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 preparations of Bullfrog stomach. The bullfrog fundic mucosa was bathed in unbuffered Ringer solution gassed with 100% O2 on the mucosal side and HCO3- Ringer's solution gassed with 95% O2/5% CO2 on the serosal side, and the acid 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) approximately 10(-3) M: (+/-)-(E)-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexnamine) caused an increase 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 stimulatory 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 scavenger) or superoxide dismutase (SOD), but not by indomethacin or methylene blue (a guanylyl cyclase inhibitor). Neoither L-NAME, L-arginine nor dibutyryl guanosine-3',5'-cyclic monophosphate (dbcGMP) has any effect on the basal acid secretion. Serosal addition of NOR-3 caused a significant increase in the luminal release of histamine, and this response was inhibited by pretreatment 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 the histamine releasing action of NO may be due to peroxynitrite produced by NO and superoxide radicals.

Bicarbonate stimulatory action of nizatidine, a histamine H(2)-receptor antagonist, in rat duodenums

Nizatidine, a histamine H(2)-antagonist, is known to inhibit acetylcholinesterase (AChE) activity and is used clinically as a gastroprokinetic agent as well as the anti-ulcer agent. We examined whether or not nizatidine stimulates duodenal HCO(3)(-) secretion in rats through vagal-cholinergic mechanisms by inhibiting AChE activity. Under pentobarbital anesthesia, a proximal duodenal loop was perfused with saline, and the HCO(3)(-) secretion was measured at pH 7.0 using a pH-stat method and by adding 10 mM HCl. Nizatidine, neostigmine, carbachol, famotidine or ranitidine was administered i.v. as a single injection. Intravenous administration of nizatidine (3-30 mg/kg) dose-dependently increased the HCO(3)(-) secretion, and the effect at 10 mg/kg was equivalent to that obtained by carbachol at 0.01 mg/kg. The HCO(3)(-) stimulatory action of nizatidine was observed at the doses that inhibited the histamine-induced acid secretion and enhanced gastric motility. This effect was mimicked by neostigmine (0.03 mg/kg) and significantly attenuated by bilateral vagotomy and pretreatment with atropine but not indomethacin. The IC(50) of nizatidine for AChE of rat erythrocytes was 1.4 x 10(-6) M, about 12 times higher than that of neostigmine. Ranitidine showed the anti-AchE activity and increased duodenal HCO(3)(-) secretion, similar to nizatidine, whereas famotidine had any influence on neither AChE activity nor the HCO(3)(-) secretion. On the other hand, duodenal damage induced by acid perfusion (100 mM HCl for 4 h) in the presence of indomethacin was significantly prevented by nizatidine and neostigmine, at the doses that increased the HCO(3)(-) secretion. These results suggest that nizatidine increases HCO(3)(-) secretion in the rat duodenum, mediated by vagal-cholinergic mechanism, the action being associated with the anti-AChE activity of this agent.

Stimulation by nitric oxide of HCO3- secretion in bullfrog duodenum in vitro--roles of cyclooxygenase-1 and prostaglandins

The effect of nitric oxide (NO) on HCO3- secretion was examined in vitro using the isolated preparation of bullfrog duodenum, in relation to cyclooxygenase (COX) isozymes and endogenous prostaglandins (PGs). The tissue was bathed in unbuffered Ringer solution gassed with 100% O2 on the mucosal side and HCO3- Ringer's solution gassed with 95% O2-5% CO2 on the serosal side. The HCO3- secretion was measured by a pH-stat method using 10 mM HCl as the titrant to keep the mucosal pH at 7.4. NOR-3 [(+/-)-(E)-Ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamine] was used as a NO donor and added to the serosal solution. To analyze the NOR-3 action, the effects of dibutyryl guanosine-3', 5'-cyclic monophosphate (dbcGMP), methylene blue, indomethacin (nonselective COX-inhibitor) and NS-398 (selective COX-2 inhibitor) on the HCO3- response were also examined. NOR-3 (1 x 10(-4) and 3 x 10(-4) M) caused an increase of HCO3- secretion in a dose-dependent manner, reaching the level of 2.5 times greater than basal values at 2 hr later. Likewise, dbcGMP (1 x 10(-3) M) also caused a significant increase of the duodenal HCO3- secretion. The HCO3- stimulatory action of NOR-3 was significantly attenuated by methylene blue (5 x 10(-5) M) and indomethacin (1 x 10(-5) M) but not by NS-398 (1 x 10(-5) M), and indomethacin also suppressed the HCO3- response to dbcGMP. The serosal release of PGE2 was significantly increased by both NOR-3 and dbcGMP, and these responses were inhibited by indomethacin but not NS-398. These results suggest that NO increases HCO3- secretion in Bullfrog duodenum in vitro, and this action is dependent on cGMP-related COX-1 activation and mediated by PGs.