Selectivity of dobutamine for adrenergic receptor subtypes: in vitro analysis by radioligand binding.
The cardiovascular responses elicited by dobutamine are distinctly different from those produced by other adrenergic or dopaminergic agonists. To test the hypothesis that dobutamine could have differential affinities for adrenergic receptor subtypes, and that such subtype selectivity could be related to its relatively unique pharmacologic properties, we assessed the ability of dobutamine to displace adrenergic radioligands from membrane receptors in a number of tissues of previously characterized adrenergic receptor subtype. For beta adrenergic receptors identified by (-) [(3)H]dihydroalprenolol (DHA), dobutamine had significantly greater affinity for the beta(1) subtype (K(D) = 2.5 muM in rat heart and 2.6 muM in turkey erythrocyte) than for the beta(2) subtype (K(D) = 14.8 muM in frog heart and 25.4 muM in rat lung) (P < 0.001). For alpha adrenergic receptors, dobutamine had markedly greater affinity for the alpha(1)-subtype identified by [(3)H]prazosin (K(D) = 0.09 muM in rat heart and 0.14 muM in rabbit uterus) than for the alpha(2)-subtype identified by [(3)H]dihydroergocryptine (DHE) (K(D) = 9.3 muM in human platelet) or by [(3)H]yohimbine (K(D) = 5.7 muM in rabbit uterus) (P < 0.001). Like other beta(1)-agonists, in the absence of guanine nucleotide, dobutamine competition curves for DHA binding in rat heart demonstrated two classes of binding sites, with one site of significantly higher affinity (K(D) = 0.5 muM, P = 0.008) than the single class of binding sites (K(D) = 5.2 muM) identified in the presence of guanine nucleotide. However, unlike beta(2)- or alpha(2)-agonists, dobutamine displacement of DHA binding in rat lung or of DHE binding in human platelets demonstrated only a single class of binding sites, and guanine nucleotide had only minimal effects. We conclude that dobutamine is selective for beta(1) as opposed to beta(2), and for alpha(1) as opposed to alpha(2) adrenergic receptors. Furthermore, guanine nucleotide effects on dobutamine binding, and biochemical response data in vitro suggest that dobutamine is a beta(1)-agonist, but has little intrinsic activity at beta(2) and alpha(2)-receptors. This selectivity for adrenergic receptor subtypes may be part of the basis for dobutamine's distinctive pharmacologic properties in vivo.
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