EFFECTS OF SYMPATHETIC AND CENTRAL NERYOUS SYSTEM ALTERATIONS ON THE BLOOD PRESSURE RESPONSES TO PHENTOLAMINE

¹ Department of Pharmacology, Medical University of South Carolina, Charleston, South Carolina

An injection of phentolamine (5 mg/kg) in 13 control animals resulted in increases in myocardial contractile force and heart rate with a simultaneous decrease in aortic blood pressure. If the sympathetic efferent system is blocked by reserpine, ganglionic blockade, spinal anesthesia or C₁ spinal section, phentolamine elicits an augmentation of aortic blood pressure. This is not due to a positive inotropic effect since heart force actually decreases. The pressor effect is not mediated by alpha receptors since prior blockade does not abolish the pressor effect. If the action of phentolamine is limited to the circulation of the head, a peripheral depressor response is observed. The data indicate that phentolamine appears to produce direct vasoconstriction in the absence of sympathetic tone. Phentolamine also appears to stimulate central hypotensive centers which in conjunction with intrinsic alpha blockade produces precipitous decreases in aortic blood pressure. The combination of these last two effects masks the direct vasoconstriction in normal animals.

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