Anomalous potassium channel-gating rates as functions of calcium and potassium ion concentrations.

With near normal monovalent ionic concentrations, the rate of increase of the potassium conductance after a depolarizing voltage-clamp step is slowed when the external calcium concentration is increased. This trend in the rise-time with changes in calcium is reversed when the axointernal potassium concentration is reduced (150 mM) and the periaxonal concentration is increased (50 mM); that is, the rise-time decrease with increasing calcium concentration. Furthermore, the degree of sigmoidality of the K-conductance time-course always increase when the rise-times increase for a given test potential. In the case of calcium surface-charge screening, these effects may be caused by a shifted distribution of K-ions within the channels following the altered ion gradient, and by a consequent shift in the reciprocal electrostatic interactions between the ionic charges and channel-gate charges.