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The effects of YM-14673, an analogue of thyrotropin-releasing hormone, on duodenal HCO3- secretion were characterized in comparison with those of prostaglandin E2 and carbachol in anesthetized rats. The proximal duodenum was perfused with saline (pH 4.5), the pH of the perfusate and of the transmucosal potential difference were continuously monitored, and HCO3- output was determined by back-titration of the perfusate and by pH change. YM-14673 (0.1, 0.3 and 1 mg/kg) increased these parameters in a dose-related manner; the total HCO3- output at 1 mg/kg (5.8 +/- 0.7 mumol) was about 50% of that induced by prostaglandin E2 (1 mg/kg, i.v.) and 2 times greater than that induced by carbachol (4 micrograms/kg, i.v.). These responses, induced by YM-14673, were almost completely blocked by bilateral vagotomy and significantly inhibited by atropine (0.3 mg/kg, i.v.) or cyclooxygenase inhibitors, such as indomethacin (5 mg/kg, s.c.) and acetylsalicylic acid (40 mg/kg, s.c.), but not affected by pirenzepine (1 mg/kg, i.v.), a selective M1 antagonist. None of the latter agents had any effect on the HCO3(-)-stimulating action of prostaglandin E2, while the carbachol-induced HCO3- output was significantly reduced by atropine. These results suggest that YM-14673 induces the vagal-dependent HCO3- secretion in the rat duodenum and that this mechanism is mediated by muscarinic cholinoceptors, excluding the M1-subtype, and may involve endogenous prostaglandins.

We set up a new system for measuring the gastroduodenal HCO3- responses using pH change and potential difference (PD) in the anesthetized rat. The stomach or the proximal duodenum was perfused at the flow rate of 0.7 mL/min with saline (pH 4.5), the pH of the perfusate and PD were continuously monitored, and HCO3- output was determined by back-titrating the perfusate and by measuring the area of pH change. In the case of the stomach, acid secretion was inhibited by omeprazole (60 mg/kg, i.p.). Output of both pH and HCO3- in these tissues was significantly increased by intravenous administration of prostaglandin (PGE2), 16, 16-dimethyl PGE2, carbachol, and YM-14673 (a thyrotropin-releasing hormone (TRH) analog), whereas the PD responded to these agents by a significant rise in the duodenum and decrease in the stomach. These parameters also responded to physiological stimulation such as mucosal acidification. When the area of pH change caused by various agents was plotted against the net amount of HCO3- output determined from back-titration, a significant relationship was found between these two factors (r = 0.98). These results indicate that this system using pH change may be useful for quantitative determination of HCO3- response in the gastroduodenal mucosa.

Duodenal pH, potential difference and acid-neutralizing capacity (HCO3- output) were measured in anesthetized rats, in an attempt to characterize these responses induced by cholinergic agents. When the proximal duodenum (1.7 cm) was perfused at a flow rate of 1 ml/min with saline adjusted to pH 4.5, the pH, potential difference and HCO3- output were 5.2 to 5.5, -3 to -5 mV (mucosa negative) and 1.5 to 1.8 muEq/10 min, respectively. Both carbachol (4 micrograms/kg, i.v.) and bethanechol (100 micrograms/kg, i.v.) significantly elevated all these parameters at the doses that stimulated gastric acid secretion; the net HCO3- output caused by these agents was about 35% of that produced by prostaglandin E2 (300 micrograms/kg, i.v.). These responses induced by carbachol were significantly inhibited by pretreatment with verapamil (0.2 mg/kg, i.v.), a Ca channel blocker, and atropine (0.1 mg/kg, i.v.), a nonselective M1 and M2 antagonist, while they remained unaltered in the presence of pirenzepine (1 mg/kg, i.v.), a selective M1 antagonist, or indomethacin (5 mg/kg, s.c.). However, the effects of prostaglandin E2 on duodenal pH, potential difference and HCO3- output were not significantly affected by any of these agents. These results suggest that the cholinergic agents stimulate HCO3- output in the duodenum, probably mediated by M2 receptors and through the Ca-dependent and electrogenic processes. Endogenous prostaglandins may not be involved in this mechanism.

Possible involvement of increased mucosal permeability in the stimulation by prostaglandin E2 (PGE2) of duodenal HCO3- secretion was investigated in rats. PGE2 (0.3, 1 mg/kg, s.c.) dose-dependently increased HCO3- secretion in the duodenum with a significant elevation of transmucosal potential difference (PD); the PD was increased from -4.5 +/- 0.3 mV to -10.0 +/- 1.5 mV (mucosa negative) at 1 mg/kg. These responses caused by PGE2 were abolished by sacrificing the animals with saturated KCl (i.v.). Although a significant increase of HCO3- output was observed after exposure of the mucosa to 1 M NaCl (0.5 ml), this response was accompanied by a significant reduction of PD and was not abolished after KCl injection. The mucosal permeability determined by Evans blue (1%, i.v.) was not affected by PGE2, while 1 M NaCl markedly elevated the amount of extravasated dye in both the luminal content and the mucosa. Stimulation of HCO3- output by PGE2 was significantly mitigated by ouabain (3 mg/kg, s.c.) or prior exposure of the mucosa to 1 M NaCl. These results suggest that stimulation by PGE2 of duodenal HCO3- secretion is not simply due to the increased mucosal permeability, but depends rather on both the Na/K ATPase activity and the intact perfusion of the organ. The HCO3- response as induced by 1 M NaCl may result from the increased permeability and is accompanied by a marked reduction of PD.

We set up a system to measure the luminal pH, potential difference (PD) and bicarbonate output in the anesthetized rat duodenum, and investigated these responses caused by prostaglandins (PGs) and cholinergic agents. When the proximal duodenum (1.7 cm) was perfused at a flow rate of 0.7 ml/min with saline adjusted to pH 4.5, the duodenal pH, PD and HCO3- output were 5.5 to 6.0, -4 to -6 mV and 1.2 to 1.6 muEq/10 min, respectively; they were markedly reduced by i.v. injection of saturated KCl. Both natural (PGE1, PGE2) and synthetic (PGE2, PGl2) PGs, given either s.c. or i.v., significantly elevated all these parameters, while indomethacin (s.c.) decreased the pH as well as the PD. Small but significant increases of the pH were observed after i.v. administration of cholinergic agents (carbachol, bethanechol), a GABAergic agent (baclofen) and an analogue of thyrotropin releasing hormone (YM-14673), with a temporal elevation of the PD; the degree of net HCO3- output caused by these agents was 20-50% of the values obtained with PGE2 (100 micrograms/kg, i.v.), and they were significantly reduced in the presence of atropine. These results suggest that (a) the system using pH deflections can be used to sensitively detect HCO3- output in the rat duodenum, and (b) duodenal acid neutralizing capacity may be regulated by central and peripheral cholinergic systems as well as endogenous PGs.