Sustained increases in sympathetic outflow during prolonged lower body negative pressure in humans

The purpose of this study was to determine whether prolonged unloading of cardiopulmonary baroreceptors with lower body negative pressure (LBNP) causes constant increases in sympathetic outflow to skeletal muscles. Eight healthy subjects underwent a 20-min control period followed by 20 min of 15-mmHg LBNP. This pressure was selected because it did not cause any significant change in mean arterial blood pressure (sphygmomanometry) or heart rate, suggesting that the cardiopulmonary baroreceptors were selectively unloaded and the activity of the arterial baroreceptors was unchanged. Muscle sympathetic nerve activity in the peroneal nerve (MSNA, microneurography) increased from an average of 21.8 ± 1.7 bursts/min over the last 5 min of control to 29.0 ± 2.9 bursts/min during the 1st min of LBNP (P < 0.05 LBNP vs. control). The increase in MSNA observed during the 1st min was sustained throughout LBNP. Forelimb blood flow (plethysmography) decreased abruptly at the onset of the LBNP from a control value of 4.3 ± 0.5 ml · min^-1 · 100 ml^-1 to 2.5 ± 0.2 at the 1st min; the flow then increased and remained significantly above this value, but below the control value, throughout LBNP. Similar blood flow findings were obtained in additional studies, when the hand circulation was excluded during the flow measurement. Forearm skin blood flow (laser Doppler) also decreased abruptly at the onset of LBNP and was followed by partial recovery, but these changes were too small to account for all the increases in limb blood flow over the course of LBNP. These observations demonstrate that a prolonged decrease in the activity of cardiopulmonary baroreceptors in humans is accompanied by a sustained increase in sympathetic outflow to the skeletal muscles of the limbs. They also suggest that 'sympathetic escape' may occur in human skeletal muscles.