Increase in epinephrine-induced responsiveness during microgravity simulated by head-down bed rest in humans

The epinephrine (Epi)-induced effects on the sympathetic nervous system (SN S) and metabolic functions were studied in men before and during a decrease in SNS activity achieved through simulated microgravity. Epi was infused a t 3 graded rates (0.01, 0.02, and 0.03 mu g.kg(-1).min(-1) for 40 min each) before and on the fifth day of head-down bed rest (HDBR). The effects of E pi on the SNS (assessed by plasma norepinephrine levels and spectral analys is of systolic blood pressure and heart rate variability), on plasma levels of glycerol, nonesterified fatty acids (NEFA), glucose and insulin, and on energy expenditure were evaluated. HDBR decreased urinary norepinephrine e xcretion (28.1 +/- 4.2 vs. 51.5 +/- 9.1 mu g/24 h) and spectral variability of systolic blood pressure in the midfrequency range (16.3 +/- 1.9 vs. 24. 5 +/- 0.9 normalized units). Epi increased norepinephrine plasma levels (P < 0.01) and spectral variability of systolic blood pressure (P < 0.009) dur ing, but not before, HDBR. No modification of Epi-induced changes in heart rate and systolic and diastolic blood pressures were observed during HDBR. Epi increased plasma glucose, insulin, and NEFA levels before and during HD BR. During HDBR, the Epi-induced increase in plasma glycerol and lactate le vels was more pronounced than before HDBR (P < 0.005 and P < 0.001, respect ively). Epi-induced energy expenditure was higher during HDBR (P < 0.02). O ur data suggest that the increased effects of Epi during simulated microgra vity could be related to both the increased SNS response to Epi infusion an d/or to the beta-adrenergic receptor sensitization of end organs, particula rly in adipose tissue and skeletal muscle.