| Summary: | This thesis describes work on two components of the ionic current in neurones of the snail Helix aspersa under voltage clamp conditions. Most of the work is a study of the inward calcium current (ICa), produced by depolarization, under conditions where the potassium (K) permeability was reduced by the K channel blockers tetraethylammonium (TEA) ions and 4-aminopyridine (4-AP). Under these conditions Ica activated then inactivated more slowly to a level which was still inward at the end of long depolarizations to potentials +20mV. Above this potential Ica was masked by a TEA and 4-AP resistant component of the outward current. The activation of the inward current, over a limited voltage range, was fitted in Hodgkin-Huxley terms with m2. Particular attention was paid to the mechanism of Ica decline, for which a number of possibilities were considered including; voltage- and calcium-dependent inactivation, depletion or accumulation of the permeant ion and outward current activation. Evidence from measurements of Ica decline, and 2 pulse experiments to measure the voltage-dep-endence of inactivation suggested that decline was dependent on Ca-entry and was unlikely to be caused by activation of a Ca-dependent K current. Injection of the Ca chelator EGTA caused an increase in the inward current and also slowed decline. Ca/EGTA buffers could either increase or decrease the peak inward current depending on the free [Ca] in the buffer, supporting evidence of the existence of a Ca binding site sensitive to [Ca]1. Experiments on the early outward current (IA) suggested that this current is not Ca-activated, in this tissue, and that 4-AP blocks the current in a time-dependent manner, from the inside of the cell, on depolarization.
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