Endocrine Control of the GI Tract & Salivary Physiology – Comprehensive Exam Notes

Introduction & Course Context

Session covers continuation of “endocrine control of the tract” begun last Tuesday; content up through salivary glands will appear on the upcoming mid-term (Tuesday).
Historical note: first gut hormone discovered = secretin (1902); lecture order starts with gastrin for relevance to stomach.
Families of GI hormones are grouped by peptide sequence similarity (e.g., Gastrin/CCK family, Secretin/VIP family).

Gastrin

Source
- G-cells in stomach antrum (major) & body (minor).
Stimuli for Release
- Gastric distension (mechanical stretch).
- Peptides & amino acids—aromatic AAs strongest.
- Cephalic phase triggers: thought/smell/talk of food.
Family: Gastrin–CCK family; can cross-activate CCK receptors at high levels.
Major Actions
1. Stimulates parietal cell $\text{HCl}$ secretion (requires histamine co-signal).
2. Trophic factor for gastric mucosa (maintains enzyme complement & prevents atrophy).
Mechanism of Acid Secretion
1. G-cell → blood → ECL-cell (enterochromaffin-like) → histamine.
2. Histamine + gastrin → parietal cell → $\text{H}^+$ secretion into lumen (NOT bloodstream).
Negative Feedback
- Low luminal pH activates D-cells → somatostatin → inhibits G-, ECL-, and parietal cells → ↓ gastrin, histamine, acid.
Isoforms
- $G_{17}$ (short, transient; acid-secretion).
- $G_{34}$ (longer half-life; mucosal maintenance).
Minor / Species-limited Actions
- ↓ gastric emptying (rats ≫ humans).
- Pancreatic $\text{HCO}_3^-$ release, biliary secretions (non-human models).

Cholecystokinin (CCK)

Discovery: 1928; member of Gastrin/CCK family.
Source: I-cells in proximal 2⁄3 of small intestine (duodenum & jejunum; density tapers in ileum).
Stimuli
- Proteins, small peptides, aromatic AAs.
- Fats, especially long-chain free fatty acids.
- Little/no carbohydrate effect.
Major Actions
1. Gallbladder contraction → bile into duodenum.
2. Relaxation of sphincter of Oddi (coordinates bile + pancreatic juice entry).
3. Pancreas: stimulates enzyme secretion & (potentiates) $\text{HCO}_3^-$ secretion; supports acinar growth/maintenance.
4. Central effects: ↓ meal size & food intake; ↑ energy expenditure.
Minor / debated
- Slows gastric emptying (likely significant in humans despite older literature).
Isoforms: multiple, non-integer lengths (e.g., CCK-8, CCK-58, CCK-83); 58 & 83 most abundant in plasma.

Secretin

First hormone discovered (1902); part of Secretin/VIP family.
Source: S-cells in proximal small intestine (duodenum → mid-ileum).
Stimuli
- Luminal acidity (↓ pH from gastric chyme).
- Irritants (e.g., capsaicin, possibly turmeric).
Major Actions
1. Potent stimulator of pancreatic & biliary $\text{HCO}_3^-$ secretion → neutralize acid.
- Pancreatic response is potentiated by CCK (synergism).
1. Trophic for pancreatic tissue.
Minor
- Inhibits gastric emptying via ↓ gastric contractions (species-dependent).
Form: Secretin-27 (S-27) predominant circulating peptide.

GIP (Glucose-Dependent Insulinotropic Peptide)

Historical names: originally “Gastric Inhibitory Peptide” (pharmacologic doses inhibited stomach); now re-designated for true physiologic role.
Source: K-cells, proximal small intestine (duodenum/jejunum → mid-ileum).
Stimuli: Primarily fats (also proteins); negligible carbohydrate influence.
Physiologic Action
- Incretin: enhances insulin secretion from pancreatic β-cells before plasma glucose rises.
Pharmacology
- Receptor agonists (e.g., semaglutide/Ozempic, Wegovy) used in T2DM & obesity ↓ food intake & body weight.

Motilin

Source: M-cells, proximal small intestine.
Release Condition: Fasting/post-absorptive state (interval depends on individual).
Action
- Initiates Migrating Motor Complex (MMC) → strong rhythmic gastric contractions (stomach “growling”); clears residual food/bacteria.
Clinical Note: Eating terminates MMC; water alone ineffective—requires caloric content.

Additional Paracrine / Hormone-Like Factors

Gastric Somatostatin
- D-cells (stomach).
- Trigger: low luminal pH.
- Inhibits G-, ECL-, and parietal cells (↓ gastrin, histamine, acid).
Peptide YY (PYY)
- Ileum terminal & proximal colon L-cells.
- Released when unabsorbed nutrients reach distal gut → “ileal brake” ↓ small-intestine motility.
Gastric Histamine
- ECL-cells (stimulated by gastrin).
- Paracrine stimulator of parietal HCl secretion.
- Pharmacology: $H_2$ -receptor blockers (e.g., ranitidine) reduce acid in GERD/ulcer.

Salivary Gland Physiology

Functions of Oral Cavity

Chemosensation (taste).
First site of digestion
- Mechanical: mastication.
- Chemical: salivary enzymes (minor).
Swallow initiation; speech; protection of teeth/mucosa.

Major Salivary Glands & Secretion Types

Gland	Location	Secretion Role
Parotid	Near ear	Food-stimulated, watery, $\text{HCO}_3^-$ -rich (~50 % of stimulated flow)
Submandibular	Jaw angle	Basal & stimulated
Sublingual	Floor of mouth	Basal (viscous, protein-rich); calculus deposition on lower incisors

Basal flow: viscous, low volume, protein-rich (mucoprotection).
Stimulated flow: high volume, watery, $\text{HCO}_3^-$ -rich (neutralize reflux/dietary acid).
Daily volume: $\approx 0.5\text{–}0.6\,\text{L}$ .

Saliva Composition

$\approx 99\%$ water.
Ions: $\text{Na}^+$ , $\text{K}^+$ , $\text{Cl}^-$ , $\text{HCO}_3^-$ .
Proteins (≈1 % by weight; multifunctional):
- Mucins → mucus (lubricant, antibacterial, enamel shield).
- Statherins & proline-rich proteins → inhibit bacterial growth, preserve enamel.
- Lysozyme + salivary IgG → disrupt bacterial walls, immune defense.
- Digestive enzymes:
  • Salivary amylase (starch → maltose; ~<20\% total starch digestion).
  • Lingual lipase (triglycerides; minor, enhances fat “mouth-feel”).
Enzymes function best at $\text{pH}\,7.4$ ; inactivated in gastric acid.

Bicarbonate Role

Neutralizes dietary acids & gastric reflux—protects esophagus & tooth enamel.

The Salivon (Functional Unit)

Acinus (acinar cells)
- Produces isotonic, plasma-like primary saliva (water + ions + proteins).
- Mechanisms
  - $\text{Na}^+$ moves paracellularly into lumen.
  - $\text{Cl}^-$ enters cell via $\text{Na}^+\text{–K}^+\text{–}2\,\text{Cl}^-$ cotransporter, exits through apical $\text{Cl}^-$ channels (includes CFTR).
  - Resulting osmotic gradient pulls water through tight junctions.
Duct (ductal cells)
- Modifies saliva—reabsorbs $\text{Na}^+$ / $\text{Cl}^-$ , secretes $\text{K}^+$ / $\text{HCO}_3^-$ .
- Key transporters
  - Apical $\text{NHE}$ (Na^+/H^+) exchanger removes $\text{Na}^+$ from lumen, exchanges $\text{H}^+$ (re-cycled by apical $\text{H}^+/\text{K}^+$ ATPase → K^+ added to saliva).
  - Basolateral $\text{Na}^+/\text{K}^+$ ATPase exports Na^+ to blood.
  - Apical $\text{Cl}^- / \text{HCO}<em>3^-$ exchanger (anion transporter) brings $\text{Cl}^-$ in, secretes $\text{HCO}</em>3^-$ to lumen; $\text{Cl}^-$ exits basolaterally via channels.

Flow-Rate Dependence
- Low flow (basal): saliva moves slowly → ample time for ductal modification → low $\text{Na}^+$ / $\text{Cl}^-$ , high $\text{K}^+$ .
- High flow (stimulated): limited time for exchange → ionic composition approximates plasma; $\text{HCO}_3^-$ paradoxically rises because secretion is flow-driven.

Clinical / Pathophysiological Notes

CFTR mutation (cystic fibrosis) impairs salivary $\text{Cl}^-$ handling.
$H_2$ receptor antagonists treat acid reflux by targeting histamine pathway.
Excessive spitting could lead to dehydration due to ~ $0.5\,\text{L}$ fluid loss.

Integration & Key Takeaways

Five classical GI hormones: Gastrin, CCK, Secretin, GIP, Motilin—each with distinct sources, stimuli, and target actions but extensive interplay (e.g., CCK potentiates secretin on pancreas).
Negative feedback mechanisms (somatostatin in stomach, peptide YY ileal brake) protect mucosa & regulate nutrient throughput.
Saliva illustrates coordinated exocrine secretion with flow-dependent ionic modulation; CFTR, NHE, and $\text{H}^+/\text{K}^+$ ATPase mirror transport themes repeated throughout GI tract.
Understanding hormone control, feedback loops, and transport mechanisms is critical for diagnosing/treating disorders such as GERD, ulcers, pancreatitis, diabetes, and motility syndromes.