Whole-cell patch-clamp techniques were used to measure anion currents through the plasma membrane of protoplasts of mesophyll cells of expanding pea (Pisum sativum L.) leaves. Voltage-induced changes of the currents could be modelled with single exponential activation and deactivation kinetics. The anion currents were activated at negative membrane potentials. The time constant of activation, tau(act), increased from 145 ms at -140 mV to 380 ms at -20 mV. A Boltzmann fit to the activation curve, n infinity (Delta G(Vm)/Delta G(max)), yielded a half-activation voltage of +27 mV. Opening and closing rate constants, alpha and beta respectively, were calculated from the values of tau and n(infinity). The currents depended on the presence of cytoplasmic Ca2+ concentrations higher than 10(-6) M. Including 3 mM MgATP in the intracellular solution resulted in a voltage-dependent inactivation of the anion current. The conductance-voltage relation resulting from the voltage-dependent activation and inactivation had a maximum at about -25 mV. The relations of the current in pea are discussed with respect to the anion currents in guard cells and suspension-cultured tobacco cells, and its possible role in growing leaf cells.
