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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 · min1
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 
-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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