Regulatory mechanism of acid secretion in the damaged stomach: role of endogenous nitric oxide.

Abstract

The present article overviews the regulatory mechanism of acid secretion in the stomach after damage with taurocholate (TC), one of the bile acids. Mucosal exposure of a rat stomach to 20 mmol/L TC for 30 min caused a decrease of acid secretion with a concomitant increase in nitric oxide (NO) and prostaglandin (PG) E2 (PGE2) as well as Ca2+ in the luminal contents. Prior administration of N(G)-nitro-L-arginine methyl ester (L-NAME), as well as indomethacin, significantly attenuated the reduction of acid secretion by TC and acid secretion was even increased in the presence of L-NAME. The acid stimulatory effect of L-NAME in the damaged stomach was not mimicked by aminoguanidine and was antagonized by co-administration of L-arginine but not D-arginine. Increased NO release in the damaged stomach was suppressed by pretreatment with L-NAME or co-application of EGTA and the latter also inhibited the increase in luminal Ca2+. The enhanced acid secretory response in the presence of L-NAME was also inhibited by cimetidine, FPL-52694 (a mast cell stabilizer) or sensory deafferentation. Mucosal exposure to TC caused an increase in luminal histamine output, together with a decrease in the number of mucosal mast cells in the stomach. These changes were prevented by FPL-52694 and sensory deafferentation and were also partly suppressed by indomethacin. In addition, the acid stimulatory action of L-NAME in the damaged stomach was significantly mitigated when indomethacin was administered together with L-NAME. We conclude that: (i) damage in the stomach may activate acid a stimulatory pathway in addition to a PG-, NO- and Ca2+-dependent inhibitory mechanism, but the latter effect overcomes the former, resulting in a decrease in acid secretion; (ii) acid stimulation in the damaged stomach is mediated by histamine released from the mucosal mast cell, a process interacting with capsaicin-sensitive sensory nerves; (iii) the increase in luminal Ca2+ plays a role in increasing NO production and, hence, in regulating acid secretion; and (iv) PG may have a dual role in the regulation of acid secretion in the damaged stomach: an inhibitory effect at the parietal cell and an excitatory effect, probably through enhancing the release of mucosal histamine.