ELECTROPHYSIOLOGICAL EFFECTS OF BASOLATERAL [NA+] IN NECTURUS GALLBLADDER EPITHELIUM

In Necturus gallbladder epithelium, lowering serosal [Na+] ([Na+]s) reversibly hyperpolarized the basolateral cell membrane voltage (V(cs)) and reduced the fractional resistance of the apical membrane (fR(a)). Previous results have suggested that there is no sizable basolateral Na+ conductance and that there are apical Ca2+-activated K+ channels. Here, we studied the mechanisms of the electrophysiological effects of lowering [Na+]s, in particular the possibility that an elevation in intracellular free [Ca2+] hyperpolarizes V(cs) by increasing gK+. When [Na+]s was reduced from 100.5 to 10.5 mM (tetramethylammonium substitution), V(cs) hyperpolarized from -68 +/- 2 to a peak value of -82 +/- 2 mV (P < 0.001), and fR(a) decreased from 0.84 +/- 0.02 to 0.62 +/- 0.02 (P < 0.001). Addition of 5 mM tetraethylammonium (TEA+) to the mucosal solution reduced both the hyperpolarization of V(cs) and the change in fR(a), whereas serosal addition of TEA+ had no effect. Ouabain (10(-4) M, serosal side) produced a small depolarization of V(cs) and reduced the hyperpolarization upon lowering [Na+], without affecting the decrease in fR(a). The effects of mucosal TEA+ and serosal ouabain were additive. Neither amiloride (10(-5) or 10(-3) M) nor tetrodotoxin (10(-6) M) had any effects on V(cs) or fR(a) or on their responses to lowering [Na+]s, suggesting that basolateral Na+ channels do not contribute to the control membrane voltage or to the hyperpolarization upon lowering [Na+]s. The basolateral membrane depolarization upon elevating [K+]s was increased transiently during the hyperpolarization of V(cs) upon lowering [Na+]s. Since cable analysis experiments show that basolateral membrane resistance increased, a decrease in basolateral Cl- conductance (gCl-) is the main cause of the increased K+ selectivity. Lowering [Na+]s increases intracellular free [Ca2+], which may be responsible for the increase in the apical membrane TEA+-sensitive gK+. We conclude that the decrease in fR(a) by lowering [Na+]s is mainly caused by an increase in intracellular free [Ca2+], which activates TEA+-sensitive maxi K+ channels at the apical membrane and decreases apical membrane resistance. The hyperpolarization of V(cs) is due to increases in: (a) apical membrane gK+, (b) the contribution of the Na+ pump to V(cs), (c) basolateral membrane K+ selectivity (decreased gCl-), and (d) intraepithelial current flow brought about by a paracellular diffusion potential.