DIFFERENTIATED NOREPINEPHRINE SPILLOVER IN HUMAN SKELETAL-MUSCLE

Most neurophysiological studies have shown similar sympathetic outflow to arm and leg. However, some direct microneurographic recordings ind icate differentiated sympathetic outflow to Limbs both at rest and dur ing mental stress. Hence, differentiated levels of norepinephrine (NE) spillover could prevail. By steady-state infusion of [H-3]NE and body composition determination by dual-energy X-ray absorptiometry-scan, w e simultaneously assessed arm and leg NE spillover related to 100 g ti ssue and total limb weight. NE spillover was lower in leg than arm(0.2 6 vs. 1.51 pmol . min(-1). 100 g(-1), P < 0.05), and the difference re mained when expressed as a function of total limb weight (66 vs. 137 p mol/min, P < 0.05). Fractional extraction of [H-3]NE was similar in ar m and leg. Neuronal uptake blockade by desipramine was more effective in leg than arm; fractional extraction in leg decreased by 32% (P < 0. 05) but was unaltered in arm. Thus a lower NE spillover was observed f rom leg than arm, possibly reflecting a lower sympathetic out-Bow and a more neuronally dependent reuptake.