Role of external potassium in the calcium-induced potassium efflux from human red blood cell ghosts.

Abstract

The exposure of red cell ghosts to external Ca++ and K+ leads to a rapid net K+ efflux. Preincubation of the ghosts for various lengths of time in the absence of K+ in the external medium prior to a challenge with maximally effective concentrations of Ca++ and K+ renders the ghosts unresponsive to that challenge with a half-time of about 7-10 min. Preincubation at a range of K+ concentrations for a fixed length of time (60 min) prior to the challenge revealed that K+ concentrations of about 500 microM or more suffice to maintain the K+ channel in a maximally responsive state for at least 60 min. These K+ concentrations are considerably lower than the K+ concentrations required to make the responsive channel respond with a maximal rat of K+ efflux. Thus external K+ is not only necessary to induce the permeability change but also to maintain the transport system in a functional state. The presence of Mg++ or ethylenediamine-tetraacetic acid (EDTA) in the K+-free preincubation media preserves the responsiveness to a challenge with Ca++ plus K+. In contrast to external K+, the presence of external Ca++ does not reduce but rather enhances the loss of responsiveness. An excess of EDTA prevents the effects of Ca++ while washes with EDTA after exposure to Ca++ do not reverse them. In red cell ghosts that contain Ca++ buffers, the transition from a responsive to a nonresponsive state incubation in the absence of external K+ is enhanced. The effects of incubation in the presence of Ca++ in K+-free media are reversed; external Ca++ now reduces the rate at which the responsiveness is lost. The loss of responsiveness after incubation in K+-free media prior to a challenge with external K+ and internal Ca++ does also take place when K+-efflux from red cell ghosts is measured by means of 42K+ into media that have the same K+ concentrations as the ghost interior. This confirms that the effects of K+-free incubation are due to the modification of the K+-selective channel rather than to an inhibition of diffusive Cl--efflux.