Effect of head-down tilt on basal plasma norepinephrine and renin activity in humans

The effects of loading cardiopulmonary baroreceptors on basal norepinephrine and renin activity were studied in six normal human subjects. Loading of cardiopulmonary baroreceptors was accomplished by a 60-min 30° head-down tilt with small supplemental saline infusions. Central venous pressure was measured continuously by intrathoracic catheter; arterial pressure was measured indirectly by cuff. During the tilt, central venous pressure increased from 5.1 ± 1.3 to 8.9 ± 1.7 mmHg (P < 0.001), whereas arterial pressure was unchanged. Plasma norepinephrine (185 ± 85 pg/ml) and plasma renin activity (3.9 ± 5.7 ng·ml^-1·h^-1) did not change. Moderate sustained loading of cardiopulmonary baroreceptors is therefore without effect on unstressed plasma norepinephrine and renin activity in normal humans, suggesting that the tonic inhibitory effects of these receptors on these neurohumoral control systems are not readily increased in the basal state.