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

Abstract

Turn-on of Ca currents, or activation, was compared with turn-off, or deactivation. The experiments were done on nerve cell bodies of Helix 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. The turn-off or tail currents were large and fast; this required that the limitations of the voltage clamp be established. A second micro-electrode was inserted to determine temporal and spatial control of potential, and it was found that the cells were essentially equipotential within 60 microsec during the largest tail currents. Series resistance was less than 5 k omega as measured by a small-perturbation, pseudorandom noise-current signal and presented negligible error in the measurements. Activation was complicated by the presence of asymmetry currents which required evaluation. This was done after Co replacement for Ca. The asymmetry currents were sufficiently small for their contribution to the tail currents to be ignored. Addition of Cd to Ca solutions could not be used since relatively large inward tail currents persisted in the presence of Cd. Tail currents were fitted by sums of two or three exponentials; each was sensitive to Ca-channel blockers but only two were due to closure of Ca channels. The two faster components with time constants tau F and tau S, for fast and slow respectively, were produced by brief, 3.0 msec voltage pulses and were present in all cells. The third and slowest component with time constant tau VS, for very slow, activated much more slowly and was not always present. The amplitudes of tau F and tau S were reduced by cooling and were increased when Ca was replaced by Ba extracellularly or when the external Ca concentration was increased. Hence, these components were due to closure of Ca channels. The third component activated faster in Ba solution. When fully activated it had the same amplitude in either Ba or Ca solution despite the differences in amplitude of Ba and Ca currents during the voltage-clamp step. The amplitude of the third component was also not changed by increasing external Ca concentration; hence it was not due to closure of Ca channels. Cooling also had very little effect. The third component was abolished by Ca blockers, but it does not appear to be related to previously described Ca-activated currents.(ABSTRACT TRUNCATED AT 400 WORDS)