Two calcium currents in Neanthes arenaceodentatus egg cell membranes

Two distinct types of Ca currents, Ca(I) and Ca(II), were found in the eggs of the marine polychaete Neanthes arenaceodentatus and studied under voltage-clamp conditions. Ca(I) and Ca(II) channels differ in their selectivity sequences, show different sensitivities to blocking by Cd, and have different activation thresholds. These facts indicate that the channels responsible for Ca(I) and Ca(II) currents are unique and different. Both Ca(I) and Ca(II) currents decrease with time under a maintained depolarization. This relaxation exhibits different kinetics, with those for Ca(II) being an order of magnitude slower than for Ca(I). The Ca(I) current relaxation has been shown previously to be due to a voltage-dependent inactivation. The magnitude of relaxation of the Ca(II) current elicited by a test-voltage step immediately following a conditioning-voltage step paralleled the magnitude of the peak of the Ca(II) current flowing during the conditioning pulse. The kinetics of the current relaxation depended upon bath Ca concentration, the kinetics slowing down as the bath Ca was increased. These observations are consistent with an external depletion being the cause of the current relaxation for the Ca(II) channel.