Osmotic stimulation of pepsin secretion in the rat.

The effect of the osmolarity of intragastric instillates on pepsin secretion was studied in rats anaesthetised with urethane. Irrigation of the stomach with solutions of sucrose and NaCl, resp. caused a concentration-dependent increase in pepsin output. A stimulation was observed already by hypotonic solutions and the maximal effect was obtained by 300 m-osmole/l of sucrose and by 600 m-osmole/l of NaCl (13- and 10-fold stimulation resp.). A similar time course in the increase of pepsin output was produced by hyperosmotic solutions (600 m-osmole/l) of sucrose, urea, NaCl and choline chloride. Pepsin output was stimulated maximally within 30 min and decreased thereafter, but remained at about 4--6-fold higher levels than during the previous irrigation with distilled water. Replacement of hyperosmotic instillates by distilled water reduced pepsin secretion to the initial level. Hypertonic ethanol (600 m-osmole/l) increased pepsin output only slightly. Vagotomy, pretreatment with atropine (1 mg/kg i.v.) or cimetidine (5 mg/kg i.v.), local anesthesia of the gastric mucosa with 4% lidocaine or intravenous infusion of PGE2 (2 microgram/kg X min) did not antagonise the stimulation of pepsin output induced by hyperosmotic NaCl (600 m-osmole/l). The results indicate that the increase of the osmolarity of intragastric instillates stimulates pepsin secretion in the rat without involvement of neural (vagal or local cholinergic reflexes) or hormonal mechanisms (release of gastrin) which are known to stimulate gastric secretion in the gastric phase.