Alpha adrenergic receptor subtype associated with receptor binding, Ca⁺⁺ influx, Ca⁺⁺ release and contractile events in the rabbit aorta

The alpha-adrenergic receptor mechanism in rabbit aorta was examined for the involvement of alpha-1 or alpha-2 receptor subtypes. Agonists (phenylephrine, norepinephrine and clonidine) and antagonists (prazosin and yohimbine) with known receptor subtype selectivity were used to define the contribution of alpha-1 or alpha-2 receptors to receptor-initiated cellular Ca++ influx, intracellular release of Ca++ and overall contraction. The receptor content of isolated membranes was also measured in [3H]prazosin and [3H]yohimbine radioligand binding studies. Contraction-derived K(B) values for prazosin (3 nM) or yohimbine (1 μM) were similar for all three agonists, indicating that each acted on the same alpha-1 receptor. Prazosin (10-7 M) was effective in causing inhibition of cellular Ca++ influx initiated by agonists whereas yohimbine (10-6 M) had no effect. Prazosin but not yohimbine caused a partial reduction in phenylephrine or norepinephrine-induced stimulation of 45Ca efflux rate whereas the smaller clonidine-induced stimulation was totally inhibited by prazosin and partially inhibited by yohimbine. Alpha-1 specific binding of [3H]prazosin was observed with a K(D) of 3.5 nM and maximum binding site (B(max)) of 73 fmol/mg of protein. Although no alpha-2 specific binding of [3H]yohimbine was observed, binding to a low-affinity/high-capacity class of sites was found. The results indicate the sole presence and contribution of alpha-1 receptors to Ca++ flux and contractile events in the rabbit aorta.