EFFECTS OF THE ARTERIAL VASODILATOR MINOXIDIL ON CARDIOVASCULAR STRUCTURE AND SYMPATHETIC ACTIVITY IN SPONTANEOUSLY HYPERTENSIVE RATS

Objective and design: In spontaneously hypertensive rats (SHR) arteria l vasodilators do not cause regression and might cause further progres sion of cardiac hypertrophy. To assess whether these effects extend to the vasculature, and to examine the possible mechanisms involved, car diac and mesenteric arterial structure was evaluated with respect to c hanges in cardiac volume load and cardiac and arterial sympathetic act ivity during long-term (5- and 10-week) treatment of 16-week-old SHR w ith the arterial vasodilator minoxidil, alone or in combination with t he diuretic hydrochlorothiazide. Results: Despite causing a persistent decrease in blood pressure in SHR, minoxidil further increased left a nd right ventricular weights and left ventricular internal diameter. I n combination with hydrochlorothiazide, minoxidil caused concentric, r ather than eccentric, left ventricular hypertrophy. In the mesenteric arterial bed of SHR, minoxidil increased the lumen of the superior mes enteric artery, and prevented further increases in the medial area of the large and small mesenteric arteries. The increase in lumen size of the superior mesenteric artery by minoxidil was abolished when hydroc hlorothiazide was added to the treatment. After 10 weeks' treatment wi th minoxidil, noradrenaline turnover rates were still significantly in creased in the left ventricle but were decreased in the mesenteric art eries in the SHR. Minoxidil increased plasma and blood volumes, the in creases being largely prevented by concomitant diuretic treatment. Con clusions: We conclude that there are regional differences in the respo nse of the cardiovascular system to minoxidil in SHR. Some of these di fferences may be related to differences in regional sympathetic activi ty, whereas volume load appears to play a modulatory role.