1. Different microcalorimetric techniques have been compared for the assessment in vitro of the total metabolic activity of resting skeletal muscle. Human fibre bundles were suspended in Krebs-Ringer-phosphate buffer containing glucose and insulin and the heat evolution was continuously monitored for 2–6 h. Palmitate as substrate was also tested.
2. The power signals declined rapidly when a static calorimetric method (A) was used. Two different perfusion methods (B, C) gave higher power values. Long-lasting steady states were observed with method C, where the sample was contained in a cage acting as a stirrer.
3. Significant differences were found for fibre bundles from different human muscles as well as age- and sex-related differences. The heat production in samples from the rectus abdominis muscle (method B), 0.73 mW/g muscle wet wt., was significantly higher than for the obliquus internus muscle, 0.44 mW/g, and the vastus lateralis muscle, 0.55 mW/g, but not different from the heat production value of vastus medialis, 0.66 mW/g.
4. In method C particularly, the fibre bundles are believed to be in adequate contact with the surrounding medium. With the use of a multichannel calorimeter it is possible to perform up to four experiments simultaneously, e.g. involving the calorigenic effects of pharmacological substances. The technique provides a new approach for detailed studies of muscle metabolism in physiological and pathological conditions.