A reappraisal of frog muscle chloride conductance-voltage relations at pH9.

Voltage clamp experiments in muscle show that the steady state chloride conductance at pH 9 remains rather independent of [Cl-]o, for [Cl-]o in the range 165-265 mM. The steady state conductance-voltage relation has a maximum near Vrest -20 mV. The initial-conductance-voltage relation obtained when the voltage is stepped away from a constant conditioning value (e.g. the resting potential) approaches an asymptotic maximum for hyperpolarizing steps and a minimum for depolarizing steps. The conductance declines with time if the test voltage is more negative than the resting potential, but remains constant if it is more positive. When the conditioning voltage is varied and the test kept constant the initial conductance at the step is also seen to be sigmoidal, if the test step is hyperpolarizing: for large negative conditioning steps the conductance at the test potential approaches the same asymptotic value as does the steady state relation, independent of the test voltage. At positive conditioning voltages it approaches a maximum asymptote which is dependent on the test voltage. When the test step is positive-going the initial conductance at the step is weakly dependent on the conditioning voltage and for large negative conditioning potentials is larger than predicted from the steady state relation. In summary, during hyperpolarizing voltage steps the chloride conductance seems to decline due to a "gating" phenomenon, but openstate conductance seems to be voltage-dependent, and at membrane potentials more positive than the resting it rapidly (within the settling time of the clamp) assumes a value almost independent of any preceding (conditioning) voltage.