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The effect of nitric oxide (NO) on HCO^–₃ secretion was examined in vitro using an isolated preparation of bullfrog duodenum. The tissue was bathed in unbuffered Ringer’s solution gassed with 100% O₂ on the mucosal side and HCO^–₃ Ringer’s solution gassed with 95% O₂–5% CO₂ on the serosal side. The HCO^–₃ secretion was measured by the pH-stat method using 2 mmol/l HCl as the titrant to keep the mucosal pH at 7.4. (±)-(E)-Ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamine (NOR3) was used as a NO donor and added to the serosal solution. To analyze the NOR3 action on HCO^–₃ secretion, the effects of dibutyryl adenosine-3′,5′-cyclic monophosphate (dbcAMP), dibutyryl guanosine-3′,5′-cyclic monophosphate (dbcGMP), methylene blue, and indomethacin on the HCO^–₃ response were also examined. NOR3 (1×10^–4 and 3×10^–4 mol/l) caused an increase in HCO^–₃ secretion in a dose-dependent manner, and this effect appeared with an about 30-min time lag, reaching the level of 1.5–2.5 times greater than basal values at 1–2 h later. Both dbcAMP (1×10^–3 mol/l) and dbcGMP (1×10^–3 mol/l) also caused a significant increase in HCO^–₃ secretion in bullfrog duodenums in vitro, although the onset of the HCO^–₃ response to dbcGMP was delayed as compared to the former. The stimulatory action of NOR3 on duodenal HCO^–₃ secretion was significantly attenuated by methylene blue (5×10^–5 mol/l) and indomethacin (1×10^–5 mol/l), the latter also inhibiting the HCO^–₃ response to dbcGMP. The release of prostaglandin E₂ in the serosal solution was significantly increased after addition of NOR3 (3×10^–4 mol/l) and dbcGMP (1×10^–3 mol/l) in an indomethacin-sensitive manner. These results suggest that the NO donor increases duodenal HCO^–₃ secretion in vitro, and this action of NO donor is cGMP-dependent and mediated by endogenous prostaglandins. Duodenal HCO^–₃ secretion may be regulated locally by NO/cGMP in addition to prostaglandin/cAMP.

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 PGE2 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 PGE2 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 PGE2 biosynthetic response, without influence on the NOx release. Pretreatment of the animals with L-NAME attenuated both the increase of mucosal PGE2 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 PGE2 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.

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8On the other hand, there are con¯icting data regarding the effects of PG on acid secretion. Way and Durbin 9 ®rst showed that PGE 1 inhibited histamine-stimulated