Secretin Stimulates Pancreatic Bicarbonate Secretion, Not Enzyme Secretion
The correct answer is none of the options provided are accurate representations of secretin's primary known actions. However, if forced to choose from the given options, Option B is the closest but technically incorrect—secretin primarily stimulates pancreatic bicarbonate and water secretion, not enzyme secretion. 1, 2
Primary Actions of Secretin
Secretin is a 27-amino acid gastrointestinal hormone whose principal physiological role is stimulating bicarbonate-rich pancreatic fluid secretion, not enzyme secretion. 1, 3, 2 The hormone acts through its specific G protein-coupled receptor (SCTR) to trigger:
- Pancreatic and biliary ductular bicarbonate and water secretion - This is the primary and most well-established action of secretin 4, 5
- Renal bicarbonate excretion - Recently discovered function for systemic HCO3- homeostasis 5
- Inhibition of gastric acid secretion - Not stimulation as suggested in option D 2
Why Each Option is Incorrect
Option A: Stimulates Gastrin Secretion
This is incorrect. Secretin does not stimulate gastrin secretion. In fact, secretin's role in regulating gastric acid secretion suggests an inhibitory rather than stimulatory relationship with gastric function. 2
Option B: Stimulates Pancreatic Enzyme Secretion
This is misleading and technically incorrect. While secretin does affect pancreatic secretion, its primary action is stimulating bicarbonate and water secretion, not enzyme secretion. 1, 2 Cholecystokinin (CCK), not secretin, is the primary hormone responsible for stimulating pancreatic enzyme secretion. 1 Secretin and CCK work synergistically, but their individual actions are distinct. 1
Option C: Inhibits Intestinal Motility
This is not a primary established action of secretin. While secretin plays a role in coordinated gastrointestinal motility responses, inhibition of intestinal motility is not its recognized primary function. 6, 2
Option D: Stimulates Gastric Acid Secretion
This is directly opposite to secretin's actual effect. Secretin is known to regulate (inhibit) gastric acid secretion, not stimulate it. 2
Critical Clinical Context
The fundamental action of secretin is neutralizing gastric acid entering the duodenum by stimulating bicarbonate-rich pancreatic fluid. 1, 2 This represents the classic "intestinal phase of digestion" where:
- Acidic chyme from the stomach triggers secretin release from duodenal S cells 3, 2
- Secretin acts on pancreatic ductal cells to secrete bicarbonate and water 4, 5
- This neutralizes gastric acid and creates optimal pH for pancreatic enzyme activity 2
Common Pitfall
Do not confuse secretin's action with CCK. While both are released during the intestinal phase of digestion and work synergistically, secretin primarily stimulates fluid and bicarbonate secretion, whereas CCK primarily stimulates enzyme secretion. 1 This distinction is fundamental to understanding pancreatic physiology.