Activation of a Potassium Conductance by Extracellular Alkaline pH in Oocytes of Xenopus laevis

Electrophysiological properties of Xenopus oocytes exposed to alkaline extracellular pH (pHo) were investigated by measuring whole-cell currents using the two-electrode voltage-clamp method. Alkaline pHo (8.5 - 10.5) elicited an outward current in a pHo-dependent manner with a concomitant increase in the membrane conductance. This outward-current response was dependent on K+ because it was suppressed by a K+ channel blocker tetraethy lammonium+ (20 mM), and the reversal potential of the response was in good agreement with the Nernst equation for K+. The response was not affected by pretreatment of oocytes with the acetoxymethyl ester of bis-(o-aminophenoxy)-ethane-N,N,N’N’-tetraacetic acid (10 μM), a membrane-permeant intracellular Ca2+ chelator, but it was augmented by forskolin (0.4 μM), a stimulant of adenylate cyclase. The outward-current response originates in the oocyte but not in the surrounding follicle cells because the current could still be evoked when follicle cells were removed by collagenase or when gap junctions connecting the oocyte membrane and follicle cells were blocked by 1-octanol (1 mM). It is concluded that the outward current elicited by alkaline pHo in Xenopus oocytes is dependent on the activation of K+ channels via the cAMP pathway and that the outward current originates in the oocyte rather than the surrounding follicle cells.