Paradoxical inotropic effects of clonidine and labetalol in the conscious rabbit

R. J. McRitchie, J. P. Chalmers

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    6 Citations (Scopus)


    Both clonidine and labetalol when given by bolus intravenous injection into conscious rabbits produce an initial rise in left ventricular pressure associated with a decrease in myocardial contractility as assessed by left ventricular dP/dt. While clonidine also produces a marked bradycardiac effect, labetalol causes no change in heart rate. Acute β-adrenoceptor blockade with propranolol, 1 mg/kg, i.v., does not significantly modify the response produced by either clonidine or labetalol. Following cardiac cholinergic blockade with scopolamine methylbromide, 50 μg/kg, the negative inotropic effect produced by clonidine is abolished and is replaced by a positive inotropic effect, while the negative inotropic response produced by labetalol is attenuated. Following cardiac autonomic blockade with propranolol and scopolamine methyl-bromide, both clonidine and labetalol produce a positive inotropic response. These positive inotropic responses may be due to the partial α-adrenoceptor-agonist properties which both drugs possess and which could be unmasked by cardiac autonomic blockade.

    Original languageEnglish
    Pages (from-to)818-827
    Number of pages10
    JournalJournal of Cardiovascular Pharmacology
    Issue number4
    Publication statusPublished - Jul 1981


    • Cardiac autonomic blockade
    • Clonidine
    • Inotropic effect
    • Labetalol


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