title: Alpha adrenergic receptor subtype associated with receptor binding, Ca++ influx, Ca++ release and contractile events in the rabbit aorta.
abstract: 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 KB values for prazosin (3 nM) or yohimbine (1 microM) 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 KD of 3.5 nM and maximum binding site (Bmax) 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.
mesh_terms: D000818:Animals; D001011:Aorta; D002118:Calcium / Q000378:metabolism*; D003000:Clonidine / Q000494:pharmacology; D007473:Ion Channels / Q000502:physiology*; D009119:Muscle Contraction; D009131:Muscle, Smooth, Vascular / Q000502:physiology*; D009638:Norepinephrine / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology; D011224:Prazosin / Q000494:pharmacology; D011817:Rabbits; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000502:physiology*; D015016:Yohimbine / Q000494:pharmacology
chemical_list: D007473:Ion Channels; D011942:Receptors, Adrenergic, alpha; D010656:Phenylephrine; D015016:Yohimbine; D003000:Clonidine; D002118:Calcium; D009638:Norepinephrine; D011224:Prazosin