title: Paradoxical inotropic effects of clonidine and labetalol in the conscious rabbit.
abstract: 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 beta-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 microgram/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 alpha-adrenoceptor-agonist properties which both drugs possess and which could be unmasked by cardiac autonomic blockade.
mesh_terms: D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D003000:Clonidine / Q000494:pharmacology*; D004983:Ethanolamines / Q000494:pharmacology*; D006339:Heart Rate / Q000187:drug effects; D007741:Labetalol / Q000494:pharmacology*; D009200:Myocardial Contraction / Q000187:drug effects*; D011433:Propranolol / Q000494:pharmacology; D011817:Rabbits; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects; D011943:Receptors, Adrenergic, beta / Q000187:drug effects; D012601:Scopolamine / Q000494:pharmacology; D013268:Stimulation, Chemical
chemical_list: D004983:Ethanolamines; D011942:Receptors, Adrenergic, alpha; D011943:Receptors, Adrenergic, beta; D011433:Propranolol; D012601:Scopolamine; D003000:Clonidine; D007741:Labetalol