Modulation by endogenous nitric oxide of acid secretion induced by gastric distention in rats: enhancement by nitric oxide synthase inhibitor.

The mechanism underlying acid hypersecretion induced by gastric distention was investigated in rats, especially in relation to endogenous nitric oxide (NO). Under urethane anesthesia, rat stomach was distended by instillation of saline (1-10 ml) through the acute fistula that was provided through a pylorus. Gastric samples were collected every 1 h, and the acid secretion was measured by titration with 100 mM NaOH. Gastric acid secretion was increased by distention, and the degree of stimulation was dependent on the volume of saline instillation; a maximal response occurred with 6-ml instillation, which maintained the intraluminal pressure of about 20 cm H(2)O. The increased acid secretory response induced by distention was completely blocked by omeprazole and significantly mitigated by vagotomy, sensory deafferentation, atropine, or famotidine but markedly enhanced by the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). On the other hand, the enhanced acid response in the presence of L-NAME occurred in an L-arginine-sensitive manner and was almost totally abolished by vagotomy and sensory deafferentation as well as by atropine. Gastric distention increased the release of NO metabolites and histamine into the gastric lumen. The NO metabolite release in the distended stomach was significantly decreased by vagotomy or L-NAME, whereas the histamine output was decreased by vagotomy but increased by L-NAME in an L-arginine-sensitive manner, respectively. These results suggest that 1) gastric distention increases acid secretion, initially through the perception by sensory neurons of the mechanical stimulation and mainly through the efferent vagocholinergic pathway, with the process being modified by endogenous NO, and 2) this molecule, released in a vagal-dependent manner, exerts a negative influence on acid secretion, at least in part by suppressing histamine release from the histamine-containing cells.