Relationship between intracellular pH and chloride in Xenopus oocytes expressing the chloride channel ClC-0.

During maturation of oocytes, Cl(-) conductance (G(Cl)) oscillates and intracellular pH (pH(i)) increases. Elevating pH(i) permits the protein synthesis essential to maturation. To examine whether changes in G(Cl) and pH(i) are coupled, the Cl(-) channel ClC-0 was heterologously expressed. Overexpressing ClC-0 elevates pH(i), decreases intracellular Cl(-) concentration ([Cl(-)](i)), and reduces volume. Acute acidification with butyrate does not activate acid extrusion in ClC-0-expressing or control oocytes. The ClC-0-induced pH(i) change increases after overnight incubation at extracellular pH 8.5 but is unaltered after incubation at extracellular pH 6.5. Membrane depolarization did not change pH(i). In contrast, hyperpolarization elevates pH(i). Thus neither membrane depolarization nor acute activation of acid extrusion accounts for the ClC-0-dependent alkalinization. Overnight incubation in low extracellular Cl(-) concentration increases pH(i) and decreases [Cl(-)](i) in control and ClC-0 expressing oocytes, with the effect greater in the latter. Incubation in hypotonic, low extracellular Cl(-) solutions prevented pH(i) elevation, although the decrease in [Cl(-)](i) persisted. Taken together, our observations suggest that KCl loss leads to oocyte shrinkage, which transiently activates acid extrusion. In conclusion, expressing ClC-0 in oocytes increases pH(i) and decreases [Cl(-)](i). These parameters are coupled via shrinkage activation of proton extrusion. Normal, cyclical changes of oocyte G(Cl) may exert an effect on pH(i) via shrinkage, thus inducing meiotic maturation.