Impedance of the electrogenic Cl⁻ pump inAcetabularia: Electrical frequency entrainements, voltage-sensitivity, and reaction kinetic interpretation

Summary

Reaction kinetic analysis of the electrical properties of the electrogenic Cl⁻ pump inAcetabularia has been extended from steady-state to nonsteady-state conditions: electrical frequency responses of theAcetabularia membrane have been measured over the range from 1 Hz to 10 kHz at transmembrane potential differences across the plasmalemma (V _m ) between −70 and −240 mV using voltage-clamp techniques. The results are well described by an electrical equivalent circuit with three parallel limbs: a conventional membrane capacitancec _m , a steadystate conductanceg _o (predominantly of the pump pathway plus a minor passive ion conductance) and a conductanceg _s in series with a capacitancec _p which are peculiar to the temporal behavior of the pump. The absolute values and voltage sensitivities of these four elements have been determined:c _m of about 8 mF m⁻² turned out to be voltage insensitive; it is considered to be normal.g _o is voltage sensitive and displays a peak of about 80 S m⁻² around −180 mV. Voltage sensitivity ofg _s could not be documented due to large scatter ofg _s (around 80 S m⁻²).c _p behaved voltage sensitive with a notch of about 20 mF m⁻² around −180 mV, a peak of about 40 mF m⁻² at −120 mV and vanishing at −70 mV. When these data are compared with the predictions of nonsteady-state electrical properties of charge transport systems (U.-P. Hansen, J. Tittor, D. Gradmann, 1983,J. Membrane Biol. in press), model “A” (redistribution of states within the reaction cycle) consistently provides magnitude and voltage sensitivity of the elementsg _o ,g _s andc _p of the equivalent circuit, when known kinetic parameters of the pump are used for the calculations. This analysis results in a density of pump elements in theAcetabularia plasmalemma of about 50 nmol m⁻². The dominating rate constants for the redistribution of the individual states of the pump in the electric field turn out to be in the range of 500 sec⁻¹, under normal conditions.