Electrophysiology of Flounder Intestinal Mucosa. I. Conductance Properties ofthe Cellular and Paracellular Pathways

We evaluated the conductances for ion flow across the cellular and paracellular pathways of flounder intestine using microelectrode techniques and ion-replacement studies. Apical membrane conductance properties are dominated by the presence of Ba-sensitive K channels. An elevated mucosal solution K concentration, [K] _m , depolarized the apical membrane potential (ψ _a ) and, at [K] _m less than 40 mM, the K dependence of ψ _a was abolished by 1-2 mM mucosal Ba. The basolateral membrane displayed Cl conductance behavior, as evidenced by depolarization of the basolateral membrane potential (ψ _b ) with reduced serosal Cl concentrations, [Cl] _s . ψ _b was unaffected by changes in [K] _s or [Na] _s . From the effect of mucosal Ba on transepithelial K selectivity, we estimated that paracellular conductance (G _p ) normally accounts for 96% of transepithelial conductance (G _t ). The high G _p attenuates the contribution of the cellular pathway to ψ _t while permitting the apical K and basolateral Cl conductances to influence the electrical potential differences across both membranes. Thus, ψ _a and ψ _b (approximately 60 mV, inside negative) lie between the equilibrium potentials for K (76 mV) and Cl (40 mV), thereby establishing driving forces for K secretion across the apical membrane and Cl absorption across the basolateral membrane. Equivalent circuit analysis suggests that apical conductance (G _a ≅ 5 mS/cm ² ) is sufficient to account for the observed rate of K secretion, but that basolateral conductance (G _b ≅ 1.5 mS/cm ² ) would account for only 50% of net Cl absorption. This, together with our failure to detect a basolateral K conductance, suggests that Cl absorption across this barrier involves KCl co-transport.