FORCE-DEPENDENT AND FORCE-INDEPENDENT HEAT-PRODUCTION IN SINGLE SLOW-TWITCH AND FAST-TWITCH MUSCLE-FIBERS FROM XENOPUS-LAEVIS

1. The origin of labile heat production, i.e. a heat component which r apidly decays after the onset of stimulation, and of stable (maintenan ce) heat production was investigated in intact single fast-twitch (typ e 1) and slow-twitch (type 3) iliofibularis muscle fibres from Xenopus laevis, at 20 degrees C, by varying stimulation frequency and by vary ing sarcomere length and the concentration of 2,3-butanedione 2-monoxi me (BDM) added. 2. The labile heat produced consisted of a force-indep endent and a force-dependent part. The average parvalbumin (PA) conten t found in type 1 fibre bundles (0.84 +/- 0.08 mM; mean +/- S.E.M.; n = 5) and in type 3 fibre bundles (0.12 +/- 0.02 mM; n = 5) indicates t hat the force-independent labile heat is explained by Ca2+-Mg2+ exchan ge on PA, and amounts to a molar enthalpy change of -78 kJ (mol PA)(-1 ). 3. Force-dependent labile heat during fused contractions was simila r to the calculated heat production resulting from the formation of fo rce-generating cross-bridges, assuming an enthalpy change associated w ith cross-bridge formation of -30 kJ mol(-1). 4. Activation heat, i.e. the part of the total stable heat that is not related to the contract ile apparatus, and of which the calcium sequestration by the sarcoplas mic reticulum is the most important contributor, determined by varying sarcomere length or BDM concentration, was identical. For fused contr actions the fraction activation heat of the stable maintenance rate of heat production was 34 +/- 4% (mean +/- S.E.M.; n = 13) in type 1 fib res, and 52 +/- 4% (n = 15) in type 3 fibres. In unfused contractions this was 48 +/- 5% (n = 13) in type 1 fibres, and 35 +/- 2% (n = 11) i n type 3 fibres. 5. From the force-dependent stable rate of heat produ ction the economy of cross-bridge cycling, expressed as the force-time integral for a single myosin head per ATP molecule hydrolysed, was ca lculated. It follows that cross-bridge interaction in type 3 fibres is more economical than in type 1 fibres, and that fused contractions ar e more economical than unfused contractions.