Calcium current in molluscan neurones: measurement under conditions which maximize its visibility.

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Abstract
Membrane currents were studied in isolated somata of molluscan neurons from Archidoris montereyensis and Anisodoris nobilis. Under voltage clamp, inward current displayed a 2-phase time course and in some cases a clear reversal potential difference could be shown for the fast and slow phases. The slower phase was carried predominantly by Ca2+. The apparent magnitude of the slower phase was greatly influenced by conditions which altered K+ current flow. Blocking voltage-dependent K+ conductances, either by appropriate conditioning polarizations or by tetraethylammonium (TEA) ion, enhanced the magnitude, while conditions which augmented K+ current made the slow phase disappear. A fraction of the membrane K conductance was TEA insensitive. This fraction could be blocked by procedures which prevented internal levels of Ca from increasing during the voltage clamp pulse. Three such procedures were demonstrated (replacement of external Ca by Mg, internal buffering by EGTA [ethyleneglycol-bis(.beta.-aminoethylether)N,N,N''N''-tetraacetic acid] and replacement of Ca by permeant Ba). Internal EGTA buffering or external Ba in combination with external TEA produced an extreme change in membrane current as compared with the normal time course. Membrane current, when activated by pulses up to +50 mV, was net inward and showed only fractional inactivation over time courses running to several seconds. Pulses to voltages greater than +60 mV resulted in outward current. Under normal conditions the Ca conductance has the extended time course clearly evident under the above modified conditions but the Ca flux component is easily missed. In agreement with several prior studies a rise in internal Ca is causally related to a rise in K+ conductance [gK]. A transmembrane flux of Ca can be uncoupled from the gK increase by appropriate buffering of internal Ca. The transient K+ current, IA, which bears a resemblance to Ca-dependent K+ transients in some muscle cells did not depend upon internal Ca but instead is a voltage-activated mechanism.