POLARIZED DISTRIBUTION OF L-TYPE CALCIUM CHANNELS IN EARLY SEA-URCHINEMBRYOS

Using the whole cell clamp technique, we have measured calcium-depende nt currents and steady-state conductance in early sea urchin blastomer es. The calcium currents in M phase decreased from 8.5 mu A/cm(2) at t he four-cell stage to 5.4 mu A/cm(2) at the eight-cell stage. In 16-ce ll stage embryos, calcium currents mere 7.4 mu A/cm(2) in the mesomere s, 2.3 mu A/cm(2) in the macromeres, and were not detected in the micr omeres. In contrast, the micromeres had a two-to threefold higher stea dy-state conductance than the mesomeres or macromeres, which may be du e to potassium ion conductivity. Nifedipine, an L-type channel antagon ist, delays cleavage division at a concentration of 0.05-0.1 mM and ca uses developmental defects, such as poor skeletal differentiation in l ater sea urchin embryos.