BETA-ADRENERGIC STIMULATION OF ENERGY-EXPENDITURE AND FOREARM SKELETAL-MUSCLE METABOLISM IN LEAN AND OBESE MEN

The effect of beta-adrenergic stimulation on whole body energy expendi ture and forearm skeletal muscle metabolism was investigated in lean a nd obese men. Whole body energy expenditure was determined during rest and during intravenous infusion of increasing doses of the nonselecti ve beta-agonist isoprenaline (Iso). Forearm skeletal muscle metabolism was investigated with Iso infusion with and without simultaneous infu sion of the beta(1)-blocker atenolol (AT) by measuring skeletal muscle blood flow (SMBF) and arteriovenous concentration differences of vari ous metabolites. The changes in SMBF were estimated from forearm total (venous occlusion plethysmography, skin (laser doppler), and fat tiss ue blood flow (Xe-133 washout). The increase in whole body energy expe nditure with Iso was similar in lean and obese subjects. With Iso, the rise in arterial or arterialized glycerol and nonesterified fatty aci ds (NEFA) was lower in obese than lean subjects, which may reflect a l ower beta-adrenergically mediated lipolysis in obesity. During infusio n of increasing doses of Iso, the respiratory exchange ratio decreased significantly in lean subjects but not in the obese subjects, which i ndicates a more pronounced increase in fat oxidation in lean subjects. This is confirmed by the data on skeletal muscle metabolism, where NE FA uptake was increased in lean subjects, whereas the obese subjects s howed a tendency toward an increased glucose uptake and a significantl y increased lactate release. With Iso plus AT (mainly beta(2)-adrenerg ic stimulation), both groups showed an increased skeletal muscle lacta te release. In conclusion, although the thermogenic response to Iso wa s similar in lean and obese subjects, the utilization of fat seems to be impaired in obesity.