Vascular Reactivity in Experimental Hypertension Measured after Hexamethonium

ONE of the few observations made in hypertension upon which there is almost universal agreement is the increased response of the blood pressure to pressor drugs. It is found in nearly all types of hypertension, both experimentally and clinical.>9 Nevertheless, the mechanisms responsible for it have not been adequately investigated. I have studied the vascular reactivity in rabbits, comparing the blood pressure responses to pressor and depressor drugs in the same animal in the normotensive and hypertensive states.10' 12 The animals were studied in the conscious state and the blood pressure was recorded by a condenser manometer from the central artery of the ear. In order to overcome the homeostatic reflexes which would modify the blood pressure response, all experiments were performed after large doses of hexamethonium had been administered. From this work and from the literature there are certain characteristics of the increased vascular reactivity which provide some evidence of its real nature: 1. In both normotensive and hypertensive animals the blood pressure responses are not related to the initial level of blood pressure. 2. The increased vascular reactivity in hypertension occurs in response to a large variety of vasoconstrictor agents, epinephrine, norepinephrine, posterior pituitary extract, tyramine and renin.' 3, 10, 11 3. These effects are not, in