Stimulation by nitric oxide of gastric acid secretion in bullfrog fundic mucosa in vitro

We examined the effect of NO on acid secretion in vitro using isolated prep arations of Bullfrog stomach. The bullfrog fundic mucosa was bathed in unbu ffered Ringer solution gassed with 100% O-2 on the mucosal side and HCO3- R inger's solution gassed with 95% O-2/5% CO2 on the serosal side, and the ac id secretion was measured at pH 5.0 using the pH-stat method and by adding 5 mM NaOH. Serosal addition of a NO donor NOR-3 (10(-5) similar to 10(-3) M : (+/-)-(E)-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexanamine) caused an incr ease of acid secretion in a dose-dependent manner, the effect lasting about 1 hr and reaching a maximal level of 2-fold the basal values. The acid sti mulatory effect of NOR-3 was mimicked by another NO donor SNAP (10(-3) mol/ L: S-nitroso-O-N-acetyl-penicillamine) and markedly and markedly inhibited by prior administration of cimetidine (10(-5) mol/L) as well as compound 48 /80 (the mast cell degranulator). Likewise, the increased acid response to NOR-3 was significantly mitigatd by pretreatment with carboxy-PTIO (a NO sc avenger) or superoxide dismutase (SOD), but not by indomethacin or methylen e blue (a guanylyl cyclase inhibitor). Neoither L-NAME, L-arginine nor dibu tyryl guanosine 3',5'-cyclic monophosphate (dbcGMP) has any effect on the b asal acid secretion. Serosal addition of NOR-3 caused a significant increas e in the luminal release of histamine, and this response was inhibited by p retreatment with either compound 48/80, carboxy-PTIO or SOD. These results suggest that the NO donor increases gastric acid secretion in the isolated frog stomach in vitro, and this action is mediated by endogenous histamine released from mast cells, the process being cGMP-independent but requiring the presence of superoxide radicals. In addition, it was speculated that th e histamine releasing action of NO may be due to peroxynitrite produced by NO and superoxide radicals.