Shigeru Kagawa

Stimulation by capsaicin of duodenal HCO3(-) secretion via afferent neurons and vanilloid receptors in rats: comparison with acid-induced HCO3(-) response.

We compared the HCO3− secretory response to capsaicin and mucosal acidification in rat duodenums, especially the relation to vanilloid receptor type 1 (VR1). 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. The secretion was stimulated by exposing the loop to capsaicin (0.03–0.3 mg/ml) or 10 mM HCl for 10 min. Indomethacin subcutaneously or ruthenium red intravenously, a nonspecific VR1 antagonist, was given 60 or 10 min, respectively, before exposure to capsaicin or acid, while l-NAME was given intravenously 3 hr before these treatments. Capsazepine, another VR1 antagonist, was coapplied to the loop for 10 min with capsaicin or acid. Luminal application of capsaicin increased the secretion of HCO3− in a dose-dependent manner; this effect was markedly attenuated by chemical ablation of capsaicin-sensitive afferent neurons (CSN) as well as pretreatment with ruthenium red or capsazepine, and significantly mitigated by indomethacin or l-NAME (in an l-arginine-sensitive manner). The HCO3− secretion was also stimulated by mucosal acidification, and this response was attenuated by both capsaicin pretreatment, indomethacin and l-NAME, but not ruthenium red or capsazepine. Mucosal application of capsaicin as well as acid increased the mucosal PGE2 content, and these effects were both significantly attenuated by indomethacin and l-NAME. These results suggest that both capsaicin and acid cause the CSN-dependent increase in duodenal HCO3− secretion mediated by NO and PG, yet the mode of their action differs in terms of the ruthenium red or capsazepine sensitivity. Although luminal H+ plays a modulatory role in duodenal HCO3− secretion, it is unlikely that the action results from the interaction of H+ with the ruthenium red- or capsazepine-sensitive site of VR1.

COX and NOS isoforms involved in acid-induced duodenal bicarbonate secretion in rats

Duodenal HCO3− secretion increases in response to luminal acid, mediated by endogenous nitiric oxide (NO) as well as prostaglandins (PGs). In this study, we examined the effects of various inhibitors of cyclooxygenase (COX) or NO synthase (NOS) on the acid-induced HCO3− secretion in rats and determined the enzyme isoforms responsible for this response. A proximal duodenal loop was perfused with saline under urethane anesthesia, 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 mM HCl for 10 min. Indomethacin, SC-560 (a selective COX-1 inhibitor) and rofecoxib (a selective COX-2 inhibitor) were given intraduodenally 1 hr before exposure to 10 mM HCl, while NG-nitro-l-arginine methyl ester (l-NAME: a nonselective NOS inhibitor) and aminoguanidine (a relatively selective inhibitor of iNOS) were given subcutaneously 3 hr before the acidification. The mucosal acidification increased the HCO3− secretion, with a rise in mucosal PGE2 content and luminal release of NO. The HCO3− secretory and PGE2 biosynthetic responses were significantly inhibited by indomethacin and SC-560, while rofecoxib had no effect on these responses. On the other hand, l-NAME, but not aminoguanidine, attenuated NO release following the acidification, resulting in inhibition of the acid-induced HCO3− secretion in a l-arginine-sensitive manner. Neither COX-2 nor iNOS mRNAs were observed in the mucosa before and 1 hr after acidification, while the gene expression of COX-1 and nNOS was constitutively detected in the mucosa and appeared to be slightly up-regulated after the acid stimulation. These results suggest that COX-1 and cNOS play as the respective key enzyme responsible for producing PG and NO following the duodenal acidification, both of which are involved in the mechanism for the acid-induced HCO3− secretion in the duodenum.

Effect of Lafutidine, a Histamine H2-Receptor Antagonist, on Gastric Mucosal Blood Flow and Duodenal HCO3− Secretion in Rats: Relation to Capsaicin-Sensitive Afferent Neurons

Lafutidine is a new type of antiulcer drug, possessing both an antisecretory effect, exerted via a blockade of the histamine H2 receptor, and gastroprotective activity, mediated by capsaicin-sensitive afferent nerves (CSN). In the present study, we examined the effect of lafutidine on gastric mucosal blood flow (GMBF) and duodenal HCO3− secretion (DAS) under basal and acid-stimulated conditions in rats. Under urethane anesthesia, GMBF was measured using a laser Doppler flowmeter in a chambered stomach before and after exposure to 20 mM taurocholate (TC) plus 50 mM HCl, while DAS was measured in a proximal duodenal loop before and after mucosal acidification (10 mM HCl for 10 min) by titrating the perfusate at pH 7.0 using a pH-stat method and by adding 10 mM HCl. Lafutidine given intraperitoneally affected neither GMBF nor DAS under basal conditions, but augmented an increase in both GMBF and DAS induced by mucosal acidification. Although the acid-induced GMBF and DAS responses were significantly mitigated by both indomethacin and sensory deafferentation but not by ruthenium red (RT), the vanilloid receptor (VR)-1 antagonist, the responses were preserved in lafutidine-treated animals, even in the presence of indomethacin. Both GMBF and DAS were significantly increased by local application of capsaicin, the responses being attenuated by indomethacin and RT as well as sensory deafferentation. Lafutidine augmented the GMBF and DAS responses to capsaicin and preserved the responses, even in the presence of indomethacin. Capsaicin evoked an increase in [Ca2+]i in rat VR1-transfected HEK293 cells, while lafutidine had no effect by itself on [Ca2+]i in these cells and did not affect the increase in [Ca2+]i evoked by capsaicin. In conclusion, these results suggest that lafutidine mimics endogenous effects of prostaglandins to augment the GMBF and DAS responses to acid or capsaicin, probably by sensitizing CSN through an unknown site other than VR1. The luminal H+ itself is not a ligand for the RT-sensitive site of VR1 but plays a modulator role in the CSN-mediated physiological responses.

Bicarbonate stimulatory action of nizatidine, a histamine H2-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.