DIVALENT-CATIONS SELECTIVELY ALTER THE VOLTAGE-DEPENDENCE OF INACTIVATION OF A-CURRENTS IN CHICK AUTONOMIC NEURONS

A-currents were studied in acutely dissociated chick autonomic neurons . Switching from salines containing 4 mM Mg2+ to salines containing 4 mM Ca2+ caused a large positive shift in the voltage dependence of ste ady-state inactivation, but had no effect on the voltage dependence or kinetics of activation, deactivation, the rate at which channels beca me inactivated, or the rate at which channels recovered from inactivat ion. This effect saturated with increasing concentrations of Ca2+. Oth er group IIA divalent cations were less effective than Ca2+, in the or der Ca2+ > Ba2+ > Mg2+ = Sr2+. Application of 4 nM Mg2+ partially anta gonized the effects of 4 mM Ca2+. The results suggest that divalent ca tions selectively alter the voltage dependence of steady-state inactiv ation by acting at sites accessible from the outside of the cell.