Abstract

This study demonstrates that the tachycardia produced by systemic injections of the β-adrenoceptor agonist, isoproterenol (10 μg/kg, i.v.), in conscious rats were not reduced after injection of the selective β 1-adrenoceptor antagonist, atenolol (1 mg/kg, i.v.), or after subsequent injection of the β 1,2-adrenoceptor antagonist, propranolol (1 mg/kg, i.v.). The hypotensive responses produced by isoproterenol were slightly diminished by atenolol and markedly diminished by propranolol. The tachycardia produced by catecholamines released for cardiac sympathetic nerve terminals were blocked by atenolol. These results suggest that the hypotensive actions of a 10 μg/kg dose of isoproterenol are mediated by activation of β 1,2-adrenoceptors whereas the increases in heart rate may be due to activation of another type of β-adrenoceptor in cardiac pacemaker cells.

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