1. Atrial natriuretic factor has been suggested to affect human sympathetic nervous system activity. The interaction between atrial natriuretic factor and the sympathetic nervous system has not been fully elucidated yet, but may occur at different sites. We studied this modulator effect at the level of the forearm vascular bed: the forearm vasoconstrictor response was examined after α-adrenergic sympathetic stimulation in healthy subjects during the loco-regional administration of atrial natriuretic factor, sodium nitroprusside and placebo. As a sympathetic stimulation test, the technique of the lower body negative pressure (–20 mmHg) was used.

2. Lower body negative pressure increased the forearm vascular resistance by +37 ± 8% during concomitant intra-arterial infusion of placebo (n = 10). During a predilator state achieved by infusion of atrial natriuretic factor (10 ng min−1 100 ml−1 forearm volume) into the brachial artery, lower body negative pressure subsequently induced a forearm vasoconstrictor response of +153 ± 22% (P < 0.05 versus placebo), whereas this was +64 ± 14% when predilatation was achieved by infusion of an equipotent vasodilator dose of sodium nitroprusside (P > 0.1 versus placebo; P < 0.05 versus atrial natriuretic factor). The potentiation of the forearm vasoconstrictor response to lower body negative pressure by atrial natriuretic factor only occurred in the experimental and not in the contralateral arm. According to calculations on simultaneously sampled arterial and venous plasma catecholamine concentrations, the augmented forearm vasoconstrictor response seemed not to be caused by an increased release of noradrenaline.

3. In a second experiment (n = 10), we demonstrated that atrial natriuretic factor did not alter the forearm vasoconstrictor response to the intra-arterial administration of exogenous noradrenaline, thereby excluding a postsynaptic modulation by atrial natriuretic factor. In the presence of atrial natriuretic factor, the subsequent infusion of two dosages of noradrenaline (0.1 and 1.0 ng min−1 100 ml−1 forearm volume) increased the forearm vascular resistance by +118 ± 29% and +387 ± 79%, respectively, whereas these numbered +129 ± 18% and + 316 ± 41% when atrial natriuretic factor was replaced by an equipotent dose of sodium nitroprusside.

4. We conclude that atrial natriuretic factor potentiates the forearm vasoconstrictor response to α-adrenergic sympathetic stimulation in man, and that this effect may be located at the level of the forearm vascular bed. This potentiation cannot be explained by an increased vascular α-adrenergic sensitivity. More detailed studies are needed to analyse the effects of atrial natriuretic factor on local noradrenaline kinetics to elucidate influences on sympathetic neurotransmission.

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