ObjectiveTo evaluate the influence of blood flow on measurements of regional sympathetic nerve activity by radiotracer methodology ([3H]noradrenaline).DesignTen healthy men were studied under two conditions of elevated forearm blood flow: mental stress (Stroop colour word conflict test) and an intra-arterial infusion of sodium nitroprusside.MethodsArterial blood pressure was measured invasively and forearm blood flow with strain-gauge plethysmography. Arterial and venous plasma adrenaline and noradrenaline were measured with high-performance liquid chromatography, and regional and total noradrenaline spillover were calculated.ResultsDuring mental stress, mean arterial pressure increased by 17%, heart rate by 16 beats/min, forearm blood flow by 117%, while forearm vascular resistance decreased by 44% (P< 0.001 for all). Sodium nitroprusside increased forearm blood flow dose-dependently, but elicited only minor effects on systemic haemodynamics. Mental stress increased arterial plasma noradrenaline by 52% (P< 0.001), and total body noradrenaline spillover by 75% (P< 0.001). During sodium nitroprusside infusion, arterial plasma noradrenaline increased only slightly and total body noradrenaline spillover was unaffected Forearm noradrenaline overflow increased from 5.4 ± 0.9 to 16.9 ± 2.6 pmol/min per l (P< 0.001) during mental stress and from 6.6 ± 0.8 to 16.9 ± 3.7 pmol/min per l (P< 0.001) during the second dose-step of sodium nitroprusside infusion. By intra-individual comparisons of forearm noradrenaline overflow increases during mental stress and during sodium nitroprusside infusion, with similar forearm blood flow increases, the flow dependence of forearm noradrenaline overflow was estimated. During mental stress, about 60% (median value, range 29–112%) of the increase in forearm noradrenaline overflow was attributed to the increase in forearm blood flow, whereas 40% was considered to reflect increased sympathetic nerve activity.ConclusionsThere seems to be a considerable flow dependence of the regional overflow of noradrenaline, that is, a component of simple wash-out of noradrenaline from the forearm tissues during vasodilation. However, the present results still indicate that sympathetic nerve activity in the forearm is increased during mental stress, justifying the radiotracer technique for semiquantitative measurements, also during vasodilation.