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Howard Hughes Medical Institute, Department of Neurobiology and Behavior, State University of New York, Stony Brook 11794-5230.

1. Calcium currents were recorded in dissociated bull-frog sympathetic neurones (BSNs) through patch pipettes using discontinuous voltage clamp. Activation kinetics were examined by analysing turn-on and turn-off currents. 2. After short depolarizing pulses turn-off tail currents were fitted with the sum of two exponentials. The fast component (time constant, tau approximately 240 microseconds at -40 mV) was undoubtedly due to the closure of calcium channels. The significance of a small and slower component is discussed. 3. Neither activation nor deactivation time courses changed as channels inactivated during progressively longer pulses or when the holding potential was less negative. No specific component was selectively suppressed by these manipulations. 4. Steady-state activation of the Ca2+ current was described by the Boltzmann distribution raised to the second power. Currents had an apparent threshold at -30 mV and were half-activated at +5 mV. 5. Calcium current turned on following m2 kinetics throughout the range of activation. The slowest time constant was around 1.2 ms between 0 and +10 mV. Turn-on was faster at negative or more positive potentials. 6. The time course of decay of tail currents became progressively faster at more negative potentials. 7. The instantaneous current-voltages (I-V) curve was obtained from tail current measurements and fitted by a modified constant-field equation. 8. The measured peak I-V curve could be reconstructed from the activation curve and the instantaneous I-V curve. 9. The activation kinetics of the calcium current in BSNs were consistent with the existence of a single kinetic class of channels and can be described with a simple m2 Hodgkin-Huxley model.

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