Calcium tail currents were measured in axotomized Aplysia neurones L2-L6 using a two-electrode voltage clamp and micro-electrodes of specially low resistance. Measurements were made at -40 mV following depolarizing pulses of 7 or 10 ms duration in the presence of 45 microM-tetrodotoxin and 200 mM-tetraethylammonium. Symmetrical currents were eliminated by addition of digitally stored current traces produced in response to equivalent hyperpolarizations. The remaining current, identified as a tail current, was blocked by replacement of extracellular calcium with cobalt or manganese. Computer fits showed that the tail current closely approximated the sum of two exponentially decaying components. The first had a time constant, tau 1, of 0.38 +/- 0.05 ms, which may have been frequency-limited by the speed of the clamp; the second had a time constant, tau 2, of 2.0 +/- 0.8 ms. A more slowly decaying third tail current component (tau 3 = 30 ms), which developed more slowly, may be related to the non-specific current rather than the calcium current. The tau 1 and tau 2 components of the tail current lost amplitude with increasing pulse duration along an approximately bi-exponential time course that resembled the time course of relaxation of the calcium current during a prolonged depolarization. The slow third component of the tail current showed no such inactivation. The amplitudes of the first and second components of the calcium tail current both increased as sigmoidal functions of the test pulse voltage, reaching half maximum at +20 mV and plateauing above +60 mV. The voltage dependencies of the two components were similar. The rate of decay of the tau 1 component increased with increasing temperature and with increasing negative potential, whereas tau 2 showed little dependence on these parameters. The rates of decay of neither the tau 1 nor the tau 2 component were affected by large changes in the amplitude of the test depolarization or in the amplitude of the tail current or by injection of calcium ions or EGTA. Thus, the kinetics of the tail current as resolved under our conditions appear to be virtually independent of calcium-mediated inactivation. Activation time constants (tau m) predicted from tau 1 are 3 to 5 times longer than the values of tau m determined from the half-time to peak of activation. These kinetics are slower than those reported for Limnaea by factors of 2.5 to 3.5.(ABSTRACT TRUNCATED AT 400 WORDS)
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