Abstract
Characterization of adrenergic receptors in membranes from the rat seminal vesicle was studied by radioligand binding assay. Seminal vesicle membranes contained saturable and high affinity binding sites for the β-adrenergic receptor antagonist 3H-dihydroalprenolol (3H-DHA)and for the α-adrenergic antagonist 3H-prazosin. The observed order of potency for adrenergic agonists in competing for the 3H-DHA binding sites: isoproterenol > epinephrine ≈ salbutamol > norepinephrine indicates that these membrane receptors have the properties of β2-adrenergic receptors. α1-Adrenergic receptors were defined mainly as α1A subtypes by demonstrating their insensitivity to pretreatment with chlorethylclonidine and the different rank orders of antagonist affinities. No significant binding sites for the α2-adrenergic receptor agonist 3H-clonidine were observed. The GTP-induced reduction in the affinity of α1-adrenergic receptors for epinephrine was significantly reversed by the muscarinic cholinergic agonist carbachol. Atropine effectively antagonized this effect of carbachol on the competitive inhibition of 3H-prazosin binding by epinephrine in the presence of GTP, which suggests that muscarinic cholinergic receptors regulate the affinity of α1-adrenergic receptors by modulating the effect of guanine nucleotides. The effect of GTP on decreasing the affinity of β2-adrenergic receptors was not influenced by the addition of carbachol.
