Functional versus structural changes of forearm vascular resistance in hypertension

Structural changes in resistance vessels have been considered an important factor in triggering and maintaining chronic hypertension in humans and in experimental animals. To determine whether the increased forearm vascular resistance observed following vasodilator maneuvers in hypertensive patients is predominantly due to structural or to functional changes, we examined the influence of different vasodilator stimuli on forearm blood flow and blood pressure in 22 male patients with established essential hypertension and in 22 age-matched normotensive men (age range, 28-52 years). Blood pressure was measured directly, and blood flow was measured by venous occlusion plethysmography. The maneuvers applied were 1) arterial occlusion combined with handgrip exercise and local heating, 2) intra-arterial infusion of the calcium entry blocker nifedipine, 3) intra-arterial infusion of the nonspecific vasodilator sodium nitroprusside, 4) arterial occlusion initiated after intra-arterial infusion of nifedipine. Vascular resistance during vasodilation induced by arterial occlusion or infusion of nifedipine or sodium nitroprusside remained significantly higher in the hypertensive than in the normotensive subjects. However, the maximal vasodilation achieved by the combination of arterial occlusion and nifedipine resulted in a similar resistance in both groups (1.6 +/- 0.2 in the hypertensive vs 1.4 +/- 0.2 mm Hg/ml/min/100 ml tissue in the normotensive subjects. These data suggest that there is an important functional component of the elevated resistance in patients with essential hypertension.