Two different conductances contribute to the anion currents in Coffea arabica protoplasts

The anion conductance of the plasma membrane of Coffea arabica protoplasts was isolated and characterized using the whole-cell patch clamp technique. Voltage pulse protocols revealed two components: a voltage-gated conductance (G(s)) and a voltage-independent one (G(l)). G(s) is activated upon depolarization (e-fold activation every +36 mV) with time constants of 1 sec and 5 sec at all potentials. G(l) and G(s) also differ by their kinetic and biophysical properties. In bi-ionic conditions the current associated with G(s) shows strong outward rectification and its permeability sequence is F- > NO3- > Cl-. In the same conditions the current associated with G(l) does not rectify and its permeability sequence is F(-)much greater than NO3- = Cl-. Furthermore, at potentials over +50 mV G(s), but not G(l), increases with a time constant of several minutes. Finally the gating of G(s) is affected by stretch of the membrane, which leads to an increased activation and a reduced voltage sensitivity. Anion conductances similar to the ones described here have been found in many plant preparations but G(l)-type components have been generally interpreted as the background activation of the slow voltage-gated channels (corresponding to G(s)). We show that in coffee protoplasts G(l) and G(s) are kinetically and biophysically distinct, suggesting that they correspond to two different molecular entities.