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 alpha-2 agonist clonidine caused dose-dependent biphasic response with increased glycerol levels at low clonidine concentrations and decreased glycerol levels at concentrations > 10(-7) mol/liter. Similar results were observed with epinephrine plus propranolol. Clonidine action was unaffected in the presence of labetalol (beta-/alpha-1 antagonist) but completely blunted by the presence of yohimbine (alpha-2 antagonist). The pseudolipolytic effect of clonidine was significantly more pronounced in gluteal as compared with abdominal adipose tissue. When clonidine was added together with the vasodilating agents nitroprusside or hydralazine, the pseudolipolytic effect was abolished and a dose-dependent decrease in dialysate glycerol was observed at all clonidine concentrations (10(-10)-10(-4) mol/liter). When ethanol was added to the perfusate to monitor blood 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 an 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 combination with antilipoiysis.