Central and peripheral adrenergic mechanisms regulating gastric secretion in the rat

Subcutaneous injection of sympathomimetic agents reduced gastric secretion in pylorus-ligated rats, and the decreasing order of activity was: isoproterenol, norepinephrine, naphazoline and phenylephrine. The effect of naphazoline and phenylephrine was antagonized with 4 mg/kg s.c. of phentolamine, and that of isoproterenol with 4 mg/kg s.c. of propranolol. Thus there exist separate, alpha and beta adrenergic receptors which control gastric secretion in the rat. Intracerebroventricular (i.c.v.) injection of the sympathomimetic agents also reduced secretion, naphazoline being the most potent. The ED50 of i.c.v. naphazoline was 12.3 times less than that of s.c. dose. The antisecretory effect of i.c.v. naphazoline was antagonized with i.c.v. phentolamine (0.016 and 0.064 mg/kg), but not with propranolol. These results suggest that naphazoline stimulates central alpha adrenergic receptors which has a tonic inhibiting role in rat gastric secretion. Analogous to the results with naphazoline, i.c.v. phentolamine, but not propranolol, blocked an antisecretory effect of chlorpromazine. Phentolamine administered s.c. also reduced the antisecretory activity, but the dose required for the antagonism was 250 times that of i.c.v. phentolamine. An antisecretory effect of imipramine was not blocked by phentolamine or propranolol. These results suggest that an activation of central alpha adrenergic receptors is important for the antisecretory effect of chlorpromazine.