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

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 + > Na + ≃ Cs + ≃ Li + . It is blocked by Cs + and by Ba 2+ in a voltage dependent manner, exhibiting a `long-pore9 behavior, similarly to various types of K + channels in animal systems. Cadmium, known for its blockage of Ca 2+ channels in animal systems, and Gd 3+ , closely related to La 3+ , which also blocks Ca 2+ channels in animal 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 Ca 2+ channels, and preventing the influx of Ca 2+ , on which the activation of the K + channels may be dependent.