Competitive Interaction of Amiloride and Verapamil with α1 Adrenoceptors in Vascular Smooth Muscle

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Abstract
Summary: 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 α1 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 α1 adrenergic receptor were 1.04 ± 0.037 μM and 32.6 ± 0.59 μM, respectively. Verapamil (10 μM) and amiloride (100 μM) 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 α1 adrenoceptor affinity in addition to blocking Na+ and Ca2+ channels, respectively.