Muscle sympathetic activity and plasma concentrations of noradrenaline.

Microelectrode recordings of sympathetic vasoconstrictor activity in muscle nerve fascicles in man indicate that the outflow of sympathetic impulses is modulated by arterial and cardiopulmonary baroreceptors. Variations in the strength of the activity correlate closely to variations of diastolic blood pressure but there is no significant correlation between mean levels of sympathetic activity and blood pressure. In a given individual the level of activity is reproducible over many months but there are large interindividual differences. In the recumbent posture there is a linear correlation between the level of activity at rest and the plasma concentration of noradrenaline in forearm venous blood, both in normotensive subjects and in patients with essential hypertension. Preliminary data indicate that an isometric hand grip at 30% of maximal power induces approximately parallel changes of muscle sympathetic activity and plasma concentrations of noradrenaline. It is suggested that sympathetic outflow to muscles is an important determinant of the level of circulating plasma noradrenaline.