K+ channel gating: C-type inactivation is enhanced by calcium or lanthanum outside

Abstract

Many voltage-gated K+ channels exhibit C-type inactivation. This typically slow process has been hypothesized to result from dilation of the outer-most ring of the carbonyls in the selectivity filter, destroying this ring’s ability to bind K+ with high affinity. We report here strong enhancement of C-type inactivation upon extracellular addition of 10–40 mM Ca2+ or 5–50 µM La3+. These multivalent cations mildly increase the rate of C-type inactivation during depolarization and markedly promote inactivation and/or suppress recovery when membrane voltage (Vm) is at resting levels (−80 to −100 mV). At −80 mV with 40 mM Ca2+ and 0 mM K+ externally, ShBΔN channels with the mutation T449A inactivate almost completely within 2 min or less with no pulsing. This behavior is observed only in those mutants that show C-type inactivation on depolarization and is distinct from the effects of Ca2+ and La3+ on activation (opening and closing of the Vm-controlled gate), i.e., slower activation of K+ channels and a positive shift of the mid-voltage of activation. The Ca2+/La3+ effects on C-type inactivation are antagonized by extracellular K+ in the low millimolar range. This, together with the known ability of Ca2+ and La3+ to block inward current through K+ channels at negative voltage, strongly suggests that Ca2+/La3+ acts at the outer mouth of the selectivity filter. We propose that at −80 mV, Ca2+ or La3+ ions compete effectively with K+ at the channel’s outer mouth and prevent K+ from stabilizing the filter’s outer carbonyl ring.