Ion channels in the plasma membrane of Amaranthus protoplasts: one cation and one anion channel dominate the conductance.

This report details preliminary findings for ion channels in the plasma membrane of protoplasts derived from the cotyledons of Amaranthus seedlings. The conductance properties of the membrane can be described almost entirely by the behavior of two types of ion channel observed as single channels in attached and detached patches. The first is a cation-selective outward rectifier, and the second a multistate anion-selective channel which, under physiological conditions, acts as an inward rectifier. The cation channel has unit conductance of approx. 30 pS (symmetrical 100 K+) and relative permeability sequence K+ greater than Na+ much greater than Cl- (1:0.16:0.03): whole-cell currents activate in a time-dependent manner, and both activation and deactivation kinetics are voltage dependent. The anion channel opens for hyperpolarized membrane potentials, has a full-level conductance of approx. 200 pS and multiple subconductance states. The number of subconductances does not appear to be fixed. When activated the channel is open for long periods, though shuts if the membrane potential (Vm) is depolarized; at millimolar levels of [Ca2+]cyt this voltage dependency disappears. Inward current attributable to the anion channel is not observed in whole-cell recordings when MgATP (2 mM) is present in the intracellular solution. By contrast the channel is active in most detached patches, whether MgATP is present or not on the cytoplasmic face of the membrane. The anion channel has a significant permeability to cations, the sequence being NO3- greater than Cl- greater than K+ greater than Aspartate (2.04:1:0.18 to 0.09:0.04). The relative permeability for K+ decreased at progressively lower conductance states. In the absence of permeant anions this channel could be mistaken for a cation inward rectifier. The anion and cation channels could serve to clamp Vm at a preferred value in the face of events which would otherwise perturb Vm.