Competitive interaction of amiloride and verapamil with alpha 1-adrenoceptors in vascular smooth muscle.

We have characterized [3H]prazosin binding to purified plasma membranes isolated from bovine carotid arteries and studied the effects of Na+ and Ca2+ channel blockers on [3H]prazosin binding to alpha 1-adrenoceptors. Amiloride and verapamil competitively inhibited the specific binding of [3H]prazosin to purified plasma membranes isolated from bovine carotid artery in a dose-dependent manner. The Ki values of verapamil and amiloride for alpha 1-adrenergic receptor were 1.04 +/- 0.037 microM and 32.6 +/- 0.59 microM, respectively. Verapamil (10 microM) and amiloride (100 microM) caused a 6-fold and 2.7-fold decrease in affinity of [3H]prazosin binding, respectively, with no change in the number of binding sites. The inhibition of [3H]prazosin binding by amiloride and verapamil could be reversed after the membranes were washed. Another Ca2+-channel blocker, nifedipine, and a Na+-channel blocker, furosemide, did not significantly inhibit [3H]prazosin binding up to 0.1 mM concentrations. Our results suggest that amiloride and verapamil may produce vascular smooth muscle relaxation by modulating alpha 1-adrenoceptor affinity in addition to blocking Na+ and Ca2+ channels, respectively.