Calcium currents and conductances in the muscle membrane of the crayfish

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Abstract
Membrane currents in Ca type muscle membrane of the crayfish Astacus fluviatilis were analyzed by a method in which a membrane microarea was isolated by circulating sucrose rings contacting the fiber perpendicular to the fiber surface. The early Ca inward currents were separated from the total membrane currents by subtraction of the early and delayed K currents from the total membrane current. The isolated Ca currents showed a time course characteristic for a transient change of Ca conductance. The presence of inactivation was further checked by the time course of the tail currents at the end of voltage clamp pulses of variable duration. The reversal potential of the early Ca currents determined from the current-voltage relations was +85 .+-. 4.2 mV. The Ca potentials were used to express the Ca currents in the form of chord conductances. Ca conductances (gCa) as functions of time and voltage were described quantitatively on the assumption that gCa [Ca conductance] was determined by 2 variables (m and h), according to the equation gCa = m6h.hivin.gCa where .hivin.gCa was a constant and m and h obeyed first order differential equations of the Hodgkin-Huxley type. The activation parameters of gCa were determined by fitting the solutions of these equations to the experimental gCa values. This method was used to check the inactivation parameters. The inactivation parameters of gCa were obtained from the inactivation curves, determined for several membrane potentials by variation of the duration of the conditioning step. The average Ca conductance constants were tabulated and compared with Na conductance constants in excitable membranes.