Interaction of the Depolarization-Activated K+ Channel of Samanea saman with Inorganic Ions: A Patch-Clamp Study

Abstract

A depolarization-activated K+ channel capable of carrying the large K+ currents that flow from shrinking cells during movements of Samanea saman leaflets has been described in the plasmalemma of Samanea motor cell protoplasts (N Moran et al [1988] Plant Physiol 88:643-648). We now characterize this channel in greater detail. It is selective for K+ over other monovalent ions, with the following order of relative permeability: K+ > Rb+ .BETA. Na+ .simeq. Cs+ .simeq. Li+. It is blocked by Cs+ and by Ba2+ in a voltage dependent manner, exhibiting a ''long-pore'' behavior, similarly to various types of K+ channels in animal systems. Cadmium, known for its blockage of Ca2+ channels in animals systems, and Gd3+, closely related to La3+, which also blocks Ca2+ channels in animals cells, both block K+ currents in Samanea in a voltage-independent manner, and without interfering with the kinetics of the currents. The suggested mechanism of block is either (a) by a direct interaction with the K+ channel, but external to its lumen, or, alternatively, (b) by blocking putative Ca2+ channels, and preventing the influx of Ca2+ on which the activation of the K+ channels may be depending.