Rates of skeletal muscle and adipose tissue glycerol release in nonobese and obese subjects

Skeletal muscle and adipose tissue lipolysis rates were quantitatively comp ared in 12 healthy nonobese and 14 insulin-resistant obese subjects for 3.5 h after an oral glucose load using microdialysis measurements of interstit ial glycerol concentrations and determinations of local blood flow with Xe- 133 clearance in the gastrocnemius muscle and in abdominal subcutaneous adi pose tissue. Together with measurements of arterialized venous plasma glyce rol, the absolute rates of glycerol mobilization were estimated. In the bas al state, skeletal muscle and adipose tissue glycerol levels were 50% highe r (P < 0.05-0.01) and adipose tissue blood flow (ATBF) and muscle blood flo w (MBF) rates were 30-40% lower (P < 0.02-0.05) in obese versus nonobese su bjects. After glucose ingestion, adipose tissue glycerol levels were rapidl y and transiently reduced, whereas in muscle, a progressive and less pronou nced fall in glycerol levels was evident. MBF remained unchanged in both st udy groups, whereas ATBF increased more markedly (P < 0.01) in the nonobese versus obese subjects after the oral glucose load. The fasting rates of gl ycerol release per unit of tissue weight from skeletal muscle were between 20 and 25% of that from adipose tissue in both groups. After glucose ingest ion, the rates of glycerol release from skeletal muscle and from adipose ti ssue were almost identical in nonobese and obese subjects, However, the kin etic patterns differed markedly between tissues; in adipose tissue, the rat e of glycerol mobilization was suppressed by 25-30% (P < 0.05) after glucos e ingestion, whereas no significant reduction was registered in skeletal mu scle. We conclude that significant amounts of glycerol are released from sk eletal muscle, which suggests that muscle lipolysis provides an important e ndogenous energy source in humans. In response to glucose ingestion, the re gulation of skeletal muscle glycerol release differs from that in adipose t issue; although the rate of glycerol release from adipose tissue is clearly suppressed, the rate of glycerol mobilization from skeletal muscle remains unaltered. In quantitative terms, the rate of glycerol release per unit of tissue weight in adipose tissue and in skeletal muscle is similar in nonob ese and obese subjects in both the postabsorptive state and after glucose i ngestion.