Effect of naloxone on vagally-induced gastric acid secretion in rats

Previous studies in man and dogs have demonstrated that endogenous opioids participate in the stimulation of cephalic phase gastric acid secretion during indirect activation of the vagus. Since the effect of naloxone during direct vagal activation is unknown gastric acid secretion was assessed during electrical stimulation of the distal cut ends of both vagal nerves. In overnight fasted anesthetized rats the distal ends of the bisectioned cervical vagi were stimulated with 10V, 5Hz, 5 msec for 15 min. Vagal stimulation elicited an increase of gastric acid secretion by 5.6 mumol/min. Naloxone (1 mumol/kg.h) augmented gastric acid secretion significantly. Since this effect of naloxone was in contrast to previous data possible mechanisms of action of naloxone were examined that might help to explain this apparently inhibitory action of endogenous opioids on vagally-induced gastric acid secretion. The additional infusion of atropine or hexamethonium abolished the stimulatory effect of naloxone on vagally-induced acid secretion completely indicating that the action of naloxone depends on cholinergic background activity. Combined blockade of alpha- and beta-adrenergic receptors with phetolamine and propanolol reduced vagally-induced acid secretion in controls and during naloxone to a similar degree. Both adrenergic blocking agents also reduced the residual acid secretion observed during atropine or atropine + naloxone infusion. Measurements of plasma gastrin levels suggested that the naloxone-induced changes of acid secretion were not due to alterations of gastrin secretion. In summary these data demonstrate that vagally-induced acid secretion in anesthetized rats is largely due to cholinergic mechanisms with a small but separate contribution of adrenergic mechanisms. Endogenous opioids are activated during peripheral vagal stimulation attenuating vagally-induced acid secretion by modulation of cholinergic but not adrenergic mechanisms.