin-like activity in addition to insulin.
• Activity not suppressed by anti-insulin antibodies is called:
• Nonsuppressible insulin-like activity (NSILA).

1.2 Source of NSILA

• NSILA persists after pancreatectomy.
• It is due to:
• Insulin-like growth factor I (IGF-I)
• Insulin-like growth factor II (IGF-II)
• IGFs are polypeptides.

1.3 Plasma distribution of IGFs

• Small amounts → free in plasma
• Low-molecular-weight fraction
• Large amounts → protein-bound
• High-molecular-weight fraction

1.4 Why pancreatectomy still causes diabetes

• Despite persistence of NSILA:
• IGF-I and IGF-II have weak insulin-like activity
• Their main role is other specific functions, not glucose homeostasis
• Therefore:
• Insulin deficiency → diabetes mellitus

2. Metabolism of insulin (circulatory fate)

2.1 Half-life

• Insulin half-life in humans: ~5 minutes

2.2 Cellular handling

1. Insulin binds to insulin receptors
2. Receptor-insulin complex is internalized
3. Insulin is degraded by proteases
4. Degradation occurs in endosomes formed by endocytosis

3. Overall physiologic role of insulin

3.1 Nature of effects

• Effects are far-reaching and complex
• Classified as:
• Rapid
• Intermediate
• Delayed

3.2 Core metabolic outcome

• Best-known effect: hypoglycemia
• Additional effects on:
• Amino acid transport
• Electrolyte transport
• Enzyme activity
• Growth

3.3 Net effect

• Storage of carbohydrate, protein, and fat
• Therefore insulin is called:
• “Hormone of abundance”

4. Glucose transport into cells

4.1 Basic mechanisms of glucose entry

• Facilitated diffusion
• Secondary active transport with Na⁺
• Intestine
• Kidneys

5. GLUT transporters – structure & classification

5.1 General properties

• GLUTs:
• Mediate facilitated diffusion
• Span membrane 12 times
• Amino & carboxyl terminals inside the cell
• GLUTs:
• No homology with SGLTs
• SGLT-1 & SGLT-2:
• Perform Na⁺-glucose cotransport
• Also have 12 transmembrane domains
• Operate in:
• Intestine
• Renal tubules

5.2 GLUT family

• Seven GLUTs (GLUT-1 to GLUT-7)
• Size: 492–524 amino acids
• Different Km values
• Each evolved for specific tasks

6. GLUT-4: insulin-sensitive glucose transport

6.1 Tissue distribution

• Skeletal muscle
• Cardiac muscle
• Adipose tissue
• Other insulin-sensitive tissues

6.2 Intracellular pool

• GLUT-4 stored in cytoplasmic vesicles

6.3 Insulin-dependent mechanism

1. Insulin binds its receptor
2. Activates phosphatidylinositol 3-kinase
3. Vesicles move rapidly to membrane
4. Vesicles fuse with membrane
5. GLUT-4 inserted into membrane → ↑ glucose entry

6.4 Termination of insulin action

• GLUT-4–containing membrane patches are:
• Endocytosed
• Returned to vesicles
• Vesicles are ready for next insulin exposure

6.5 Other GLUTs

• Most non-insulin-sensitive GLUTs:
• Constitutively expressed in cell membrane

7. Regulation after glucose entry

7.1 Phosphorylation control

• In insulin-responsive tissues:
• Glucose phosphorylation is regulated by other hormones
• Growth hormone & cortisol:
• Inhibit glucose phosphorylation in certain tissues

7.2 Rate-limiting steps

• Transport usually not rate-limiting
• Exception: B cells
• Transport is rate-limiting

8. Insulin and hepatic glucose uptake

• Insulin increases glucose entry into liver
• Not via GLUT-4 increase
• Mechanism:
• Insulin induces glucokinase
• ↑ Glucose phosphorylation
• ↓ Intracellular free glucose
• Facilitates glucose entry

9. Exercise-induced glucose uptake (insulin-independent)

• Insulin-sensitive tissues also have:
• GLUT-4 vesicles that move to membrane during exercise
• This process:
• Is independent of insulin
• Explains why exercise lowers blood glucose
• Likely mediator:
• AMP-activated protein kinase (AMPK)

10. Insulin preparations & pharmacokinetics

10.1 Time course

• IV insulin:
• Max glucose fall at ~30 minutes
• Subcutaneous insulin:
• Max glucose fall at 2–3 hours

10.2 Types of preparations

• Modified by:
• Protamine or polypeptide complexing
• Amino acid substitutions
• Categories:
• Rapid-acting
• Intermediate-acting
• Long-acting (24–36 h)

11. Relation of insulin to potassium balance

11.1 Effect on K⁺

• Insulin causes K⁺ entry into cells
• ↓ Extracellular K⁺ concentration

11.2 Clinical applications

• Insulin + glucose infusion:
• Lowers plasma K⁺
• Used for temporary treatment of hyperkalemia
• Especially in renal failure

11.3 Diabetic ketoacidosis

• Insulin treatment may cause:
• Hypokalemia

11.4 Mechanism

• Insulin increases activity of:
• Na⁺/K⁺-ATPase
• Result:
• ↑ K⁺ pumped into cells

12. Other metabolic and growth actions

12.1 Carbohydrate & lipid metabolism

• Activates glycogen synthase → glycogen storage
• Activates glycolytic enzymes
• Inhibits:
• Phosphorylase
• Gluconeogenic enzymes
• Promotes:
• Conversion of glucose to two-carbon fragments
• Lipogenesis

12.2 Protein synthesis

• ↑ Amino acid entry
• ↑ Protein synthesis
• ↓ Protein degradation

12.3 Growth effects

• Adequate intracellular glucose:
• Has protein-sparing effect
• Diabetes in children:
• Causes failure to grow
• Insulin:
• Stimulates growth in immature hypophysectomized rats
• Almost equal to growth hormone

13. Temporal classification of insulin actions (Table 24–1)

Rapid (seconds)

• ↑ Transport of:
• Glucose
• Amino acids
• K⁺
• Into insulin-sensitive cells

Intermediate (minutes)

• ↑ Protein synthesis
• ↓ Protein degradation
• ↑ Glycolytic enzymes
• ↑ Glycogen synthase
• ↓ Phosphorylase
• ↓ Gluconeogenic enzymes

Delayed (hours)

• ↑ mRNAs for:
• Lipogenic enzymes
• Other enzymes

14. Tissue-specific effects of insulin (Table 24–2)

Adipose tissue

• ↑ Glucose entry
• ↑ Fatty acid synthesis
• ↑ Glycerol phosphate synthesis
• ↑ Triglyceride deposition
• ↑ Lipoprotein lipase
• ↓ Hormone-sensitive lipase
• ↑ K⁺ uptake

Muscle

• ↑ Glucose entry
• ↑ Glycogen synthesis
• ↑ Amino acid uptake
• ↑ Ribosomal protein synthesis
• ↓ Protein catabolism
• ↓ Release of gluconeogenic amino acids
• ↑ Ketone uptake
• ↑ K⁺ uptake

Liver

• ↓ Ketogenesis
• ↑ Protein synthesis
• ↑ Lipid synthesis
• ↓ Glucose output due to:
• ↓ Gluconeogenesis
• ↑ Glycogen synthesis
• ↑ Glycolysis

General

• ↑ Cell growth

15. Glucose transporters summary (Table 24–3)

Secondary active transport (Na⁺-glucose cotransport)

• SGLT-1
• Km: 0.1–1.0 mM
• Small intestine, renal tubules
• SGLT-2
• Km: 1.6 mM
• Renal tubules

Facilitated diffusion

• GLUT-1
• Basal uptake
• Km: 1–2
• Placenta, BBB, brain, RBCs, kidneys, colon, others
• GLUT-2
• B-cell glucose sensor
• Transport out of intestinal & renal epithelium
• Km: 12–20
• B cells, liver, intestine, kidneys
• GLUT-3
• Basal uptake
• Km <1
• Brain, placenta, kidneys, others
• GLUT-4
• Insulin-stimulated uptake
• Km: 5
• Skeletal muscle, cardiac muscle, adipose tissue
• GLUT-5
• Fructose transport
• Km: 1–2
• Jejunum, sperm
• GLUT-6
• Function unknown
• Brain, spleen, leukocytes
• GLUT-7
• Glucose-6-phosphate transporter in ER
• Liver

🔑 Core exam logic lock (single-sentence synthesis)

Insulin is a short-lived anabolic hormone that promotes storage of glucose, fat, and protein by receptor-mediated signaling that drives GLUT-4 translocation, enzyme activation, K⁺ influx, and gene transcription, effects that cannot be replaced by circulating IGFs despite their weak insulin-like activity.

INSULIN: FATE, ACTIONS & GLUCOSE TRANSPORT — COMPLETE MASTER TABLE (ZERO OMISSION)

MECHANISM OF ACTION – INSULIN RECEPTORS (Logic-Based Note)

1. Distribution of Insulin Receptors

• Insulin receptors are present on many different cell types.
• Importantly, they are found even on cells where insulin does NOT increase glucose uptake.
• Therefore, insulin has multiple actions beyond glucose transport (growth, anabolic effects, signaling).

2. Structure of the Insulin Receptor

• The insulin receptor has a molecular weight ≈ 340,000.
• It is a tetrameric receptor composed of:
• 2 α (alpha) subunits
• 2 β (beta) subunits
• All four subunits are:
• Synthesized from a single mRNA
• Then proteolytically cleaved
• And held together by disulfide bonds
• Both α and β subunits are glycoproteins.

3. Genetics of the Insulin Receptor

• The insulin receptor gene:
• Has 22 exons
• Is located on chromosome 19 in humans

4. Location and Function of Subunits

• α subunits
• Located extracellularly
• Responsible for binding insulin
• β subunits
• Span the cell membrane
• Their intracellular portions have tyrosine kinase activity
• Both α and β subunits are glycosylated
• Sugar residues extend into the interstitial fluid

5. Activation of the Receptor (Key Signaling Step)

• When insulin binds to the α subunits:
• It activates the tyrosine kinase activity of the β subunits
• This causes:
• Autophosphorylation of β subunits on tyrosine residues
• This autophosphorylation is:
• Essential for insulin’s biological effects

6. Downstream Signaling Effects

• Autophosphorylation of the receptor leads to:
• Phosphorylation of some cytoplasmic proteins
• Dephosphorylation of other proteins
• These downstream changes occur mostly on serine and threonine residues

7. Insulin Receptor Substrates (IRS)

• IRS-1 (Insulin Receptor Substrate-1) mediates some insulin effects in humans
• However:
• Other effector systems also exist
• Evidence from animal models:
• Insulin receptor gene knockout mice:
• Severe intrauterine growth retardation
• CNS and skin abnormalities
• Death at birth due to respiratory failure
• IRS-1 knockout mice:
• Only moderate intrauterine growth retardation
• Survive
• Are insulin-resistant
• Otherwise nearly normal
• This shows:
• Insulin receptor signaling is broader and more critical than IRS-1 alone.

8. Growth-Promoting and Anabolic Pathway

• The growth-promoting protein anabolic effects of insulin are mediated via:
• Phosphatidylinositol 3-kinase (PI3K) pathway
• In invertebrates:
• This pathway is involved in:
• Growth of nerve cells
• Axon guidance in the visual system

9. Comparison with Other Receptors

• The insulin receptor:
• Is very similar to the IGF-I receptor
• Is different from the IGF-II receptor
• Other receptors:
• Many growth factor receptors and oncogene receptors also have tyrosine kinase activity
• However:
• Their amino acid composition differs significantly from the insulin receptor

10. Receptor Internalization and Turnover

• After insulin binds:
• Insulin receptors aggregate in membrane patches
• They are taken into the cell via receptor-mediated endocytosis
• Inside the cell:
• Insulin–receptor complexes enter lysosomes
• Receptors are either:
• Broken down
• Or recycled
• The half-life of the insulin receptor ≈ 7 hours

🔑 Exam Logic Locks

• Insulin receptor = tetramer (α₂β₂) with tyrosine kinase activity
• Autophosphorylation on tyrosine residues is mandatory
• IRS-1 is important but not exclusive
• PI3K pathway = growth + anabolic effects
• Receptor undergoes endocytosis and recycling
• Chromosome 19, 22 exons, half-life ~7 h

INSULIN RECEPTOR — MECHANISM OF ACTION (COMPLETE MASTER TABLE)

CONSEQUENCES OF INSULIN DEFICIENCY

(“Starvation in the midst of plenty”)

1. CORE DEFECT IN DIABETES (FOUNDATION)

Insulin deficiency (absolute or relative) causes two fundamental abnormalities:

1. ↓ Entry of glucose into peripheral tissues
• Skeletal muscle
• Cardiac muscle
• Smooth muscle
• Adipose tissue
2. ↑ Net release of glucose from the liver
• Due to:
• Loss of insulin inhibition
• Excess glucagon
• Stress hormones (catecholamines, cortisol, GH)

➡ Result:

Extracellular glucose excess + intracellular glucose deficiency

This paradox explains all downstream effects.

2. GLUCOSE TOLERANCE ABNORMALITY

What happens after glucose intake:

• Plasma glucose:
• Rises higher
• Falls more slowly
• Basis of oral glucose tolerance test (OGTT) diagnosis

Mechanisms:

1. ↓ Peripheral glucose utilization
• Insulin-dependent tissues cannot take up glucose
2. ↑ Hepatic glucose output (major contributor)
• Liver:
• Takes up glucose
• Stores as glycogen
• Releases glucose via glucose-6-phosphatase
• Insulin normally:
• ↑ Glycogenesis
• ↓ Hepatic glucose output
• This control is lost in:
• Type 1 DM → insulin absent
• Type 2 DM → insulin resistance
3. Glucagon excess
• Stimulates gluconeogenesis
4. Stress hormones
• Catecholamines, cortisol, GH → ↑ glucose output

Unaffected processes:

• Intestinal glucose absorption → normal
• Renal tubular glucose reabsorption → normal
• Brain glucose uptake → normal
• RBC glucose uptake → normal

3. EFFECTS OF HYPERGLYCEMIA

A. Osmotic & Renal Effects

• Hyperglycemia → ↑ plasma osmolality
• Renal glucose threshold exceeded → glycosuria
• Glucose in urine → osmotic diuresis
• Consequences:
• Massive water loss
• Na⁺ and K⁺ loss
• Dehydration → polydipsia

Energy loss:

• 4.1 kcal lost per gram of glucose excreted
• Increasing food intake:
• Worsens hyperglycemia
• Increases glycosuria
• Does not prevent weight loss

B. HbA1c Formation

• Episodic hyperglycemia → non-enzymatic glycation of hemoglobin A
• Forms HbA1c
• Reflects:
• Integrated glycemic control over 4–6 weeks
• Reduced by good insulin control

4. EFFECTS OF INTRACELLULAR GLUCOSE DEFICIENCY

Cellular energy failure:

• Glucose unavailable inside cells
• Energy derived from:
• Protein catabolism
• Fat catabolism

Appetite dysregulation:

• Hypothalamic satiety centers affected due to:
• ↓ glucose sensing
• ↓ insulin, leptin, CCK signaling
• Feeding center uninhibited → hyperphagia

Glycogen depletion:

• Liver glycogen ↓
• Skeletal muscle glycogen ↓

5. CHANGES IN PROTEIN METABOLISM

A. Increased protein breakdown

• Amino acids:
• Oxidized to CO₂ + H₂O
• Converted to glucose (gluconeogenesis)

B. Causes of ↑ gluconeogenesis:

1. Hyperglucagonemia
2. ↑ Glucocorticoids (severe illness)
3. ↓ Protein synthesis in muscle
• Amino acids accumulate in blood
4. Alanine
• Particularly efficient glucose precursor
5. ↑ Enzyme activity
• Phosphoenolpyruvate carboxykinase (OAA → PEP)
• Fructose-1,6-diphosphatase
• Glucose-6-phosphatase
6. ↑ Acetyl-CoA
• ↓ Lipogenesis → acetyl-CoA accumulates
• Activates pyruvate carboxylase (pyruvate → OAA)

Net effect:

• Protein depletion
• Muscle wasting
• ↓ Resistance to infections

6. FAT METABOLISM IN DIABETES

Key abnormalities:

1. ↑ Lipolysis
2. ↓ Fatty acid & triglyceride synthesis
3. ↑ Ketone body formation

Diabetes is often more a lipid disorder than a carbohydrate disorder.

Normal glucose fate:

• 50% → CO₂ + H₂O
• 5% → glycogen
• 30–40% → fat

In diabetes:

• <5% → fat
• ↓ oxidation
• Glycogen unchanged
• → glucose accumulates in blood → urine

Lipase dysregulation:

• Hormone-sensitive lipase
• Normally inhibited by insulin
• Active in insulin deficiency → ↑ FFA
• Lipoprotein lipase
• ↓ activity → ↓ triglyceride clearance

FFA consequences:

• Plasma FFA > doubled
• Parallels glucose level
• Better indicator of disease severity than glucose

Acetyl-CoA overload:

• FA → acetyl-CoA
• TCA cycle capacity exceeded
• Conversion to fatty acids impaired due to:
• ↓ Acetyl-CoA carboxylase
• Excess acetyl-CoA → ketone bodies

7. KETOSIS

Ketone bodies:

• Acetoacetate
• β-Hydroxybutyrate
• Acetone

Normal:

• Important fuel in fasting
• Up to 50% of metabolism in fasted dogs

Diabetes:

• Safe fat catabolism limit: 2.5 g/kg/day
• Production exceeds utilization
• Ketones accumulate → ketosis

8. ACIDOSIS (DIABETIC KETOACIDOSIS)

Mechanism:

• Ketone bodies = organic acids
• H⁺ buffered initially
• Buffering capacity exceeded → metabolic acidosis

Consequences:

• Kussmaul breathing (deep, rapid respiration)
• Acidic urine
• Renal excretion of ketone anions with:
• Na⁺
• K⁺
• → electrolyte loss + dehydration
• → hypovolemia → hypotension
• → CNS depression → coma

Medical emergency

Most common early cause of death in diabetes

Electrolyte status:

• Total body Na⁺ ↓
• Plasma Na⁺ may ↓ if Na⁺ loss > water loss
• Total body K⁺ ↓
• Plasma K⁺ often normal due to:
• Acidosis-induced K⁺ shift out of cells
• Lack of insulin-mediated K⁺ entry

9. COMA IN DIABETES

Causes:

1. Acidosis + dehydration
2. Hyperosmolar coma
• Extremely high glucose → ↑ plasma osmolality
3. Lactic acidosis
• Tissue hypoxia
4. Cerebral edema
• ~1% of children with DKA
• Mortality ~25%

10. CHOLESTEROL METABOLISM

• Plasma cholesterol ↑
• Due to:
• ↑ VLDL
• ↑ LDL
• Causes:
• ↑ hepatic production
• ↓ clearance
• Leads to:
• Accelerated atherosclerosis
• Major long-term complication

11. FINAL INTEGRATED SUMMARY (EXAM CORE)

• Insulin deficiency →
• ↓ Peripheral glucose uptake
• ↑ Hepatic glucose output
• → Hyperglycemia → glycosuria → osmotic diuresis → dehydration → polydipsia
• Intracellular glucose deficiency →
• Hyperphagia
• ↑ Protein & fat catabolism
• Protein loss → wasting + infection susceptibility
• Fat breakdown → FFA ↑ → acetyl-CoA overload → ketone bodies
• Ketosis → metabolic acidosis → electrolyte loss → coma → death
• Only insulin corrects the fundamental defect

Supportive therapy (fluids, Na⁺, K⁺, alkali) treats consequences,

insulin treats the cause.

🧠 CONSEQUENCES OF INSULIN DEFICIENCY — MASTER INTEGRATED TABLE (ZERO OMISSION)

🔒 ULTIMATE EXAM LOCK (One-liner)

Insulin deficiency causes intracellular starvation despite extracellular excess → hyperglycemia, osmotic diuresis, protein & fat catabolism, ketosis, acidosis, electrolyte loss, coma — and only insulin corrects the cause.

INSULIN EXCESS → HYPOGLYCEMIA

Core Principle (Big Picture)

• All consequences of insulin excess are due to hypoglycemia affecting the nervous system.
• The brain depends almost entirely on glucose for energy (except after prolonged fasting).
• Neural tissue has minimal carbohydrate reserves, so continuous plasma glucose supply is essential.

WHY THE NERVOUS SYSTEM IS AFFECTED FIRST

• Brain:
• Uses glucose as the only significant fuel (non-fasted state).
• Has very limited glycogen stores.
• Therefore:
• Even short-lasting hypoglycemia → rapid neural dysfunction.

SYMPTOMS OF INSULIN EXCESS (HYPOGLYCEMIA)

1. Early Phase – AUTONOMIC SYMPTOMS

Cause: Falling plasma glucose → activation of autonomic nervous system.

Symptoms:

• Palpitations
• Sweating
• Nervousness

Important logic point:

• These symptoms appear at glucose levels slightly lower than the level at which autonomic activation begins.
• Reason:
• Threshold for symptoms is slightly above the threshold for initial autonomic activation.

2. Intermediate Phase – NEUROGLYCOPENIC SYMPTOMS

Cause: Inadequate glucose delivery to brain neurons.

Symptoms:

• Hunger
• Confusion
• Cognitive impairment
• Other higher-function abnormalities

3. Severe Phase – CNS FAILURE

At even lower plasma glucose levels:

• Lethargy
• Coma
• Convulsions
• Death (if untreated)

TREATMENT LOGIC

• Hypoglycemia requires immediate glucose replacement.
• Methods:
• Oral glucose
• Glucose-containing drinks (e.g., orange juice)

Clinical note:

• Symptoms often disappear dramatically after glucose.
• HOWEVER:
• If hypoglycemia is severe or prolonged, residual effects may persist:
• Intellectual dulling
• Prolonged coma

COMPENSATORY MECHANISMS IN HYPOGLYCEMIA

1. FIRST DEFENSE: INSULIN SHUTOFF

• Endogenous insulin secretion stops as plasma glucose falls.
• Complete inhibition occurs at ≈ 80 mg/dL.

2. COUNTERREGULATORY HORMONE RESPONSE

Hypoglycemia stimulates secretion of four hormones:

1. Glucagon
2. Epinephrine
3. Growth hormone
4. Cortisol

Hormonal Actions (Mechanism-based)

Glucagon

• Increases hepatic glucose output
• Acts via:
• ↑ Glycogenolysis

Epinephrine

• Also increases hepatic glucose output
• Acts via:
• ↑ Glycogenolysis
• Note:
• Epinephrine response is reduced during sleep

Growth Hormone

• ↓ Glucose utilization in peripheral tissues

Cortisol

• Similar to growth hormone
• ↓ Peripheral glucose utilization

KEY EXAM LOGIC: WHICH HORMONES MATTER MOST?

• Epinephrine and glucagon are the critical counterregulatory hormones.
• Rule:
• ↑ Either one → plasma glucose decline is reversed
• Failure of both → little or no compensatory rise in glucose
• Growth hormone and cortisol:
• Supplementary, not primary

TIMING OF RESPONSES (VERY HIGH-YIELD)

• Autonomic discharge + counterregulatory hormone release occur at higher glucose levels
• Cognitive deficits and severe CNS symptoms occur at lower glucose levels

CLINICAL SIGNIFICANCE IN DIABETES

• In insulin-treated diabetics:
• Autonomic symptoms act as a warning signal to ingest glucose.
• Problem:
• In long-term, tightly controlled diabetics:
• Autonomic symptoms may not occur
• Leads to hypoglycemia unawareness
• Major clinical risk

ONE-LINE EXAM LOCK 🔒

Insulin excess causes hypoglycemia, and because the brain depends almost exclusively on glucose, early autonomic symptoms precede neuroglycopenia, while epinephrine and glucagon form the primary counterregulatory defense.

REGULATION OF INSULIN SECRETION (ZERO-OMISSION, LOGIC-BASED)

1) Basal levels + daily output (numbers first)

• The fasting peripheral venous plasma insulin measured by radioimmunoassay in normal humans is 0–70 μU/mL (0–502 pmol/L).
• Basal secretion rate is about 1 unit/hour.
• After ingestion of food, insulin secretion rises 5-fold to 10-fold.
• Therefore, the average insulin secreted per day in a normal human is about 40 units (287 nmol).

2) What increases vs decreases insulin secretion (Table logic)

A) Stimulators

• Glucose
• Mannose
• Amino acids: leucine, arginine, others
• Intestinal hormones: GIP, GLP-1 (7–36), gastrin, secretin, CCK; others?
• α-Adrenergic stimulators: norepinephrine, epinephrine
• β-Keto acids
• Acetylcholine
• Glucagon
• Cyclic AMP and various cAMP-generating substances
• β-Adrenergic stimulators
• Theophylline
• Sulfonylureas

B) Inhibitors

• Somatostatin
• 2-Deoxyglucose
• Mannoheptulose
• β-Adrenergic blockers (propranolol)
• Galanin
• Diazoxide
• Thiazide diuretics
• K+ depletion
• Phenytoin
• Alloxan
• Microtubule inhibitors
• Insulin

3) Plasma glucose control is the key driver (precision + sequence)

• Glucose acts directly on pancreatic B cells to increase insulin secretion.
• The glucose response is biphasic:
• First phase: rapid and short-lived spike
• Second phase: slower onset but prolonged increase

A) Step-by-step mechanism (why glucose causes secretion)

• Glucose enters B cells via GLUT-2 transporters.
• Glucose is phosphorylated by glucokinase.
• It is metabolized in the cytoplasm to pyruvate.
• Pyruvate enters mitochondria and is metabolized to CO2 and H2O via the citric acid cycle.
• This leads to ATP generation by oxidative phosphorylation.
• ATP enters cytoplasm and inhibits ATP-sensitive K+ channels.
• K+ efflux decreases → the B cell depolarizes.
• Depolarization opens voltage-gated Ca2+ channels → Ca2+ influx.
• Ca2+ influx triggers exocytosis of a readily releasable pool of insulin granules → initial spike (first phase).

B) Why the second phase happens (glutamate “priming” pathway)

• Citric acid cycle metabolism also increases intracellular glutamate.
• Glutamate acts on a second pool of secretory granules, committing them to a releasable form.
• Proposed mechanism: glutamate may decrease pH inside secretory granules, which is necessary for granule maturation.
• Release of this primed pool produces the prolonged second phase.
• Therefore, glutamate functions as an intracellular second messenger that primes granules for secretion.

C) System-level accuracy (feedback precision)

• Feedback control between plasma glucose and insulin is highly precise.
• Plasma glucose and insulin levels parallel each other with remarkable consistency.

4) Protein + fat derivatives (why AA and ketoacids stimulate)

• Insulin normally:
• Stimulates amino acid incorporation into proteins.
• Opposes fat catabolism that produces β-keto acids.
• So it fits that:
• Arginine, leucine, and certain other amino acids stimulate insulin secretion.
• β-keto acids (example: acetoacetate) also stimulate secretion.
• Shared mechanism with glucose:
• When metabolized, these substrates generate ATP → ATP closes ATP-sensitive K+ channels → depolarization → Ca2+ influx → insulin release.
• Additional arginine-specific mechanism:
• L-arginine is a precursor of NO, and NO stimulates insulin secretion.

5) Oral hypoglycemic agents (what they do + where they act)

A) Sulfonylureas (mechanism + limitation)

• Examples listed: tolbutamide, acetohexamide, tolazamide, glipizide, glyburide.
• They are orally active hypoglycemic agents that lower blood glucose by increasing insulin secretion.
• They work only if there are remaining B cells.
• They are ineffective:
• after pancreatectomy
• in type 1 diabetes
• Mechanism:
• They bind to the ATP-inhibited K+ channels in B cell membranes and inhibit channel activity.
• This depolarizes the B cell membrane → increases Ca2+ influx → increases insulin release.
• This occurs independent of increases in plasma glucose.

B) Persistent hyperinsulinemic hypoglycemia of infancy (key disease logic)

• Defined by:
• Plasma insulin remains elevated despite hypoglycemia.
• Cause:
• Mutations in genes for various B-cell enzymes that decrease K+ efflux via the ATP-sensitive K+ channels.
• Treatment:
• Diazoxide to increase K+ channel activity.
• If severe: subtotal pancreatectomy.

C) Metformin (biguanide) (insulin-independent action + risk)

• Metformin acts in the absence of insulin.
• Primary action:
• Reduces gluconeogenesis → decreases hepatic glucose output.
• It may be combined with a sulfonylurea in type 2 diabetes.
• Adverse effect:
• Can cause lactic acidosis, but incidence is usually low.

D) Thiazolidinediones (troglitazone + related) (receptor + consequence)

• Example given: troglitazone (Rezulin) and related thiazolidinediones.
• Used because they increase insulin-mediated peripheral glucose disposal → reduce insulin resistance.
• Mechanism:
• Bind and activate PPARγ in the nucleus.
• PPARγ is part of the superfamily of hormone-sensitive nuclear transcription factors.
• Activation has a unique ability to normalize a variety of metabolic functions.

6) cAMP pathway (how “second messenger” amplifies secretion)

• Any stimulus that increases cAMP in B cells increases insulin secretion.
• Examples listed:
• β-adrenergic agonists
• glucagon
• phosphodiesterase inhibitors such as theophylline

7) Catecholamines have dual effects (α2 vs β, net effect rule)

• Catecholamines:
• Inhibit insulin secretion via α2-adrenergic receptors.
• Stimulate insulin secretion via β-adrenergic receptors.
• Net effect:
• Epinephrine and norepinephrine usually inhibit insulin secretion.
• Critical exception (exam trap):
• If catecholamines are infused after α-adrenergic blockade, inhibition converts to stimulation.

8) Autonomic nerve control (vagus vs sympathetic, and galanin)

A) Parasympathetic (vagus)

• Branches of the right vagus nerve innervate pancreatic islets.
• Parasympathetic stimulation increases insulin secretion via M4 receptors.
• Atropine blocks this response.
• Acetylcholine stimulates insulin secretion.
• Mechanism:
• Like glucose, ACh increases cytoplasmic Ca2+.
• But ACh does it by activating phospholipase C.
• PLC produces IP3, and IP3 releases Ca2+ from the endoplasmic reticulum.

B) Sympathetic

• Sympathetic stimulation usually inhibits insulin secretion.
• Mechanism:
• Norepinephrine acts on α2-adrenergic receptors.
• If α receptors are blocked:
• Sympathetic stimulation increases insulin secretion via β2-adrenergic receptors.

C) Galanin (autonomic peptide inhibitor)

• Galanin exists in some autonomic nerves innervating islets.
• It inhibits insulin secretion by activating K+ channels that are normally inhibited by ATP.
• Therefore:
• Denervated pancreas still responds to glucose,
• but autonomic innervation participates in overall regulation.

9) Potassium status (K+ depletion → impaired secretion → diabetic curve)

• K+ depletion decreases insulin secretion.
• K+-depleted patients (example: primary hyperaldosteronism) develop diabetic glucose tolerance curves.
• These curves return to normal with K+ repletion.

Therapeutic highlight (diuretics link)

• Thiazide diuretics cause urinary loss of K+ and Na+.
• They decrease glucose tolerance and worsen diabetes.
• Main reason:
• Their K+-depleting effects.
• Additional possibility:
• Some may cause pancreatic islet cell damage.
• Clinical substitution:
• Use potassium-sparing diuretics such as amiloride in diabetics who require diuretics.

10) Intestinal hormones / “incretin effect” (oral > IV)

• Oral glucose produces a greater insulin response than IV glucose.
• Oral amino acids also produce a greater insulin response than IV amino acids.
• This led to the concept of GI mucosal factors that stimulate insulin secretion.

Gut peptides with insulinotropic action (listed)

• Glucagon
• Glucagon derivatives
• Secretin
• CCK
• Gastrin
• GIP

Key specificity point (why GIP matters most among these)

• Many stimulate insulin, but GIP is the only one that stimulates insulin secretion when given at doses that match the blood levels seen after an oral glucose load.

GLP-1 (7–36) (newer focus and potency)

• GLP-1 (7–36) is a product of preproglucagon.
• B cells have receptors for:
• GLP-1 (7–36)
• GIP
• GLP-1 (7–36) is more potent than GIP as an insulinotropic hormone.
• Mechanism (shared with GIP):
• Both act by increasing Ca2+ influx through voltage-gated Ca2+ channels.

Note on other regulators mentioned

• Possible roles of pancreatic somatostatin and glucagon in insulin regulation are discussed elsewhere.

11) Long-term changes in B-cell responsiveness (history + exhaustion + “meta-” terms)

A) Secretory history changes the response magnitude

• Insulin response to a stimulus depends partly on prior secretory history.
• High-carbohydrate diet for weeks causes:
• Higher fasting insulin levels.
• Greater insulin response to a glucose load,

compared with an isocaloric low-carbohydrate diet.

B) B-cell hypertrophy and exhaustion

• B cells can hypertrophy with stimulation.
• With marked or prolonged stimulation they may become exhausted and stop secreting:
• B cell exhaustion.
• Pancreatic reserve is large, so exhaustion is difficult in normal animals.
• If reserve is reduced (e.g., partial pancreatectomy):
• Chronic elevation of plasma glucose can exhaust remaining B cells.

C) How experimental diabetes is precipitated in limited reserve animals

• Diabetes can be induced by chronically raising plasma glucose using:
• anterior pituitary extracts
• growth hormone
• thyroid hormones
• prolonged continuous glucose infusion alone

D) Reversible vs permanent hormone-induced diabetes (naming rules)

• Early hormone-precipitated diabetes is often reversible.
• With prolonged treatment it becomes permanent.
• Transient diabetes is named for the agent:
• “hypophysial diabetes”
• “thyroid diabetes”
• Permanent diabetes persisting after stopping treatment uses prefix meta-:
• “metahypophysial diabetes”
• “metathyroid diabetes”

E) Protective effect of insulin co-administration

• If insulin is administered along with diabetogenic hormones:
• B cells are protected.
• Likely reason: plasma glucose is lowered.
• Diabetes does not develop.

F) Genetic control of B-cell reserve (IRS knockouts)

• Genetic factors can influence B-cell reserve.
• In IRS-1 knockout mice:
• Robust compensatory B-cell response occurs.
• In IRS-2 knockout mice:
• Compensation is reduced.
• A more severe diabetic phenotype occurs.

Ultra-high-yield mechanism chain (single line)

• Glucose/AA/ketoacids → ATP ↑ → ATP-sensitive K+ channels close → depolarization → voltage-gated Ca2+ opens → Ca2+ influx → granule exocytosis (first phase), while glutamate primes second pool → prolonged second phase.

ZERO-OMISSION MASTER TABLE (MECHANISM-FIRST)

A. INSULIN EXCESS → HYPOGLYCEMIA (PATHOPHYSIOLOGY + CLINICAL)

B. COUNTERREGULATION IN HYPOGLYCEMIA

C. INSULIN SECRETION — BASELINE & QUANTITATION

D. REGULATORS OF INSULIN SECRETION

1. STIMULATORS vs INHIBITORS

E. GLUCOSE-INDUCED INSULIN SECRETION (BIPHASIC — FULL MECHANISM)

F. AMINO ACIDS & β-KETO ACIDS

G. DRUGS AFFECTING INSULIN

1. Sulfonylureas

2. Persistent Hyperinsulinemic Hypoglycemia of Infancy

3. Metformin

4. Thiazolidinediones

H. cAMP & CATECHOLAMINES

I. AUTONOMIC CONTROL

J. POTASSIUM STATUS

K. INCRETIN EFFECT

L. LONG-TERM B-CELL DYNAMICS

FINAL EXAM LOCK 🔒

Insulin excess → hypoglycemia → early autonomic warning → neuroglycopenia → CNS failure, while insulin secretion is governed by ATP-sensitive K⁺ channel closure, Ca²⁺ influx, and biphasic granule release, with glucagon and epinephrine as the dominant counterregulators.

GLUCAGON (ZERO-OMISSION, LOGIC-BASED COMPLETE NOTE)

1) CHEMISTRY / SOURCE / PROCESSING (what it is, where it comes from, why different tissues give different peptides)

Core identity

• Human glucagon is a linear polypeptide with a molecular weight of 3485.
• It is produced by:
• A cells of the pancreatic islets
• The upper gastrointestinal tract
• It contains 29 amino acid residues.
• All mammalian glucagons appear to have the same structure.

Preproglucagon (the master precursor)

• Human preproglucagon is a 179–amino-acid protein.
• It is found in:
• Pancreatic A cells
• L cells in the lower gastrointestinal tract
• The brain
• It is the product of a single mRNA, but it is processed differently depending on the tissue.

Tissue-specific processing products

In pancreatic A cells

• Preproglucagon is processed primarily to:
• Glucagon
• Major proglucagon fragment (MPGF)

In intestinal L cells (lower GI)

• Preproglucagon is processed primarily to:
• Glicentin (glucagon extended by extra amino acids at either end)
• GLP-1
• GLP-2
• Some oxyntomodulin is also formed.
• In both A and L cells, residual glicentin-related polypeptide (GRPP) is left.

Functional notes on the processing products

• Glicentin has some glucagon activity.
• GLP-1 and GLP-2 have no definite biologic activity by themselves.
• However:
• GLP-1 is further processed by removing its amino-terminal residues to form GLP-1 (7–36).
• GLP-1 (7–36) is a potent stimulator of insulin secretion and also increases glucose utilization.
• GLP-1 and GLP-2 are also produced in the brain:
• Brain GLP-1 function is uncertain.
• Brain GLP-2 appears to mediate a pathway from nucleus tractus solitarius (NTS) to the dorsomedial nuclei of the hypothalamus.
• Injection of GLP-2 lowers food intake.
• Oxyntomodulin inhibits gastric acid secretion:
• Its physiologic role is unsettled.
• GRPP has no established physiologic effects.

2) ACTIONS (what glucagon does overall)

Big metabolic themes

• Glucagon is:
• Glycogenolytic
• Gluconeogenic
• Lipolytic
• Ketogenic

Receptor type

• It acts on G protein–coupled receptors with a molecular weight of about 190,000.

3) LIVER SIGNALING MECHANISMS (how it raises plasma glucose)

Pathway 1: Gs → adenylyl cyclase → cAMP → PKA

• In the liver:
• Glucagon activates Gs.
• This activates adenylyl cyclase.
• Intracellular cAMP increases.
• cAMP activates protein kinase A (PKA).
• PKA activates phosphorylase → increases glycogen breakdown → increases plasma glucose.

Pathway 2: PLC → Ca2+ rise → glycogenolysis

• Glucagon also acts on different glucagon receptors on the same hepatic cells to activate phospholipase C (PLC).
• PLC signaling increases cytoplasmic Ca2+.
• Increased cytoplasmic Ca2+ also stimulates glycogenolysis.

4) HOW GLUCAGON SHIFTS METABOLISM TOWARD GLUCOSE PRODUCTION (key enzymatic control points)

PKA reduces glucose-6-phosphate “use” and pushes it toward release

• PKA decreases metabolism of glucose-6-phosphate by:
• Inhibiting the conversion of phosphoenolpyruvate (PEP) to pyruvate.
• PKA also decreases fructose 2,6-diphosphate.
• This inhibits conversion of:
• fructose 6-phosphate → fructose 1,6-diphosphate
• Result:
• Glucose-6-phosphate builds up.
• This leads to increased glucose synthesis and release.

5) WHAT GLUCAGON DOES NOT DO (muscle)

• Glucagon does not cause glycogenolysis in muscle.

6) OTHER METABOLIC EFFECTS (beyond glycogen)

Gluconeogenesis + metabolic rate

• Glucagon increases gluconeogenesis in the liver from available amino acids.
• It elevates the metabolic rate.

Ketogenesis mechanism

• Glucagon increases ketone body formation by decreasing malonyl-CoA levels in the liver.

Lipolysis link

• Glucagon has lipolytic activity, which contributes to increased ketogenesis (lipolysis details referenced elsewhere).

Calorigenic effect (why metabolic rate increases)

• The calorigenic action is not due to hyperglycemia itself.
• It is probably due to increased hepatic deamination of amino acids.

7) HEART EFFECTS (pharmacologic, not physiologic control)

• Large doses of exogenous glucagon produce a positive inotropic effect on the heart.
• It does not increase myocardial excitability.
• Likely mechanism: increases myocardial cAMP.
• Use has been advocated in heart disease, but:
• There is no evidence for a physiologic role of glucagon in regulating cardiac function.

Other endocrine effects stimulated by glucagon

• Glucagon stimulates secretion of:
• Growth hormone
• Insulin
• Pancreatic somatostatin

8) METABOLISM / CLEARANCE (half-life + portal logic + cirrhosis)

• Glucagon half-life in circulation is 5–10 minutes.
• It is degraded by many tissues, especially the liver.
• Because glucagon enters the portal vein and reaches the liver first:
• Peripheral blood levels are relatively low.
• In cirrhosis:
• Peripheral glucagon rise after stimuli is exaggerated.
• Presumed reason: decreased hepatic degradation.

9) REGULATION OF GLUCAGON SECRETION (drivers + neural + gut + fasting + exercise)

A) Glucose relationship (core feedback)

• Secretion is:
• Increased by hypoglycemia
• Decreased by a rise in plasma glucose

B) Insulin-linked inhibition via GABA (B cell → A cell paracrine logic)

• Pancreatic B cells contain GABA.
• With hyperglycemia:
• Insulin secretion increases.
• At the same time, GABA is released.
• GABA inhibits glucagon secretion by acting on A cells via GABAA receptors.
• GABAA receptors are Cl– channels.
• Cl– influx hyperpolarizes A cells → glucagon secretion decreases.

C) Sympathetic stimulation (β predominance for glucagon output)

• Sympathetic stimulation increases glucagon secretion.
• Mediated via β-adrenergic receptors and cAMP.
• A cells mirror B cells in receptor logic:
• β-adrenergic stimulation increases secretion
• α-adrenergic stimulation inhibits secretion
• Net physiologic effect during sympathetic activation (no blocking drugs):
• Glucagon secretion increases
• Therefore β effect predominates in glucagon-secreting cells.
• Stresses (and possibly exercise and infection) stimulate glucagon at least partly via the sympathetic system.

D) Vagal stimulation

• Vagal stimulation also increases glucagon secretion.

E) Protein and amino acids (why this prevents hypoglycemia after protein)

• A protein meal and infusion of various amino acids increase glucagon secretion.
• Particularly potent: glucogenic amino acids because they are converted to glucose in the liver under glucagon influence.
• Listed glucogenic amino acids (explicit):
• alanine, serine, glycine, cysteine, threonine
• The post-protein-meal glucagon rise is beneficial because:
• Amino acids stimulate insulin secretion.
• Glucagon prevents hypoglycemia.
• Meanwhile insulin promotes storage of absorbed carbohydrates and lipids.

F) Starvation pattern (peak day 3 then decline)

• Glucagon increases during starvation.
• Peaks on the third day of a fast, at maximal gluconeogenesis.
• After that:
• Plasma glucagon declines as fatty acids and ketones become main energy sources.

G) Exercise balancing mechanism

• During exercise:
• Glucose utilization increases.
• This is balanced by increased glucose production caused by increased circulating glucagon.

H) Gut mediation of amino-acid effect (oral > IV)

• Oral amino acids stimulate glucagon more than IV amino acids.
• Suggests a glucagon-stimulating factor from GI mucosa.
• GI hormones effects:
• CCK and gastrin increase glucagon secretion.
• Secretin inhibits glucagon secretion.
• Because CCK and gastrin rise with a protein meal:
• Either could mediate the GI component of the glucagon response.
• Somatostatin inhibition is referenced separately.

10) TABLE 24–5: FACTORS AFFECTING GLUCAGON SECRETION (exact list)

Stimulators

• Amino acids (particularly glucogenic: alanine, serine, glycine, cysteine, threonine)
• CCK, gastrin
• Cortisol
• Exercise
• Infections
• Other stresses
• β-Adrenergic stimulators
• Theophylline
• α-Adrenergic stimulators
• Acetylcholine

Inhibitors

• Glucose
• Somatostatin
• Secretin
• FFA
• Ketones
• Insulin
• Phenytoin
• GABA

Special note (FFA/ketone inhibition can be overridden)

• FFA and ketones inhibit glucagon secretion.
• But this can be overridden:
• Plasma glucagon is high in diabetic ketoacidosis.

11) INSULIN–GLUCAGON MOLAR RATIOS (why both hormones must be considered)

Opposing roles (storage vs release)

• Insulin is:
• Glycogenic
• Antigluconeogenetic
• Antilipolytic
• Antiketotic
• Overall: hormone of energy storage
• Glucagon is:
• Glycogenolytic
• Gluconeogenetic
• Lipolytic
• Ketogenic
• Overall: hormone of energy release
• Therefore:
• Must interpret physiology in terms of both hormones together, conveniently using their molar ratio.

Why ratios fluctuate

• Ratios fluctuate because secretion of both hormones depends on conditions preceding the stimulus.

Numeric ratios given (must memorize)

• Balanced diet: insulin–glucagon molar ratio ≈ 2.3
• Arginine infusion (fed state): increases both hormones → ratio rises to 3.0
• After 3 days starvation: ratio falls to 0.4
• Arginine infusion after 3 days starvation: ratio lowers further to 0.3
• Constant glucose infusion: ratio is 25
• Protein meal during constant glucose infusion: ratio rises to 170
• Because insulin rises sharply and the usual glucagon response to protein is abolished.

Functional interpretation

• During starvation (energy needed):
• Ratio is low → favors glycogen breakdown and gluconeogenesis.
• When energy mobilization need is low:
• Ratio is high → favors deposition/storage of glycogen, protein, and fat.

12) CLINICAL BOX 24–4 (two clinical correlations)

A) Macrosomia in infants of diabetic mothers

• Infants of diabetic mothers often have:
• High birth weight
• Large organs (macrosomia)
• Mechanism:
• Excess fetal circulating insulin
• Due partly to fetal pancreas stimulation by high maternal blood glucose and amino acids.
• Placenta handling of insulin:
• Free insulin in maternal blood is destroyed by placental proteases.
• Antibody-bound insulin is protected and can reach the fetus.
• Therefore:
• Fetal macrosomia also occurs when women develop antibodies against animal insulins and continue receiving animal insulin during pregnancy.

B) GLUT-1 deficiency

• Defective transport of glucose across the blood–brain barrier.
• Findings:
• Low CSF glucose with normal plasma glucose.
• Seizures.
• Developmental delay.

Exam-lock one-liner (compression without omission of the key logic)

• Glucagon is a 29–amino-acid A-cell peptide derived from tissue-specific processing of preproglucagon, and via hepatic GPCR signaling (Gs–cAMP–PKA plus PLC–Ca2+) it promotes glycogenolysis, gluconeogenesis, lipolysis and ketogenesis, with secretion driven mainly by hypoglycemia, amino acids, autonomic input and stress, and its physiologic meaning is best interpreted with insulin using the insulin–glucagon molar ratio.

🧬 GLUCAGON — COMPLETE MASTER TABLE (ZERO OMISSION)

🔒 FINAL EXAM LOCK (ONE-LINE)

Glucagon is a 29-AA A-cell peptide derived from tissue-specific processing of preproglucagon that, via hepatic GPCR signaling (Gs–cAMP–PKA and PLC–Ca²⁺), drives glycogenolysis, gluconeogenesis, lipolysis and ketogenesis; its secretion is governed by hypoglycemia, amino acids, autonomic input and stress, and its physiologic meaning is understood only in relation to insulin via the insulin–glucagon molar ratio.

OTHER ISLET HORMONES + OTHER HORMONES/EXERCISE + HYPOGLYCEMIA + DIABETES + OBESITY/METABOLIC SYNDROME (ZERO-OMISSION, LOGIC-BASED)

1) OTHER ISLET CELL HORMONES (what else the islet secretes and why it matters)

• Besides insulin and glucagon, pancreatic islets secrete:
• Somatostatin
• Pancreatic polypeptide
• Somatostatin may also act within the islets to adjust the pattern of hormones secreted in response to stimuli.

SOMATOSTATIN (D-cell hormone)

2) Forms and location

• Somatostatin 14 (SS 14) and somatostatin 28 (SS 28) (amino-terminal–extended form) are found in D cells of pancreatic islets.

3) Core action in islets (paracrine inhibition)

• Both SS 14 and SS 28 inhibit secretion of:
• Insulin
• Glucagon
• Pancreatic polypeptide
• They act locally inside the islets in a paracrine fashion.

4) SS 28 vs SS 14 (receptor + potency)

• SS 28 is more active than SS 14 at inhibiting insulin secretion.
• SS 28 apparently acts via SSTR5 receptor.

5) Somatostatinoma (clinical consequences)

• Somatostatin-secreting pancreatic tumors (somatostatinomas) cause:
• Hyperglycemia
• Other manifestations of diabetes
• These manifestations disappear when the tumor is removed.
• Additional features:
• Dyspepsia due to:
• Slow gastric emptying
• Decreased gastric acid secretion
• Gallstones precipitated by:
• Decreased gallbladder contraction
• due to inhibition of CCK secretion

6) What stimulates pancreatic somatostatin secretion

• Increased by several of the same stimuli that increase insulin secretion:
• Glucose
• Amino acids, particularly:
• Arginine
• Leucine
• Also increased by CCK.
• Somatostatin is released from:
• Pancreas
• Gastrointestinal tract

into the peripheral blood.

PANCREATIC POLYPEPTIDE (F-cell hormone)

7) Chemistry + family relations

• Human pancreatic polypeptide:
• Linear polypeptide
• 36 amino acid residues
• Produced by F cells in islets
• Closely related to:
• Polypeptide YY (GI peptide)
• Neuropeptide Y (brain and autonomic nervous system)
• Shared structural feature:
• All end in tyrosine
• All are amidated at the carboxyl terminal