Identification and Characterization of Alpha 1 Adrenergic Receptors in the Canine Prostate Using [ 125 I]-Heat

Abstract

We have recently utilized radioligand receptor binding methods to characterize muscarinic cholinergic and alpha adrenergic receptors in human prostate adenomas. The primary advantages of radioligand receptor binding methods are that neurotransmitter receptor density is quantitated, the affinity of unlabelled drugs for receptor sites is determined, and receptors can be localized using autoradiography on slide-mounted tissue sections. Recently, [¹²⁵I]-Heat, a selective and high affinity ligand with high specific activity (2200 Ci/mmole) has been used to characterize alpha₁ adrenergic receptors in the brain. In this study alpha₁ adrenergic receptors in the dog prostate were characterized using [¹²⁵I]-Heat. The Scatchard plots were linear indicating homogeneity of [¹²⁵I]-Heat binding sites. The mean alpha₁ adrenergic receptor density determined from these Scatchard plots was 0.61 ± 0.07 fmol/mg. wet wt. ± S.E.M. The binding of [¹²⁵I]-Heat to canine prostate alpha₁ adrenergic binding sites was of high affinity (K_d = 86 ± 19 pM). Steady state conditions were reached following an incubation interval of 30 minutes and specific binding and tissue concentration were linear within the range of tissue concentrations assayed. The specificity of [¹²⁵I]-Heat for alpha₁ adrenergic binding sites was confirmed by competitive displacement assays using unlabelled clonidine and prazosin. Retrospective analysis of the saturation experiments demonstrated that Bmax can be accurately calculated by determining specific [¹²⁵I]-Heat binding at a single ligand concentration. [¹²⁵I]-Heat is an ideal ligand for studying alpha₁ adrenergic receptors in the prostate and its favorable properties should facilitate the autoradiographic localization of alpha₁ adrenergic receptors in the prostate.