SIMULATED MICROGRAVITY INCREASES BETA-ADRENERGIC LIPOLYSIS IN HUMAN ADIPOSE-TISSUE

The effect of a sustained decrease in sympathetic nervous activity, ac hieved through 5-day head-down bed rest (HDBR), on the beta-adrenergic Lipolytic activity of sc adipose tissue was studied in eight healthy men. The in situ beta-adrenoceptor (AR) sensitivity was studied using the microdialysis method. Local perfusion of increasing concentrations of isoprenaline showed an increased beta-AR sensitivity to lipolysis (assessed by extracellular glycerol concentration) and to vascular ton e (assessed by the ethanol clearance). The adrenergic sensitivity of i solated adipocytes was studied in vitro. Basal lipolysis and the respo nse to nonselective (isoprenaline) or selective (dobutamine, terbutali ne, and CGP 12177) beta-AR agonists were increased after HDBR as was t he lipolytic effect of dibutyryl cAMP. When data were expressed as a p ercentage of the dibutyryl cAMP effect to rule out the postreceptor ev ents, basal and lipolytic responses to beta-AR agonists where similar before and during HDBR. The alpha(2)-AR-mediated antilipolytic effects of adrenaline were not modified. Lymphocyte beta-AR number was unchan ged during HDBR. Our results demonstrate that a sustained sympathoinki bition induces an increase in the lipolytic beta-adrenergic response i n adipose tissue and suggest that this hypersensitization is linked to an increase in the postreceptor steps of the lipolytic cascade in the adipocyte rather than to changes in beta-adrenoceptors.