Stimulatory effect of nitric oxide on bicarbonate secretion in bullfrog duodenums in vitro

The effect of nitric oxide (NO) on HCO3- secretion was examined in vitro us ing an isolated preparation of bullfrog duodenum. The tissue was bathed in unbuffered Ringer's solution gassed with 100% O-2 on the mucosal side and H CO3- Ringer's solution gassed with 95% O-2-5% CO2 on the serosal side. The HCO3- secretion was measured by the pH-stat method using 2 mmol/l HCl as th e titrant to keep the mucosal pH at 7.4. (+/-)-(E)-Ethyl-2-[(E)-hydroxyimin o]-5-nitro-3-hexenamine (NOR3) was used as a NO donor and added to the sero sal solution. To analyze the NOR3 action on HCO3- secretion, the effects of dibutyryl adenosine-3',5'-cyclic monophosphate (dbcAMP), dibutyryl guanosi ne-3',5'-cyclic monophosphate (dbcGMP), methylene blue, and indomethacin on the HCO, response were also examined. NOR3 (1 x 10(-4) and 3 x 10(-4) mol/ l) caused an increase in HCO3- secretion in a dose-dependent manner, and th is 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 x 10(-3 ) mol/l) and dbcGMP (1 x 10(-3) mol/l) also caused a significant increase i n HCO3- secretion in bullfrog duodenums in vitro, although the onset of the HCO3- response to dbcGMP was delayed as compared to the former. The stimul atory action of NOR3 on duodenal HCO3- secretion was significantly attenuat ed by methylene blue (5 x 10(-5) mol/l) and indomethacin (1 x 10(-5) mol/l) , the latter also inhibiting the HCO3- response to dbcGMP. The release of p rostaglandin E-2 in the serosal solution was significantly increased after addition of NOR3 (3 x 10(-4) mol/l) and dbcGMP (1 x 10(-3) mol/l) in an ind omethacin-sensitive manner. These results suggest that the NO donor increas es duodenal HCO3- secretion in vitro, and this action of NO donor is cGMP-d ependent and mediated by endogenous prostaglandins. Duodenal HCO3- secretio n may be regulated locally by NO/cGMP in addition to prostaglandin/cAMP.