Quantitation of alpha 1-adrenergic receptors in porcine uterine and mesenteric arteries.

The activation of vascular alpha-adrenergic receptors may be involved in the control of uterine blood flow. A radioligand binding assay with the use of the alpha 1-adrenergic antagonist 3H-WB-4101 was established to characterize the alpha-adrenergic receptors in uterine and mesenteric arterial membranes obtained from nonpregnant pigs. Specific binding of 3H-WB-4101 was rapid, saturable, and exhibited the alpha-adrenergic agonist potency order of (-)-epinephrine inhibition constant [Ki] = 0.6 mumol/L greater than (-)-norepinephrine (Ki = 1.5 mumol/L) much greater than (-)-isoproterenol (Ki = 120 mumol/L). The alpha-adrenergic antagonist phentolamine (Ki = 6.0 nmol/L) was 200 times more potent than the beta-adrenergic antagonist (+/-)-propranolol (Ki = 1,200 nmol/L); the alpha 1-selective antagonist prazosin (Ki = 1.2 nmol/L) was 130 times more potent than the alpha 2-selective antagonist yohimbine (Ki = 160 nmol/L). Scatchard analysis, as well as iterative curve-fitting analysis, demonstrated that 3H-WB-4101 binding by arterial membranes was to a single class of binding sites. Uterine arteries exhibited greater maximal binding capacity (BMax) than that of mesenteric arteries (47.5 +/- 3.2 versus 30.9 +/- 3.6 fmol per milligram of protein, p less than 0.01), but the uterine artery dissociation constant (Kd) was higher, thus indicating a lower affinity, when compared with mesenteric artery (0.43 +/- 0.04 versus 0.33 +/- 0.04 nmol/L, p less than 0.05).