Articles

The Journal of Membrane Biology

, Volume 124, Issue 3, pp 265-273

First online:

Action potentials inAcetabularia: Measurement and simulation of voltage-gated fluxes

  • H. MummertAffiliated withInstitut für Biologie I der Universität
  • , D. GradmannAffiliated withInstitut für Biologie I der Universität

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Summary

Amounts and temporal changes of the release of the tracer ions K+ (86Rb+),22Na+, and36Cl as well as of H+ in the course of action potentials inAcetabularia have been recorded. New results and model calculations confirm in quantitative terms the involvement of three major ion transport systemsX in the plasmalemma: Cl pumps, K+ channels, and Cl channels (which are marked in the following by the prefixes,P, K andC) with their equilibrium voltages X V e 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 (P L, KL,C L=1) but voltage gating be different: the pump ( p V e about −200 mV) being inactivated (open,o→closed,c) at positive going transmembrane voltages,V m; the K+ channels (K V e about −100 mV) are inactivated at negative goingV m; and the Cl channels (C V e: around 0 mV), which are normally closed (c) at a restingV m (nearPVe) go through an intermediate open (o) state at more positiveV m before they enter a third “shut” state (s) in series. Model calculations, in which voltage sensitivities are expressed by the factorf=exp(V mF/(2RT)), simulate, the action potential fairly well with the following parameters (PKco∶10/f ks−1,PKoc∶1000·f ks−1,KKco∶200·f ks−1,Kkoc∶2/f ks−1,cKco∶500·f ks−1,CKoc∶5/f ks−1,CKso∶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+.

Key Words

Acetabularia action potential chloride channel excitation simulation transient fluxes voltage gating