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

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 (near_PV_e) 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 (_PK_co∶10/f ks⁻¹,_PK_oc∶1000·f ks⁻¹,_KK_co∶200·f ks⁻¹,_Kk_oc∶2/f ks⁻¹,_cK_co∶500·f ks⁻¹,_CK_oc∶5/f ks⁻¹,_CK_so∶0.1/f ks⁻¹,_Ck_os∶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⁺.