ROLE OF VASCULAR ALPHA-2-ADRENOCEPTORS IN REGULATING LIPID MOBILIZATION FROM HUMAN ADIPOSE-TISSUE

The role of alpha-2 adrenoceptors in lipid mobilization and blood flow was investigated in situ using microdialysis of subcutaneous adipose tissue in nonobese healthy subjects. The alpha2 agonist clonidine caus ed dose-dependent biphasic response with increased glycerol levels at low clonidine concentrations and decreased glycerol levels at concentr ations > 10(-7) mol / liter. Similar results were observed with epinep hrine plus propranolol. Clonidine action was unaffected in the presenc e of labetalol (beta- / alpha-1 antagonist) but completely blunted by the presence of yohimbine (alpha-2 antagonist). The pseudolipolytic ef fect of clonidine was significantly more pronounced in gluteal as comp ared with abdominal adipose tissue. When clonidine was added together with the vasodilating agents nitroprusside or hydralazine, the pseudol ipolytic effect was abolished and a dose-dependent decrease in dialysa te glycerol was observed at all clonidine concentrations (10(-10)-10(- 4) Mol/liter). When ethanol was added to the perfusate to monitor bloo d flow, the escape of alcohol from the dialysate was accelerated by 30 % with hydralazine or nitroprusside (P < 0.01) and 30% retarded (P < 0 .05) by clonidine (10(-10) mol/liter). Thus, the results demonstrate a n important role of blood flow for regulating lipid mobilization from adipose tissue in vivo. Alpha-2 adrenoceptor activation causes marked retention of lipids in adipose tissue due to vasoconstriction in combi nation with anti-lipolysis.