Patients with essential hypertension show impaired endothelium-dependent vasodilation induced by acetylcholine. Because dietary potassium supplementation increases endothelium-dependent relaxations to acetylcholine in hypertensive rats, we designed the present study to investigate whether potassium increases endothelium-dependent vasodilation in essential hypertensive patients. Therefore, in patients with essential hypertension (n = 13) and in normotensive control subjects (n = 13) we evaluated the effect of intrabrachial potassium chloride (0.2 mmol/min) on forearm blood flow (strain-gauge plethysmography) modifications induced by intrabrachial acetylcholine (0.15, 0.45, 1.5, 4.5, and 15 micrograms/100 mL forearm tissue per minute). In both groups of patients, potassium chloride infusion augmented local plasma potassium concentrations. Furthermore, in essential hypertensive patients but not in normotensive subjects it increased the vasodilating effect of the first three infusion rates of acetylcholine. In contrast, in seven adjunctive essential hypertensive patients, potassium chloride did not alter intrabrachial sodium nitroprusside-induced forearm vasodilation (1, 2, and 4 micrograms/100 mL forearm tissue per minute). Finally, to evaluate the role of nitric oxide on potassium-dependent facilitation of acetylcholine-induced vasodilation in essential hypertension, we studied the effect of intrabrachial NG-monomethyl L-arginine (100 micrograms/100 mL per minute) in another group of seven hypertensive patients. Vasodilation to acetylcholine was again increased by potassium chloride; NG-monomethyl L-arginine slightly blunted the vasorelaxing effect of acetylcholine but abolished the potentiating effect of potassium.(ABSTRACT TRUNCATED AT 250 WORDS)