α-Adrenoceptors in Dog Mesenteric Vessels–Subcellular Distribution and Number of [3H]Prazosin and [3H]Rauwolscine Binding Sites

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Abstract
Binding of the α-adrenergic antagonists [3H]prazosin and [3H]rauwolscine to well-characterized subcellular membrane fractions isolated from dog mesenteric arteries and veins was studied. Binding of both ligands was saturable with Kd values of 0.5 ± 0.1 nM for [3H]prazosin and 5.85 ± 0.85 nM for [3H]rauwolscine in arteries, and 0.87 ± 0.4 nM for [3H]prazosin and 6.6 ± 1.5 nM for [3H]rauwolscine in veins. In veins, the maximum number of binding sites for [3H]rauwolscine was higher than that for [3H]prazosin, whereas in arteries the maximum number of binding sites for each ligand was similar. In microsomes from dog aorta, the maximum number of bindings sites for [3H]prazosin was higher than that for [3H]rauwolscine. Neuronal membrane contamination in these studies was minimized by dissection procedures and evaluated by the comparison of [3H]saxitoxin binding in various preparations. Only mesenteric veins responded functionally to agonists acting on α2-adrenoceptors. This study thus identified two distinct populations of [3H]prazosin and [3H]rauwolscine binding sites in the plasma membranes of dog mesenteric vessels and suggests that a much higher density of α2- compared to α1-adrenoceptor binding sites is required for a contractile response.