A comparison of the energy balance in two successive isometric tetani of frog muscle

Abstract

Measurements were made of the energy produced as heat and work (h + w) and the chemical changes which occurred between the beginning and end of each of 2 periods of stimulation [in Rana temporaria muscles]. The muscles contracted tetanically under isometric conditions. Each period of stimulation (tetanus) lasted 5 s and there was an interval of 3 s between them. The tension developed in the 2nd tetanus was 91% of that in the 1st. The (h + w) was greater in each tetanus than the quantity expected from the measured chemical changes. This excess was 230 .+-. 43 mJ/g dry weight (mean .+-. SE [standard error] of mean) in the 1st tetanus, but only 89 .+-. 46 mJ/g in the 2nd tetanus. The (h + w) was divided into 2 parts, labile and stable, which were defined by Aubert. This division was made on the basis of the time course of the (h + w) production, without reference to the chemical changes. The labile part of the (h + w) had an exponentially declining rate and the stable part had a constant rate. The (h + w) was less in the 2nd tetanus than in the 1st. This was due largely to the decrease in the labile part in the 2nd tetanus; it was only 0.35 of that in the 1st tetanus. The stable part remained relatively constant, 0.83 of that in the 1st tetanus. Having divided the (h + w) into the stable and labile parts, the stable part was accounted for by the chemical change, but the labile part was not. For both tetani, the stable part of the energy was about equal to the explained energy and the labile part was about equal to the unexplained energy. The possible interpretations of of these equalities are discussed.