Differential developmental fates of the two calcium currents in early embryos of the ascidian Ciona intestinalis.

Two voltage-dependent calcium currents have been described in unfertilized eggs of the ascidian Ciona intestinalis: a low threshold, slowly activating current, and a high threshold fast one. According to the classical criteria for classification of calcium currents, they both share some of the features of L-like and T-like currents. We have studied these two calcium currents further by measuring their sensitivity to permeant ions, temperature and inhibitors. Both currents were sensitive to relatively high concentrations of nitrendipine, which was a selective blocker of the low threshold channel. The lanthanide ion gadolinium was a potent blocker of the low threshold current, and cadmium preferentially inhibited the high threshold current. The two calcium currents were regulated in a different manner after fertilization. The density of the high threshold current remained relatively constant, while the low threshold current was lost by the time of first cleavage. This loss following fertilization is similar to the loss of a low threshold sodium current in fertilized eggs of the ascidian Boltenia villosa. Block of the cell cycle with various compounds did not prevent loss of the low threshold calcium current. This observation adds weight to the hypothesis that a loss of excitability is a general property of early development. We conclude that fertilization can differentially modulate channel populations before first cleavage. The mechanism by which this occurs in the ascidian embryo has yet to be discovered.