Changes in sympathetic/parasympathic balance in response to short-term simulated weightlessness

Nine young healthy men (171.2 ± 4.4 cm, 77.4 ± 3.9 kg, 23.1 ± 18 yr) were studied before, during, and at the end of 48 hours of 6° head down bed rest (HDBR). Arterial pressure (Finapres), heart rate (BoMed), stroke volume (BoMed), cardiac output (BoMed), radial artery flow (Parks), skin perfusion of palm and calf (Perimed) and peripheral vascular resistance (calculated) were acquired noninvasivery for 20 min rest and lower body negative pressure (LBNP) sessions. Periodic blood samples were assayed for venous levels of norepinephrine (NE), epinephrine (EP), pancreatic polypeptide and plasma renin activity (PRA). In addition micro-hematocrit (HCT) determinations were used to predict shifts of plasma volume. A two factor analysis of variance was used to assess responses to -50 mmHg LBNP, 6° HDBR, and interactions between LBNP and HDBR. Both HDBR and LBNP evoked significant increases in NE, EP, PRA, and HCT (p < .05). In addition, LBNP -induced increases in NE and PRA were greater after HDBR than before while the rate of plasma filtration during LBNP remained the same after HDBR. These data suggest that head down bed rest evoked a decrease in plasma volume accompanied by an increase in sympathetic activity.