?-Adrenergic receptors in guinea-pig myocardial tissue

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Abstract
Stereospecific binding sites for (−) [3H]-alprenolol, a β-adrenergic antagonist, have been identified in guinea-pig myocardial broken cell preparations. The concentration of the sites was 0.3 pmoles per mg of protein and the dissociation constant (at 37°C) 10−8 M. A close correlation between the ability of various β-adrenergic antagonists to compete with tracer alprenolol binding and to block the response of isoprenaline-stimulated myocardial adenylate cyclase has been found. Low affinity sites for the labelled β-adrenergic antagonist in contrast to stereospecific sites are heat stable and do not discriminate between the (−) and the (+) forms of the β-adrenergic antagonists. Adenylate cyclase in guineapig myocardial tissue is poorly stimulated by isoprenaline or 5′-guanylylimidodiphosphate. This is attributed to a high basal activity which could be lowered by a preincubation at 37°C.