Separate [3H]‐nitrendipine binding sites in mitochondria and plasma membranes of bovine adrenal medulla

1 Two binding sites for the 1,4‐dihydropyridine (DHP) derivative [3H]‐nitrendipine have been found in the bovine adrenal medulla. The high‐affinity site (Kd = 0.48 nM and Bmax = 128 fmol mg−1 protein) was specifically located in purified plasma membranes. The low‐affinity site (Kd = 252nM and Bmax = 169 pmol mg−1 protein) was located only in mitochondria. Chromaffin granule membranes lacked specific binding sites for [3H]‐nitrendipine. 2 Kinetic analysis of the rates of association and dissociation of [3H]‐nitrendipine, saturation isotherms and displacement experiments with unlabelled nitrendipine and PN200‐110 revealed single, homogeneous populations of high‐ and low‐affinity sites in plasma and mitochondrial membranes, respectively. 3 The high affinity site was sensitive to Ca2+ deprivation and heating; it was practically unaffected by changes in ionic strength of the medium and its optimal pH was slightly alkaline. This site exhibited a strong DHP stereoselectivity; diltiazem increased and verapamil decreased the affinity of [3H]‐nitrendipine. 4 In contrast, binding of [3H]‐nitrendipine to the low affinity site was more heat resistant and less affected by Ca2+ removal. Its optimal pH was slightly acid and the increase in ionic strength enhanced the number of available sites. The site had no DHP stereoselectivity. Verapamil decreased the dissociation constant of [3H]‐nitrendipine acting in a non‐competitive manner; diltiazem did not affect equilibrium binding parameters of [3H]‐nitrendipine. 5 These results suggest that both binding sites reflect different receptor entities. The high‐affinity binding site corresponds to the dihydropyridine receptor associated with the L‐type calcium channel. The function of the mitochondrial, low‐affinity binding site is, at present, unknown.