THE ENERGETIC COST OF ACTIVATION OF WHITE MUSCLE-FIBERS FROM THE DOGFISH SCYLIORHINUS-CANICULA

The energetic cost of activation was measured during an isometric teta nus of white muscle fibres from the dogfish Scyliorhinus canicula. The total heat production by the fibres was taken as a measure of the tot al energetic cost. This energy consists of two parts. One is due to cr ossbridge interaction which produces isometric force, and this part va ries linearly with the degree of filament overlap in the fibres. The o ther part of the energy is that associated with activation of the cros sbridges by Ca2+, mainly with uptake of Ca2+ into the sarcoplasmic ret iculum by the ATP-driven Ca2+ pump. Total heat production was measured at various degrees of filament overlap beyond the optimum for force d evelopment. Extrapolation of heat versus force production data to eval uate the heat remaining at zero force gave a value of 34+/-5% (mean +/ - S.E.M., N=24) for activation heat as a percentage of total heat prod uction in a 2.0 s isometric tetanus. Values for 0.4 and 1.0 s of stimu lation were similar. Comparison with values in the literature shows th at the energetic cost of activation in dogfish muscle is very similar to that of frog skeletal muscle and it cannot explain the lower maximu m efficiency of dogfish muscle compared with frog muscle. The proporti on of energy for activation (Ca2+ turnover) is similar to that expecte d from a simple model in which Ca2+ turnover was varied to minimize th e total energy cost for a contraction plus relaxation cycle.