Myocardial and haemodynamic effects of phentolamine.

1. In cats anaesthetized with pentobarbitone, intravenous infusions of phentolamine ((10-50 mug/kg)/min for 5 min) increased heart rate, left ventricular dp/dt max (without increasing end-diastolic pressure), aortic dp/dt, cardiac output, myocardial blood flow and metabolic heat production.2. Phentolamine-induced increases in myocardial contractility occurred irrespective of the direction or magnitude of the blood pressure change and were maintained well beyond the actual infusion period.3. In cats treated with alprenolol, bretylium or reserpine there was no evidence of increased cardiac contractility following phentolamine administration.4. It is concluded that phentolamine, in doses less than those required to produce significant alpha-adrenoceptor blockade, increased myocardial contractility through an effect on the sympathetic nervous system.