All-or-none response of the Ca²⁺-dependent K⁺ channel in inside-out vesicles

Abstract

In several cell lines, an increase in the intracellular Ca²⁺ concentration elicits an increase in the membrane permeability to K⁺(Rb⁺) by activation of the so-called Ca²⁺-dependent K⁺ channel¹. The human red cell has been widely used to study this transport system². As Ca²⁺ (refs 3,4) and other possible modulators^5,6 act on the intracellular side of the membrane, a preparation of everted vesicles would be most useful for investigating such actions. Although several previous studies in inside-out vesicles⁷ have been unsatisfactory^8–10, a new vesiculation procedure that preserves Ca²⁺-dependent K⁺ transport has recently been reported^11,12. Using this preparation we now find that each vesicle responds to Ca²⁺ in an all-or-none fashion, the number of activated (permeable to ⁸⁶Rb) vesicles increasing with Ca²⁺ concentration between 10⁻⁸ and 10⁻⁶M. This behaviour probably reflects the all-or-none response of individual K⁺ channel units. The sensitivity to Ca²⁺ is decreased by Mg²⁺.