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

Skeletal muscle and adipose tissue lipolysis rates were quantitatively compared in 12 healthy nonobese and 14 insulin-resistant obese subjects for 3.5 h after an oral glucose load using microdialysis measurements of interstitial glycerol concentrations and determinations of local blood flow with Xe-133 clearance in the gastrocnemius muscle and in abdominal subcutaneous adipose tissue. Together with measurements of arterialized venous plasma glycerol, the absolute rates of glycerol mobilization were estimated. In the basal state, skeletal muscle and adipose tissue glycerol levels were 50% higher (P < 0.05-0.01) and adipose tissue blood flow (ATBF) and muscle blood flow (MBF) rates were 30-40% lower (P < 0.02-0.05) in obese versus nonobese subjects. After glucose ingestion, adipose tissue glycerol levels were rapidly and transiently reduced, whereas in muscle, a progressive and less pronounced fall in glycerol levels was evident. MBF remained unchanged in both study groups, whereas ATBF increased more markedly (P < 0.01) in the nonobese versus obese subjects after the oral glucose load. The fasting rates of glycerol release per unit of tissue weight from skeletal muscle were between 20 and 25% of that from adipose tissue in both groups. After glucose ingestion, the rates of glycerol release from skeletal muscle and from adipose tissue were almost identical in nonobese and obese subjects, However, the kinetic patterns differed markedly between tissues; in adipose tissue, the rate of glycerol mobilization was suppressed by 25-30% (P < 0.05) after glucose ingestion, whereas no significant reduction was registered in skeletal muscle. We conclude that significant amounts of glycerol are released from skeletal muscle, which suggests that muscle lipolysis provides an important endogenous energy source in humans. In response to glucose ingestion, the regulation of skeletal muscle glycerol release differs from that in adipose tissue; 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 nonobese and obese subjects in both the postabsorptive state and after glucose ingestion.