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Labetalol binding to specific alpha adrenergic sites and beta adrenergic sites in vitro and its antagonism of adrenergic responses in vivo



Labetalol binding to specific alpha adrenergic sites and beta adrenergic sites in vitro and its antagonism of adrenergic responses in vivo



Journal of Molecular & Cellular Cardiology 11(8): 803-812



Labetalol competed with [3H]dihydroergocryptine for specific .alpha.-adrenergic binding sites in rabbit uterine membrane preparations. Based on IC50 [median inhibitory concentration] values from binding-competition curves, labetalol had 536 times lower affinity for .alpha. receptors than phentolamine. Labetalol competed with [3H]dihydroalprenolol for specific .beta.-adrenergic binding sites in guinea pig heart and lung membranes. The drug had 53 times lower affinity for the heart .beta. receptors than propranolol, and was significantly less potent than propranolol in inhibiting stimulation of cardiac adenylate cyclase by 10-5 M isoproterenol. Labetalol had 19 times greater binding affinity for .beta. receptors in heart membranes than .alpha.-receptors in uterine membranes. In vivo studies in anesthetized dogs indicate that the blockade of .beta. adrenergic responses predominates at a lower dose of labetalol and the blockade of .alpha. responses was observed when the dose was increased 10-fold. Labetalol probably antagonizes both .alpha. and .beta. adrenoceptors, and labetalol is apparently a more potent antagonist at .beta. than .alpha. adrenergic sites.

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Accession: 005787143

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