Muscle sympathetic activity and venous plasma noradrenaline concentrations during static exercise in normotensive and hypertensive subjects.

Peroneal muscle sympathetic activity (MSA) was recorded during 2 min of isometric handgrip at 30% of maximal power and correlated with plasma levels of noradrenaline (NA) in venous blood from the contralateral forearm in 16 normotensive subjects and 15 previously untreated patients with essential hypertension. Resting values for MSA and NA were similar in the two groups and there was a significant positive correlation between the level of MSA (expressed either as bursts per 100 heart beats or bursts min-1) and the concentration of NA. Changes of MSA and NA during handgrip were similar in both groups. When monitored every minute in a subgroup of the material (n = 12), the number of sympathetic bursts min-1 increased maximally by 23% and total MSA (bursts min-1 X mean burst amplitude) by 67% during the second minute of handgrip. The maximal increase of venous NA concentrations was 21% and occurred 2 min later, probably reflecting a slow wash-out of NA from the neuroeffector junctions. It is concluded that: there is no difference in MSA between normotensive and hypertensive subjects at rest or during isometric handgrip, MSA is an important determinant of the concentration of NA in forearm venous plasma, and the relative change of the venous plasma NA concentration during isometric handgrip is considerably smaller than the change in total MSA.