Action potentials in Acetabularia: measurement and simulation of voltage-gated fluxes

Amounts and temporal changes of the release of the tracer ions K+ (86Rb+), 22Na+, and 36Cl- as well as of H+ in the course of action potentials in Acetabularia have been recorded. New results and model calculations confirm in quantitative terms the involvement of three major ion transport systems X in the plasmalemma: Cl- pumps, K+ channels, and Cl- channels (which are marked in the following by the prefixes, P, K and C) with their equilibrium voltages XVe and voltage/time-dependent conductances, which can be described by the following, first approximation. Let the maximum (ohmic) conductance of each of the three populations of transporter species be about the same (pL, KL, CL = 1) but voltage gating be different: the pump (pVe about -200 mV) being inactivated (open, o----closed, c) at positive going transmembrane voltages, Vm; the K+ channels (KVe about -100 mV) are inactivated at negative going Vm; and the Cl- channels (CVe: around 0 mV), which are normally closed (c) at a resting Vm (near pVe) go through an intermediate open (o) state at more positive Vm before they enter a third "shut" state (s) in series. Model calculations, in which voltage sensitivities are expressed by the factor f = exp(VmF/(2RT], simulate the action potential fairly well with the following parameters (pkco: 10/fks-1, pkoc: 1000.f ks-1, Kkco: 200.f ks-1, Kkoc: 2/f ks-1, ckco: 500.f ks-1, fkoc: 5/f ks-1, Cks0: 0.1/f ks-1,Ckos: 20.f ks-1). It is also shown that the charge balance for the huge transient Cl- efflux, which frequently occurs during an action potential, can be accounted for by the observation of a corresponding release of Na+.