Contribution from a pH- and tonicity-sensitive K⁺ conductance to toad translens short-circuit current

Studies of toad (Bufo marinus) lenses mounted in Ussing-type chambers revealed that: (1) the translens short-circuit current (I_sc) across the posterior surface is primarily carried by the movement of Na⁺ from the posterior bathing solution to the lens; (2) across the anterior face the majority of the I_sc is mediated by Ba²⁺-sensitive channels and the remaining current is rapidly reduced by ouabain; (3) most of the anterior K⁺ conductance is of the tonicity-sensitive, quinidine-inhibitable type (i.e. hypotonic shifts increase I_sc and hypertonic shifts decrease I_sc; quinidine pretreatment eliminates such responses); (4) ⁸⁶Rb⁺ uptake is stimulated by alkaline pH and occurs primarily across the anterior surface with quinidine the most potent inhibitor of this process; and (5) the Na⁺K⁺ pump can maintain lens [Na⁺] and [K⁺] for at least 20 hr in a Ringer's solution near neutral pH but not at pH 8·7 (a pH used in some studies with this lens). It is concluded that the I_sc can be viewed as a representation of the current across the epithelial basolateral membrane, a surface dominated by pH- and tonicity-sensitive K⁺ channels. The direction of the I_sc response to tonicity changes suggests a role for these channels in epithelial volume regulation.