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M. Henek and J. Zachar

Institute of Normal and Pathological Physiology, Slovak Academy of Sciences, 884 23 Bratislava, Sienkiewiczova 1, Czechoslovakia

1. Membrane currents in calcium type muscle membrane of the cray-fish Astacus fluviatilis were analysed by a method in which a membrane microarea was isolated by circulating sucrose rings contacting the fibre perpendicular to the fibre surface.

2. The early calcium inward currents were separated from the total membrane currents by subtraction of the early and delayed potassium currents from the total membrane current.

3. The isolated calcium currents show a time course characteristic for a transient change of calcium 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.

4. The reversal potential of the early calcium currents determined from the current—voltage relations was +85 ± 4·2 mV. The calcium potentials were used to express the calcium currents in the form of chord conductances.

5. Calcium conductances (g_Ca) as functions of time and voltage were found to be described quantitatively on the assumption that g_Ca is determined by two variables (m and h), according to the equation g_Ca = m⁶h_Ca, where _Ca is a constant and m and h obey first order differential equations of the Hodgkin—Huxley type.

6. The activation parameters of the g_Ca were determined by fitting the solutions of the above equations to the experimental values of the g_Ca. This method was also used to check the inactivation parameters.

7. The inactivation parameters of the g_Ca were obtained from the inactivation curves, which were determined for several membrane potentials by variation of the duration of the conditioning step.

8. The average calcium conductance constants were tabulated and compared with sodium conductance constants in excitable membranes.

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