Physiology of Secretions

General Introduction

The purpose of the gastrointestinal tract is to digest and absorb food and fluid; several secretions facilitate this process.

Since the GI tract is in steady state, the amount that enters the lumen must equal the amount that exits:

ENTERING LUMEN

ingested:

Food/drink – 1.50 L

secreted:

Saliva – 1.50 L
Gastric juice – 3.00 L
Pancreatic juice – 2.00 L
bile – 0.50 L

TOTAL: 8.50 L

EXITING LUMEN

absorbed:

small intestine – 5.85 L
colon – 2.50 L

eliminated:

feces – 0.15 L

TOTAL: 8.50 L

In addition to acting as a barrier, epithelium can transport substances, so it mediates absorption and secretion

Note: Diseases that alter epithelial transport can dramatically alter flow through the GI tract

CF-1 chloride channel (cystic fibrosis) knockout mice do not absorb efficiently and the bowel ruptures

inflammation of the bowel can increase secretion and cause systemic dehydration

In general, all gastrointestinal secretions share the following characteristics:

composition of the secretion varies dramatically upon rate of secretion, and
flow of secretion is inducible (up to 10 or even 50 fold) and is carefully controlled

Saliva – 1.5 L/day

Saliva consists of mostly water and salt

other constituents: amylase (CHO digestion), glycoproteins, lysozyme, peroxidase, lipase, IgA, ABO blood group antigens, R binder (a protein important for vitamin B₁₂ metabolism), and proline-rich-proteins (PRP)

basal secretion 0.1 ml/min, inducible to 4.0 ml/min

mucins are for lubrication

Functions:

(1) dilute food – also moisten and lubricate to prevent esophageal damage, and dissolve to allow taste

(2) begin digestion of carbohydrates – by means of amylase; accounts for up to 40% of total carbohydrate digestion

(3) antibacterial/immunological – defend against pathogens and retard dental caries

(4) wash out substances – e.g. gastric acid during vomiting

(5) facilitate speech – hence public speakers have drinking water

Salivary secretion is not essential for normal digestion and absorption of nutrients

Sjogren’s syndrome: autoimmune exocrinopathy affecting salivary and lacrimal glands

Types of Salivary Glands and Mechanism of Secretion – There are three main types of salivary glands:

(1) parotid – 25% of secretion – serous only (watery, high amylase)

(2) submaxillary – 70% of secretion – mixed serous/mucous (6:1 ratio)

(3) sublingual – 5% of secretion – predominantly mucous (high mucin content, low amylase); serous demilunes

Each of these glands contains different proportions of three cell types:

(1) acinar – near origin of gland, secrete Na⁺, Cl^-, and amylase (from zymogen granules)

water-permeable so water follows ion flow

mechanism of ion transport:

basolateral Na⁺/K⁺ pump charges a sodium gradient

this drives basolateral Na⁺/K⁺/2Cl^- cotransporter to import Cl^- ions (K⁺ ions recycle through K⁺ channels)

high intracellular Cl^- drives apical Ca⁺⁺-dependent Cl^- channels, which secrete chloride to the lumen

the charge imbalance created by this transcellular Cl^- transport causes paracellular Na⁺ migration

(2) striated – downstream – secrete K⁺ and HCO₃^-, absorb Na⁺ and Cl^-

water-impermeable so pumping ions out dilutes secretion

contain basal infoldings full of mitochondria

produce ATP for selective ion transport

mechanism of ion transport:

basolateral Na⁺/K⁺ pump charges a sodium gradient

this drives an apical Na⁺ channel to absorb Na⁺

sodium gradient also drives apical and basolateral Na⁺/H⁺ antiporters that ß intracellular pH, and this drives apical K⁺/H⁺ antiporters that secrete K⁺

Cl^- is absorbed by apical and basolateral Cl^- channels, which are driven by electrostatic force

Cl^- gradient drives Cl^-/HCO₃^- antiporters that secrete HCO₃^-

(3) intercalated – nondescript columnar cells that exist alongside striated

In order to dilute food to the osmolality of plasma, saliva is hypotonic to plasma

salt is moved out of an isotonic solution through water-impermeable epithelium (ductile cells; striated and intercalated)

The same thing is observed whenever hypotonic solution is produced – e.g. sweat glands and distal nephron when ADH is not present.

acetylcholine during parasympathetic stimulation Ý intracellular Ca⁺⁺ and activation of Cl^- and K⁺ channels

Saliva enters acinar cells isotonic to plasma, and is diluted in duct by striated cells

Therefore, ion concentrations are dependent upon flow, since faster flow means less time is spent in the duct

at low flow rates, saliva is very hypotonic (50 mM), with high K⁺ (25-30 mM) and low Na⁺, Cl^-, and HCO₃^- (10 mM)

Na⁺ and Cl^- concentrations << plasma

at high flow rates, saliva is less hypotonic (250 mM), with higher Na⁺ (100 mM), Cl^- (90 mM), and HCO₃^- (50 mM)

Na⁺ and Cl^- concentrations < plasma, K⁺ and HCO₃^- concentrations > plasma

Ý secretion of HCO₃^- causes saliva to be alkaline (about pH = 8.0) at high flow rate

Ý saliva secretion Ý HCO₃^-secretion

release of kallikreins Þ convert kininogens to bradykinin Þ vasodilation Þ Ý blood flow to salivary glands during parasympathetic stimulation

Regulation of Saliva Flow

Control of saliva flow is primarily neuronal (allows rapid stimulation) through the autonomic nervous system

parasympathetic cholinergic stimulation enhances secretion (rich in amylase, high volume, more serous)

anticholinergics produce dry mouth

sympathetic adrenergic stimulation ß secretion but Ý mucin concentration (low volume, viscous, more mucous)

Gastric Juices – 1-3.0 L/day

The stomach has several functions:

(1) Storage of food to allow slow release into the small intestine – allows fewer and larger meals

(2) Mixing of food – the stomach is a very muscular organ

(3) Acid Secretion – HCl activates pepsinogen hydrolysis to produce pepsin, and kills bacteria

NOTE: High grain diets can cause cow stomach environments to be less acidic, facilitating E. coli infections; therefore cows are not fed grain for the last few days prior to slaughter

(4) Protein Digestion – via pepsin, which is regulated by gastrin and intestinal secretions

(5) Carbohydrate Digestion – salivary amylase continues to function until pH falls below 4.0

(6) Production of Intrinsic Factor – needed for B12 absorption – only function of stomach essential for life

R binder binds B₁₂, intrinsic factor absorbs it

Parietal cells secrete 0-3 ml/min – ion composition depends on flow rate:

at low flow rate, secretion has high Cl^- (120 mM), Na⁺ (90 mM), H⁺ (20 mM), and K⁺ (10 mM)
at high flow rate, secretion has much lower Na⁺ (10 mM) and much higher H⁺ (120 mM; pH=1.0!)
secretion is always nearly isotonic to plasma (300 mOsm/L) - at high rates, HCl secretion is isoosmotic to plasma (150 mM)

Secretory Glands and Cell Types

Although anatomically divided into different segments, the stomach acts as single functional unit, the surface of the stomach is lined with a columnar epithelium with multiple gastric pits (100-200 /cm³)

these are deep invaginations into the mucosa, into which several gastric glands secrete gastric juice
the glands are divided into the isthmus (parietal and surface mucous cells), neck (parietal and mucous neck cells), and base (chief cells, some parietal and mucous cells)

Major Cell Types:

(1) mucin-secreting cells – line stomach and duodenum – packed with mucin granules; columnar epithelium

secrete unstirred mucin layer that can be several millimeters thick
also secrete HCO₃^- that titrates acid as it diffuses through mucin

(2) chief cells – synthesize and secrete pepsinogen – zymogen granules undergo Ca⁺⁺-mediated fusion with apical surface

only present at the base of the gland; functions similarly to acinar cells of pancreas

(3) parietal cells – secrete acid and makes intrinsic factor – many mitochondria for active ion transport

secretory canaliculus – multiple invaginations of apical membrane to dramatically increase surface area

produce secretions with pH as low as 1.0 – millionfold concentration of H⁺ from plasma

source of protons is carbonic anhydrase reaction (CO₂ + H₂O Þ HCO₃^- + H⁺)

mechanism of ion transport – uses four transporters:

apical K⁺/H⁺ pump pumps H⁺ into the lumen (uses ATP)

basolateral K⁺ channels allow absorbed K⁺ to leave
loss of H⁺ drives carbonic anhydrase reaction to the right, producing excess of HCO₃^- – drives basolateral Cl^-/HCO₃^- antiporter (secretes HCO₃^- into blood)

apical Cl^- channels allow Cl^- to diffuse into lumen

the K⁺/H⁺ pump (also called "proton pump") is unique to these cells

inhibited by drugs called PPI’s (proton pump inhibitors, e.g. omeprazole) which are used to treat tumors of gastrin-producing cells

HCO₃^- is secreted back into the blood, and is transported to mucin-secreting cells on the surface

(4) enteroendocrine cells – secrete biogenic amines and hormones

also called ECL (enterochromaffin like) cells – paracrine signalling via histamine

importance underestimated – probably the primary location of regulation of acid secretion
also secrete serotonin, somatostatin, VIP, bombesin, and gastrin

Regulation

Acetylcholine (parasympathetic) and Gastrin (hormone from gastric and intestinal epithelium) produce a Ca⁺⁺-mediated response in ECL cells, causing them to release histamine

Histamine then stimulates parietal cells (paracrine signalling) by a cAMP-mediated pathway
acid secretion can be attenuated by H₂ histamine receptor antagonists such as ranetidine

prostaglandin PGE₂ inhibits acid secretion by downregulating adenylate cyclase through G_i, an inhibitory G-protein

histamine cannot then stimulate parietal cells

Three Phases of Gastric Secretion

(1) cephalic – caused by chewing, swallowing, thinking about food, etc. (smell, thought, taste)

parasympathetic (vagal) input – cause parietal cells to secrete acid and G-cells to secrete gastrin
can rapidly increase acid secretion to 40% of maximum

(2) gastric – stimulated by gastric distention or presence of peptides in the stomach

causes release of acetylcholine which mimics cephalic stimulation (local or vagovagal reflex)
stimulates gastrin release

(3) intestinal – stimulated by intestinal dilation or AA/peptides in intestine

activates intestinal G-cells and endocrine cells

Acid Inhibition – Three Locations

(1) antrum of stomach – direct feedback to control acid level by inhibiting gastrin release – more inhibition at lower pH

(2) duodenum – feedback to (1) reduce acid and (2) stimulate alkaline pancreatic and biliary secretions to neutralize acid

mediated primarily by secretin

(3) intestine – fatty acids in intestine cause reduction of gastric acid secretion by means of several peptides

mediated primarily by CCK (cholecystokinin)

Some Experimental Proof of Regulation

(1) Neuronal: Sham feeding of people with esophageal stenosis (chew but not into stomach)

stimulate acid secretion only if they get a meal they like (not with cereal greul) – shows neuronal (cephalic) stimulation

(2) Hormonal: Heidenheim pouch in dog stomach (denervated segment where acid is measured)

secrete acid upon sham feeding even though denervated – shows activation by hormone (gastrin) from innervated cells
does NOT work if entire stomach is denervated

(3) Peptide Detection: several Heidenheim pouches in dog stomach – put 3% liver solution in only one pouch

secrete acid in another denervated pouch – shows gastrin secretion secreted by peptides as well as neuronal

(4) Acid Feedback: repeat above experiment with 3% liver solution at pH = 1.0

no acid secretion, so pH feedback overrides effect

Pancreas

Functions

(1) Digestive Enzymes – produces and secretes enzymes that digest protein (peptidases/hydrolases), carbohydrate (amylase), and fat (lipases), into a form that can be absorbed by the intestine into the circulation.

(2) HCO₃^- Secretion – (1) solubilize and flush enzymes out of the pancreas (2) neutralize gastric secretions (note: these previously mentioned enzymes do not work in an acidic environment).

Functional Anatomy of Pancreas:

acinar cells – primarily secrete zymogens Þ enteropeptidase converts trypsinogen to trypsin, which in turn converts pro-enzymes to enzymes

ductal cells – secrete salt and water (minor); major secretion is HCO₃^-

Phases of secretion: analogous to gastric secretion…

Phase	Stimulus	Mediators	Outcome
Cephalic	Taste, smell, chewing and swallowing of food	ACh (vagal)	Enzymes
Gastric	Gastric distension	ACh (vagal)	Enzymes
Intestinal	Acid in duodenum, fat, peptides, and amino acids	Secretin, CCK, Ach	Enzymes and fluid (HCO₃^-)

Secretion rate and pancreatic juice composition:

Pancreatic juice is characterized by:

High volume (~ 2 L/day). Note: most secretions occur in short bursts (basal rate<0.1 ml/min).

Almost same [Na⁺] and [K⁺] as plasma

Much higher [HCO₃^-] than plasma

Much lower [Cl^-] than plasma

Isotonicity

Pancreatic peptidases, amylase, and lipases. Juice contains 0.1-10% proteins. Zymogens released via exocytosis from acinar cells Þ activated subsequent to release.

Changes in ion concentrations at different flow rates:

Cellular mechanisms:

Acinar Cells (same as salivary glands) – produce a small volume of initial pancreatic secretion, which is mainly NaCl and water. Secretin and VIP potentiate acinar secretion (see below).

The apical membrane Cl^- channel is the protein CFTR. In cystic fibrosis this channel is mutated and causes a deficiency of pancreatic enzymes resulting in malabsorption and steatorrhea.

Ductal Cells (different than salivary glands) – secrete a large volume of Na⁺/HCO₃^- fluid modify the initial pancreatic secretion by secreting HCO₃^- and absorbing Cl^- via a Cl^—HCO₃^- exchange mechanism in the luminal membrane. Because the pancreatic ducts are permeable to water, H₂O moves into the lumen to make the pancreatic secretion iso-osmotic. Secretin is the most important secretagogue, can Ý pancreatic secretion x50 and [HCO₃^-] to > 150 mEq/L.

concentration depends on flow rate; Ý flow, Ý HCO₃^-, ß Cl^-; Na⁺ and K⁺ remain unchanged

Regulation of pancreatic secretion:

(1) Secretin – secreted by S cells of duodenum in response to H⁺ in duodenal lumen. Acts on the pancreatic ductal cells to increase HCO₃^- secretion. The 2^nd messenger for secretin is cAMP.

(2) CCK – secreted by I cells of duodenum in response to peptides, amino acids, and fatty acids in duodenal lumen. Acts on the pancreatic acinar cells to increase enzyme secretion (amylase, lipases, proteases). Potentiates the effect of secretin (i.e. Ý HCO₃^- secretion) by working in parallel but different pathway: the 2^nd messenger for CCK is IP₃ and Ý [Ca²⁺].

(3) ACh (via vagovagal reflexes) – is released in response to H⁺, peptides, amino acids, and fatty acids in duodenal lumen. Stimulates enzyme secretion by acinar cells and, like CCK, potentiates the effect of secretin on HCO₃^- secretion.