Evidence for Cl(-)-independent HCO3- transport in basolateral membranes of Necturus oxyntopeptic cells.

Luminal H+ secretion by gastric mucosa is accompanied by basolateral HCO3- release. A basolateral Cl-/HCO3- exchanger is known to mediate HCO3- extrusion from oxyntopeptic cells during resting and secretagogue-induced apical HCl secretion. From recent work, we hypothesized that there might be a Cl(-)-independent pathway for basolateral HCO3- exit in Necturus oxyntopeptic cells. In this study, we used a fluorescent pH indicator [2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein] to evaluate Cl(-)-independent HCO3- transport across the basolateral membranes of intact oxyntopeptic cells. Removal of serosal Cl- increased intracellular pH (pHi) (7.05 to 7.25), consistent with Cl(-)-dependent HCO3- extrusion. Removal of serosal Na+ in the absence of Cl- resulted in significant acidification of pHi (7.10 to 6.89), but studies involving amiloride, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), and 0 HCO3(-)-N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-buffered solutions suggest that Na(+)-dependent changes in pHi are due to Na+/H+ exchange. Our studies demonstrate a marked concentration-dependent alkalinization when tissues are exposed to increases in serosal K+. A substantial part of this alkalinization in response to increases in serosal K+ (pHi 7.00 to 7.46) appears to be a HCO3- exit pathway that is independent of both Na+ and Cl-, unaffected by bumetanide or amiloride, but sensitive to DIDS. We propose the presence of a Cl(-)- and Na(+)-independent K(+)-dependent HCO3- cotransporter in Necturus oxyntopeptic cell basolateral membranes.

Electrophysiological effects of cholinergic agonists in surface epithelium of Necturus gastric antrum.

Intracellular microelectrode techniques were used to characterize voltage and conductance properties of the basolateral membrane of surface epithelial cells in in vitro Necturus antral mucosa. Flux studies confirmed that this tissue secretes HCO3- under resting conditions and during response to cholinergic stimulation. In studies using intracellular microelectrodes, exposure to cholinergic agonists such as acetylcholine, bethanechol, or carbachol elicited an initial hyperpolarization followed by depolarization of the basolateral cell membranes associated with up to fourfold increases in basolateral membrane conductance. Effects of acetylcholine were dose dependent (10(-6) - 10(-4) M) and prevented by pretreatment of tissues with the nonselective muscarinic receptor blocker atropine. Some variation in this response to cholinergic stimulation was observed and appeared to be related to the season (fall/winter/early spring vs. late spring/summer). Despite such variability, circuit analysis and ion substitution studies indicated that the carbachol-induced increases in basolateral conductance were due to increases in conductance to K+ and Cl- . These increases in basolateral transport processes may serve to stabilize cell ion composition and membrane electrical properties during cholinergic stimulation of mucus and HCO3- secretions.