A description of activation and conduction in calcium channels based on tail and turn‐on current measurements in the snail.

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Abstract
Turn-on of Ca currents, or activation, was compared with turn-off, or deactivation. The experiments were done on nerve cell bodies of H. aspersa separated by dissection, voltage-clamped and internally perfused using a combined suction pipette-micro-electrode method. Ca currents were isolated by suppression of Na and K currents. At least 3 distinct exponentials were required to describe the total activation process. Two exponentials were required to fit turn-on; a 3rd, much faster exponential was revealed by turn-off. The fact that there was 3 exponentials in the system response indicates a minimum of 4 states in a transition state model of activation. The effect of cooling to slow turn-on with little effect on tail .tau. values was explained by making the leading forward rate constant of the 4-state model more temperature-sensitive than the other rate constants. The instantaneous current-voltage (I-V) curve was made up of the amplitudes associated with the fast and slow exponentials, denoted AF and AS, respectively. The instantaneous I-V curve was fitted by a modified constant field equation; the dominant AF component had the same shape. The slower component had a constant amplitude at potentials below +10 mV. These results may help explain the different instantaneous I-V curves that have been described for the Ca channel. An isochronal I-V was measured using the current at the end of 3 ms pulses. This was in good agreement with an I-V curve which was constructed from an activation curve and an instantaneous I-V curve, both of which were obtained from tail current measurements.