Journal of Applied Physiology
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J Appl Physiol 87: 1614-1620, 1999;
8750-7587/99 $5.00
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Vol. 87, Issue 5, 1614-1620, November 1999

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

P. Barbe, J. Galitzky, C. Thalamas, D. Langin, M. Lafontan, J. M. Senard, and M. Berlan

Laboratoire de Pharmacologie Médicale et Clinique, Institut National de la Santé et de la Recherche Médicale U-317, 31073 Toulouse Cedex; and Centre d'Investigation Clinique, Hopital Purpan, 31059 Toulouse Cedex, France

The epinephrine (Epi)-induced effects on the sympathetic nervous system (SNS) and metabolic functions were studied in men before and during a decrease in SNS activity achieved through simulated microgravity. Epi was infused at 3 graded rates (0.01, 0.02, and 0.03 µg · kg-1 · min-1 for 40 min each) before and on the fifth day of head-down bed rest (HDBR). The effects of Epi on the SNS (assessed by plasma norepinephrine levels and spectral analysis 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 excretion (28.1 ± 4.2 vs. 51.5 ± 9.1 µg/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) during, 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 HDBR. During HDBR, the Epi-induced increase in plasma glycerol and lactate levels was more pronounced than before HDBR (P < 0.005 and P < 0.001, respectively). Epi-induced energy expenditure was higher during HDBR (P < 0.02). Our data suggest that the increased effects of Epi during simulated microgravity could be related to both the increased SNS response to Epi infusion and/or to the beta -adrenergic receptor sensitization of end organs, particularly in adipose tissue and skeletal muscle.

adrenergic sensitivity; norepinephrine; lipid mobilization; energy expenditure; sympathoinhibition


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