Role of endogenous nitric oxide and prostaglandin in duodenal bicarbonate response induced by mucosal acidification in rats

Duodenal HCO3- secretion increases in response to mucosal acidification by luminal acid. Although this process is known to be mediated by endogenous prostaglandins (PGs), the role of nitric oxide (NO) in this response has been little studied. We examined the effects of indomethacin and N-G-nitro-L-arginine methyl ester (L-NAME) on the acid-induced HCO3- secretion in the rat duodenum, together with those on PGE(2) generation as well as luminal release of NO metabolites (NOx). A proximal duodenal loop was perfused with saline, and the HCO3- secretion was measured at pH 7.0 using a pH-stat method and by adding 10 mM HCl. Mucosal acidification was performed by exposing the loop to 10 or 100 mM HCl for 10 min. Acidification of the duodenal mucosa stimulated the HCO3- secretion, with concomitant increase of mucosal PGE(2) contents and luminal release of NOx, the response being much greater in case of 100 mM HCl. Indomethacin significantly inhibited the acid-induced HCO3- secretion as well as the PGE(2) biosynthetic response, without influence on the NOx release. Pretreatment of the animals with L-NAME attenuated both the increase of mucosal PGE, contents and luminal release of NOx following the acidification, resulting in a marked inhibition of the acid-induced HCO3- response, and these effects were significantly antagonized by coadministration of L-arginine. Duodenal HCO3- secretion was also increased by mucosal exposure to NOR-3 (a NO donor), with concomitant increase of PGE(2) generation, but these effects were mitigated in the presence of indomethacin. In addition, the duodenal damage caused by mucosal perfusion with 100 mM HCl for 4 hr was markedly aggravated by pretreatment with L-NAME as well as indomethacin. These results suggest that both endogenous NO and PGs are involved in the mechanism for the acid-induced duodenal HCO3- secretion, and that NO may increase the HCO3- secretion by stimulating PG generation.