The Action Potential in Chara corallina III.* The Hodgkin-Huxley Parameters for the Plasmalemma

Abstract

An analysis, based on the Hodgkin-Huxley (H-H) equations is given of excitation processes in the plasmalemma of cells of C. corallina. Voltage clamp data for the plasmalemma show the presence of two activation and two inactivation processes and both of these transients are modelled in a manner analagous to the gated Na+ current in squid axons. Separation of the various ionic components of the experimental clamp currents was achieved by fitting the experimental data over a wide range of potentials to the model equations. The potential dependencies of the various H-H parameters for C. corallina, determined from the analysis of experimental results, are presented. A reconstruction is also given of the action potential and it is shown to be in satisfactory agreement with the experimental data. Comparisons are made of the experimental and predicted threshold potential, refractory period and the effects of the external Cl- and Ca²+ ion concentrations on the action potential. From the H-H parameters, the time courses during an action potential of the conductances g*Cl, g*x (¿ g*Ca?), the non activated/inactivated steady-state conductance g*ss, and the corresponding ionic currents I*Cl, I*x (¿ I*Ca?) and I*ss are calculated. While in the H-H analysis the peak value of g*x is found to be very large (larger than the peak value of g*Cl), it is shown that nevertheless I*Cl and I*ss dominate during an action potential.