Binding of [3H]Nitrendipine to Proteins in the Plasma Membrane of Sea Urchin Sperm

The plasma membrane fractions of the sperm of four species of sea urchin, obtained by the method by Podell et al. (24), gave similar electrophoretic profiles of proteins. Several proteins in the membrane fraction from Hemicentrotus pulcherrimus bound ³H]nitrendipine, a specific antagonist of voltage-dependent Ca²⁺channels, added at concentration of about 10⁴times those reported to be effective in muscle and nerve cells. Nifedipine, a close analogue of nitrendipine, decreased the bindings of ³H]nitrendipine to 210, 140, 130 and 110 kDa and increased its bindings to several other proteins. Diltiazem, another type of Ca²⁺channel blocker, enhanced the bindings of ³H]nitrendipine to proteins of 210, 140, 130 and 110 kDa, and decreased its bindings to the other proteins. This effect of diltiazem on the binding of ³H]nitrendipine to proteins in the membrane fraction was similar to its effect on the mammalian excitable membrane fraction. The proteins whose binding to ³H]nitrendipine was blocked by nifedipine and enhanced by diltiazem are Ca²⁺channels.