There is controversy over whether endothelial function is impaired in human essential hypertension. All studies to date have used measurements of forearm blood flow by plethysmography to assess endothelium-dependent vasodilation and endothelial function. In contrast to these studies, which have focused on resistance vessels, we have determined what effects the endothelium has on underlying smooth muscle cells in conduit arteries by measuring arterial compliance of the radial arteries (change in diameter of radial artery over pressure for each arterial pulse). In 13 normotensive healthy subjects and 11 young patients with essential hypertension, arterial compliance of the radial artery was assessed directly with a new high-precision ultrasonic device (NIUS 02) after infusion of acetylcholine (endothelium-dependent response) or sodium nitroprusside (endothelium-independent response). Arterial compliance of the radial artery was similar at baseline and with increasing doses of acetylcholine and sodium nitroprusside in normotensive and in hypertensive subjects. The increase in arterial compliance from baseline at each individual concentration of acetylcholine and sodium nitroprusside was the same in both normotensive and hypertensive subjects. However, after a single oral dose of a combination of the angiotensin converting enzyme inhibitor spirapril and the calcium entry blocker isradipine, the increase in arterial compliance in response to the maximum dose of intraarterial acetylcholine was enhanced in normotensives (0.38 +/- 1.23 to 0.76 +/- 1.01 mm2/mm Hg x 10(-3), P < .05), but not in hypertensives (+0.41 +/- 1.26 to 0.36 +/- 1.31 mm2/mm Hg x 10(-3), not significant), and differed significantly between normotensive and hypertensive subjects (P < .05). Thus, pharmacologic stimulation disclosed a blunted response of endothelium-dependent action in the arterial compliance of the conduit arteries in hypertensive subjects. This suggests an impaired endothelial function reserve in persons with essential hypertension.