Importance of cardiac and vascular beta-receptors in the action of phentolamine.

To determine to what extent the effects of phentolamine (PHE) are mediated by the cardiac and vascular beta-receptors, the action of the drug was studied in anaesthetized cats pretreated with beta-blockers. Blockade of the cardiac beta-receptors with either propranolol or metoprolol entirely prevented the increases in heart rate, dP/dt and cardiac output caused by PHE. In animals pretreated with propranolol (in a dose blocking the vascular beta-receptors) or metoprolol (in a purely cardio-selective dose) PHE significantly lowered blood pressure to the same extent as in untreated controls. The effects of PHE on the heart are thus conditional on the integrity of cardiac beta-receptors, whereas it seems improbable that vascular beta-receptors have any part in its vasodilative action. Evidence presented in the literature rather suggests a predominantly central mode of action.