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We noninvasively investigated the effects of a single 30-min i.v. infusion of a 2-mg dose of niravoline, a new selective kappa-opioid agonist, on systemic and regional (brachial artery) hemodynamics, on plasma levels of the main hormones regulating the cardiovascular system, on diuresis and on plasma and urinary osmolalities and electrolytes. This was a placebo-controlled, randomized, double-blind, crossover study performed in 12 healthy volunteers. Compared with placebo, niravoline induced a significant, early and potent diuresis, which peaked within 2 hr (urine output increased 2.4-fold) and lasted for 4 hr. Niravoline significantly decreased, between 0 and 2 hr, urine osmolality (-71%) and sodium (-38%) and potassium (-29%) excretion and significantly increased plasma osmolality and natremia, without changing kalemia. Niravoline induced a slight, but significant, increase in blood pressure (+8% at 0.5 hr), which disappeared within 2 hr. Because heart rate, stroke volume and cardiac output were not modified, this effect was due to an increase in total peripheral resistance (+22% at 0.5 hr). Niravoline did not modify brachial artery diameter and flow and corresponding vascular resistance. Niravoline tended to decrease plasma vasopressin levels and urinary excretion and significantly increased plasma levels of norepinephrine (+44% at 0.5 hr), active renin (+22% at 1.25 hr), aldosterone (+52% at 1.25 hr) and atrial natriuretic factor (+20% at 2 hr). We conclude that niravoline induces a potent aquaretic effect associated with antinatriuresis and antikaliuresis. These main effects are accompanied by a stimulation of the sympathetic and reninangiotensin systems and a slight and transient increase in blood pressure.