Circulatory and alpha‐adrenoceptor blocking effects of phentolamine.

1 Intravenously administered phentolamine provoked immediate decreases in diastolic blood pressure but increases in heart rate and cardiac output. 2 These immediate circulatory effects had largely disappeared twenty minutes after administration and at this time phentolamine did not inhibit increases in blood pressure which were provoked during hand immersion in ice-cold water. 3 Log dose-response curves of noradrenaline induced increases in systolic and diastolic pressure 20 min after intravenous phentolamine were shifted to the right in a parallel manner compared with the curves before phentolamine administration. 4 It was concluded that the immediate and short acting effects induced by phentolamine are due to a non-specific vasodilator effect but in addition phentolamine causes a longer acting alpha-adrenoceptor blockade at vascular adrenoceptor sites. However, by producing both pre- and post-synaptic alpha-adrenoceptor blockade this may explain why this drug exerts only a weak antihypertensive effect.