Comparison of Sympathetic Nerve Activity in Normotensive and Hypertensive Subjects

A microneurographic technique was used to record multiunit sympathetic activity in skin and muscle nerves of 24 healthy subjects and 21 hypertensive subjects. In both groups, the sympathetic activity recorded during rest appeared in bursts following one of two highly different temporal patterns--one characteristic for muscle nerves and the other characteristic for skin nerves. Muscle nerve sympathetic activity probably consisting of vasoconstrictor impulses, occurred in bursts that followed the pulse rhythm and waxed and waned in inverse relation to spontaneous blood pressure fluctuations. Transient random elevations of blood pressure above a certain level caused total suppression of the sympathetic bursts. This inhibitory blood pressure level was higher in hypertensive subjects than it was in normotensive subjects, suggesting an elevated baroreflex working range in hypertension. No other important differences in muscle nerve sympathetic activity were noted between the two groups. Skin nerve sympathetic activity, probably consisting of both vasoconstrictor and sudomotor impulses, had a similar temporal pattern in normotensive and hypertensive subjects at rest: bursts of variable duration occurred randomly or were loosely related to the respiratory rhythm. In both groups of subjects, the strength of this activity increased in response to arousal stimuli, mental stress, and body cooling. These findings emphasize the necessity of having highly standardized experimental conditions in future studies aimed at a quantitative comparison of the absolute strength of sympathetic activity in normotensive and hypertensive subjects.