Indirect evidence for release of endothelium-derived relaxing factor in human forearm circulation in vivo. Blunted response in essential hypertension.

TLDR

Acetylcholine is known to release endothelium‑derived relaxing factor (EDRF) in isolated vessels, and in vivo its vasodilatory effect may involve EDRF, prostacyclin, or prejunctional adrenergic inhibition. The study aimed to determine whether acetylcholine releases EDRF in human forearm circulation in vivo and whether this response is altered in essential hypertension. Acetylcholine was infused into the brachial artery while forearm blood flow was measured by venous occlusion plethysmography, and the roles of prostacyclin and adrenergic pathways were tested with intravenous acetylsalicylic acid and phentolamine. Acetylcholine increased forearm blood flow and reduced vascular resistance in controls, effects unchanged by aspirin or phentolamine, but the resistance reduction was blunted in hypertensive patients, indicating EDRF‑mediated vasodilation that is impaired in essential hypertension.

Abstract

In isolated blood vessels, acetylcholine releases endothelium-derived relaxing factor (EDRF). In vivo, the vasodilator action of acetylcholine may be mediated by EDRF, but prostacyclin or prejunctional inhibition of adrenergic neurotransmission may also be involved. Therefore, we investigated whether acetylcholine releases EDRF in humans in vivo and, if so, whether the response altered in essential hypertension. Acetylcholine was infused into the brachial artery, and forearm blood flow measured by venous occlusion plethysmography. In control subjects, acetylcholine (0.02-16 micrograms/min/100 ml tissue) increased flow from 2.4 +/- 5.0 to 20.6 +/- 5.2 ml/min/100 ml tissue (n = 14; p less than 0.05) and decreased forearm vascular resistance from 42.0 +/- 4.1 to 6.0 +/- 1.4 units (p less than 0.03), a response comparable to that of sodium nitroprusside (0.6 micrograms/min ml tissue). Acetylsalicylic acid (500 mg i.v.) given to block vascular prostacyclin production did not alter the response (n = 14). alpha-Adrenoceptor blockade by phentolamine (12 micrograms/min/100 ml tissue) did not prevent the increase in flow evoked by acetylcholine. In hypertensive patients, the decrease in forearm vascular resistance induced by acetylcholine but not evoked by sodium nitroprusside was reduced as compared with controls (14.5 +/- 3.1 and 6.1 +/- 1.6 units, respectively; n = 8; p less than 0.05). Thus, the vascular effects of acetylcholine in the human forearm circulation are independent of prostaglandins and adrenergic neurotransmission and therefore are most likely to be mediated by EDRF; the acetylcholine-induced release of EDRF is blunted in patients with essential hypertension.