BINDING AND FUNCTIONAL-CHARACTERIZATION OF ALPHA-2-ADRENERGIC RECEPTOR SUBTYPES ON PIG VASCULAR ENDOTHELIUM

Alpha-2 adrenergic receptor subtypes were characterized in membranes o f pig vascular endothelium using [H-3]rauwolscine. Alpha-2 adrenergic receptor subtypes that mediate endothelium-dependent vascular relaxati on were studied in vitro by using ring segments of pig epicardial coro nary arteries. SpecifiC [H-3]rauwolscine binding in endothelial membra nes was saturable and to a single class of high-affinity sites with a mean K(D) of 0.217 +/- 0.05 nM and B(max) of 156 +/- 28 fmol/mg of pro tein. Nonlinear regression analysis indicated that competition binding curves for drugs that distinguish the alpha-2A adrenergic receptor su btype from the alpha-2C adrenergic receptor subtype fit best to two-si te binding models. K(l) values for drugs in binding to endothelial alp ha-2 adrenergic receptors correlated well with their K(l) values for a lpha-2A (r = .98) and alpha-2C (r = .97) adrenergic receptor subtypes identified in other tissues. Vascular endothelium contained 23% alpha- 2A and 77% alpha-2C adrenergic receptors. In the presence of indometha cin, the rank order of potency for agonists that cause endothelium-dep endent vascular relaxation was p-iodoclonidine > clonidine > UK-14,304 > guanabenz > epinephrine > norepinephrine. K(B) values for antagonis t inhibition of epinephrine-induced, endothelium-dependent vascular re laxation correlated best with K(l) values for antagonist binding at th e alpha-2A adrenergic receptor subtype. These results suggest that the alpha-2A and alpha-2C adrenergic receptor subtypes are present on pig vascular endothelium and that the alpha-2A adrenergic receptor subtyp e mediates indomethacin-insensitive, endothelium-dependent relaxation of pig epicardial coronary arteries.