Some observations on the behaviour of chloride current--voltage relations in Xenopus muscle membrane in acid solutions.

Chloride current--voltage relations in Xenopus laevis muscle membrane have been investigated in phosphate-buffered solution (pH 5.2--5.4) using a three-microelectrode voltage clamp. Resting chloride conductance in these conditions is about 10(-4) S/cm2, approximately 1/10th that at pH 8.8. When the membrane potential is stepped from the holding (resting) potential to a more negative voltage the current rises from the initial to the steady state. The instantaneous current--voltage relation is linear and the steady-state relation shows inward-going rectification. As hyperpolarization appears to "activate" the chloride conductance, the "availability" of chloride current has been measured at the beginning of a voltage step to a standard test potential following conditioning at a variety of potentials. The relationship between the test current and the conditioning voltage is sigmoid. The normalized sigmoid curve has the same slope (absolute value) but opposite sign to that obtained in the same experiment at pH 8.8. In mildly acidic solutions (pH 6.4) the current wave form is diphasic: current initially falls then rises to the steady state. This combination of transients militates against the idea that transients are due solely to accumulation--depletion effects in restricted spaces ("unstirred layers") and a hypothesis is qualitatively outlined in which pH-and voltage-dependent effects are ascribed to a single type of channel whose orientation in the membrane is unconstrained.