Identification of ω‐Conotoxin Binding Sites on Adrenal Medullary Membranes: Possibility of Multiple Calcium Channels in Chromaffin Cells

Abstract

Binding of 125I-omega-conotoxin GVIA and [3H]nitrendipine to membranes from bovine adrenal medulla was investigated to test for the presence of N- and L-type Ca2+ channels in adrenal chromaffin cells. Saturable, high-affinity binding sites for 125I-omega-conotoxin and [3H]nitrendipine were detected in a membrane fraction from adrenal medulla. [3H]Nitrendipine binding sites were found to have a KD of 500 +/- 170 pM and a Bmax of 26 +/- 11 pmol/g of protein. 125I-omega-Conotoxin binding sites had a KD of 215 +/- 56 pM and a Bmax of 105 +/- 18 pmol/g of protein, about four times the number of sites found for [3H]nitrendipine. 125I-omega-Conotoxin binding was potently inhibited by unlabeled toxin and Ca2+ but was unaffected by dihydropyridines, verapamil, and diltiazem. [3H]Nitrendipine binding was not affected by omega-conotoxin, whereas it was inhibited by other dihydropyridines. Bay K 8644 potentiated K+-evoked cytosolic Ca2+ transients measured by fura-2 fluorescence, and this potentiation was completely blocked by nifedipine. In contrast, omega-conotoxin had no effect on Bay K 8644-evoked Ca2+ transients. Thus, the binding sites for omega-conotoxin and for nitrendipine appear to be different. The results confirm the presence of L-type Ca2+ channels and open the possibility of N-type Ca2+ channels as the omega-conotoxin binding sites in chromaffin cell membranes.