Displacement by α-Adrenergic Agonists and Antagonists of 3H-Prazosin Bound to the α-Adrenoceptors of the Dog Aorta and the Rat Brain

Abstract

Abstract-To characterize the a,-adrenoceptors in the dog aorta and the rat brain and to assess the antagonistic potencies of various chemicals inclusive of newly synthesized ones, radioligand binding assays were performed, and the potencies thus obtained were compared with those obtained from pharmacological obser vations.Reproducible binding to and displacement from the dog aorta of 3H prazosin were observed.The rank orders of the inhibition of 3H-prazosin binding to the membrane preparations of the aorta expressed as IC50 (the concentration of drugs inhibiting 50% of maximal specific binding of 3H-prazosin) were: prazosin> YM09538 > phentolamine > yohimbine > phenoxybenzamine > labetalol >S-596> dibenamine > K-351 > propranolol > hydralazine > N-696 for a-blockers and ( epinephrine > clonidine > I-norepinephrine > phenylephrine > 1-isoproterenol for agonists.IC50 values of phenoxybenzamine, labetalol, dibenamine and K-351 obtained in the brain preparations were higher than those obtained in the pre paration of the aorta. There was a good correlation (r=0.90) between the IC50 values obtained in the dog aorta and in the rat brain, suggesting that the a, -adrenergic receptors in the dog aorta and the rat brain resemble each other. Good correlations (aorta, r=0.97 and brain, r=0.94) were also observed between IC50 values derived from the binding assay in the aorta or the brain and the pA2 values obtained as regards to the contractile response of the rat aorta to phenylephrine.Thus, this method could be useful for the assessment of newly synthesized chemicals as a adrenergic antagonists.

Discovery of Quinazoline-Based Fluorescent Probes to α1-Adrenergic Receptors

“…21,22 All compounds demonstrated up to nanomolar affinities with three α 1 -AR subtypes, which are close to the positive control, phentolamine (Table 1). Compound 1e is approximately 100-fold less potent than compound 1c because of the lack of carbonyl group.…”

Effects of bunitrolol on adrenergic and serotonergic receptors.

“…In the case of dog brain, the membrane sus pension (0.25 mg of protein) was incubated with constant shaking for 30 min at 23 °C with 1.0 nM 3H-DHA in a total volume of 0.5 ml containing 60 mM Tris-HCI, 20 mM MgC12 buffer (pH 7.4), and the indicated concentration of unlabelled drugs. al Adrenoceptor binding assay using the rat brain and the dog aorta was carried out by the methods described previously (13). The membrane suspensions (0.25 mg of protein) from dog brain were incubated for 60 min at 23°C with 0.2 nM 3H-prazosin in a total volume of 1 ml containing 15 mM Tris-HCI, 5 MM MgC12 buffer (pH 7.4).…”

“…Pharmacological observations: (9-Blocking actions were determined as described pre viously (12,19), and a,-blocking actions were studied as described previously (13,20). The right and left atria of the guinea pig were used for the assessment of the antagonistic potencies against the positive chronotropic and inotropic actions of isoproterenol (81 effect).…”

“…Preparation of the membrane-enriched fraction: The membrane-enriched fractions from the rat brain and heart used for 3H-DHA binding were prepared by the methods described previously (10)(11)(12). The mem brane-enriched fractions from the rat brain and dog aorta used for 3H-prazosin binding were prepared by the methods described previously (13). The membrane-enriched fractions from the dog brain used for 3H-DHA binding, those from the brain used for 3H prazosin binding, and those from the dog and rat brain used for 3H-p-aminoclonidine binding were prepared by the following methods.…”

Substance P Injected into the Hypothalamic Supraoptic Nucleus Causes Antidiuresis through the Release of Arginine-Vasopressin in Water-Loaded and Ethanol-Anesthetized Rats

“…In the canine aorta, the existence of an a1-adrenoceptor was demonstrated by both radio ligand binding assay and pharmacological methods (2). Recently, an increasing number of a1-adrenoceptor sub classifications have been proposed through evidence ob tained in pharmacological and/or radioligand binding studies (3 10).…”