Identification of β-adrenergic receptor binding sites in rat brain microvessels, using [125I]iodohydroxybenzylpindolol

Brain microvessels were prepared from rat cerebral cortex. The purity was confirmed by phase-contrast microsocpy and by the measurement of an enzymatic marker, γ-glutamyltranspeptidase. The microvessel preparation was subjected to radioreceptor assay using a ¹²⁵I-labelled β-adrenergic antagonist, hydroxybenzylpindolol (IHYP). The binding was linear with protein concentration up to at least 80 μg per tube. It was saturated at 200 pM IHYP concentration. The K(D) value calculated by Scatchard analysis was 69.4 ± 9.9 pM. The maximum binding (B(max)) was 107 ± 4 fmol/mg protein. The binding reached equilibrium within 30 min and was dissociated by addition of (-)-propranolol. The inhibitory effects of isomers of propranolol and isoproterenol on this binding showed that (-)-isomers were two orders of magnitude more potent than the (+)-isomers. Other neurotransmitters did not affect IHYP binding. The characteristics of the binding, saturability, high affinity, reversibility and stereospecificity, suggest that IHYP is bound to β-adrenergic receptor sites located on brain microvessels.