Potassium-efflux channels in extensor and flexor cells of the motor organ of Samanea saman are not identical. Effects of cytosolic calcium

Leaflet movements in the mimosa-family tree Samanea saman stem from coordin ated volume changes of cells in the leaf motor organs in the adaxial and ab axial motor cells ("flexors" and "extensors"). Shrinking, initiated by diss imilar light signals in extensors and in flexors, depends in both cell type s on K+ efflux via depolarization-dependent potassium (K-D) channels. To co mpare between flexor and extensor K-D channels and to test for a possible i nteraction of these channels with the Ca2+-mobilizing phosphoinositide casc ade evoked in these motor cells by the "shrinking signals," we probed the c hannels with varying (5 nM-3 mM) cytosolic free-Ca2+ concentration ([Ca2+]( cyt)) in patch-clamped inside-out excised membrane patches. Ca2+ was not re quired for K-D channel activation. [Ca2+](cyt) of 600 nM decreased the mean number of open K-D channels in flexors, as monitored at -30 mV. Detailed a nalysis revealed that in flexors millimolar [Ca2+](cyt) decreased the maxim um number of open channels, but simultaneously increased K-D channel openin g probability by negatively shifting the half-maximum-activation voltage by 40 to 50 mV. Thus, the promoting and the inhibitory effects at millimolar [Ca2+](cyt) practically cancelled-out. In contrast to flexors, none of the gating parameters of the extensor K-D channels were affected by [Ca2+](cyt) . Irrespective of [Ca2+](cyt), the steady-state gating of extensor K-D chan nels was slightly but significantly more voltage sensitive than that of fle xors. The unitary conductances of flexor and extensor K-D channels were sim ilar and decreased by approximately 20% at millimolar [Ca2+](cyt). It is in triguing that the extensor K-D channels were significantly less K+ selectiv e than those in flexors.