Characterization of [H](±)Carazolol Binding to /?-Adrenergic Receptors Application to Study of /?-Adrenergic Receptor Subtypes in Canine Ventricular Myocardium and Lung

'H](±)Carazolol, " newly available ^-adrenergic receptor antagonist, has been used to characterize /f-adrenergic receptor subtypes present in membrane vesicles derived from canine ventricular myocardium and canine lung. [H](±)Carazolol binding is saturable, of high affinity, and in displaceable by /J-adi-e.nergic agents in accordance with their known pharmacological potencies. The interaction of carazolol with /J-adrenergic receptors is stereospeclflc; the (—) stereoisomer demonstrates greater potency than the ( + ) stereoisomer. Kinetic analysis of [H}(±)carazolol interaction with /3-adrenergic receptors suggests the presence of two phases of interaction, consistent with initial rapidly reversible "low" affinity association of ligand with receptor, followed by Isomerization to form a high affinity, slowly reversible complex. Through use of a [*H](±)carazolol binding assay based on the high affinity complex, pharmacological specificities of £-adrenergic receptor populations of canine myocardium and lung were quantified. Analysis using computer-assisted techniques suggests a fi\/fi\ receptor ratio of approximately 85%/15% for canine myocardium and 5%/95% for canine lung. In the absence of added guanine nucleotides, comparison of potencies of /J-adrenergic agonists in the two membrane systems suggests significant /S, selectivity of Msoproterenol and /-eplnephrine, and nonselectivity of norepinephrine. In the presence of saturating levels of guanine nucleotides, comparison of agonist potencies confirms the non-selectivity of /-isoproterenol and /-epinephrine, and /3i selectivity of norepinephrine. These results demonstrate that the state of guanine nucleotide regulation of receptors should be defined when examining agonist selectivities for /8-adrenergic receptor subtypes in vitro. Circ Res 49: 326-336, 1981