SUMMARY1. Frog semitendinosus muscles were stretched to various lengths beyond the rest length (lo) and their initial heat and isometric tension production were measured.2. As the overlap between the thick and thin filaments is reduced, the initial twitch heat and tension decline in a linear manner. At a point at which the twitch tension approaches zero, the initial heat is 30 % of that seen at lo. It is concluded that this heat is the activation heat and reflects the energetics of calcium release and reaccumulation. The initial heat at shorter sarcomere lengths appears to be the sum of the activation heat plus a heat production associated with the interaction of the thick and thin filaments.3. A similar relationship between heat and tension production is seen in tetanic contractions.4. The time course of activation heat production in a twitch can be resolved into two phases: a temperature insensitive (Q10 < 1.3) 'fast' phase (with a time constant of 45 msec) and a temperature sensitive (Q10 = 2.8) 'slow' phase (with a time constant of 330 msec at 00 C).5. Measurements of the creatine phosphate (PC) hydrolysis by muscles contracting isometrically at various muscle lengths at and beyond lo, indicate an enthalpy change of -11-2 kcal/mole PC hydrolysed. The enthalpy change for the ATP hydrolysis by muscles stretched so that little or no tension was produced with stimulation was -9 9 kcal/mole ATP hydrolysed. It is concluded that the net activation heat is produced by the hydrolysis of PC or ATP.

SUMMARY1. The average resting heat production of a muscle under zero tension is 24*8 mcal/g muscle . min at 20°C. In the majority of muscles exomined the resting heat production increases when the resting tension and muscle length are increased.2. The relation between actively developed tension and heat produced is similar to that existing in skeletal muscle. The plot of heat against developed tension can be obtained either by altering muscle length or by varying the stimulus frequency.3. The mean maximum total efficiency work/(work + heat) in the work experiments was 11-6 %. The total energy produced (work+ heat) correlates with the load rather than with the work done. 4. In isotonic contractions more heat is liberated than the heat versus tension plot predicts. This extra heat is load-dependent.

A method has been developed to measure myocardial heat production simultaneously with mechanical (developed tension, rate of contraction and relaxation) and metabolic parameters in the arterially perfused interventricular septum of the rabbit. The system allows control of rest tension, frequency of contraction, temperature, and composition of the perfusate. The technique is based on the differential measurement of the heat flux from the muscle to the calorimetric bath. This technique is able to resolve changes in heat production as small as 0.06 mW. The resting heat production measured with the present calorimeter (1.62 +/- 0.1 mW/g wet tissue) agrees with that obtained with thermopiles and with that calculated from measurements of oxygen consumption. The heat per contraction (6.5 +/- 1.4 mJ/g wet tissue) also agreed with that measured with thermopiles in rabbit papillary muscles. The heat production measured at 22 degrees C under severe hypoxia can be fully explained by the addition of the expected change of enthalpy due to the reaction 0.5 glucose-lactate, calculated on the basis of the lactate measured in the perfusate, and the expected change of enthalpy of oxygen consumption [assuming that all remaining O2 in perfusate (0.09 vol%) is used for combustion of glucose]. These results clearly demonstrated the feasibility of this method for the correlation of changes in energy turnover, through the measurement of the heat production, with mechanical and metabolic processes on-line in arterially perfused septum.

Recent data obtained from Rana temporaria sartorius muscles during an isometric tetanus indicate that the time-course of phosphocreatine (PC) splitting cannot account for the total energy (heat + work) liberation (Gilbert et al. 1971. J. Physiol. (Lond.). 218:163). As this conclusion is important to an understanding of the chemical energetics of contraction, similar experiments were performed on unpoisoned, oxygenated Rana pipiens sartorius muscles. The muscles were tetanized (isometrically) at 0°C for 0.6, 1, or 5 s; metabolism was rapidly arrested by freezing the muscles with a specially designed hammer apparatus, and the frozen muscles were chemically analyzed. Comparable myothermal measurements were made on frogs from the same batch. Results of these experiments indicate: (a) The energy liberation parallels the PC and ATP breakdown with a proportionality constant of --10.7 kcal/mol; (b) comparably designed experiments with sartorius muscles of R. temporaria revealed that the ratio of energy liberation to PC splitting was significantly greater than that observed in R. pipitns sartorius muscles; (c) there is no systematic difference between experiments in which metabolism was arrested by the hammer apparatus and others using a conventional immersion technique.

A number of investigators have succeeded in preparing isolated cardiac cells by enzymatic digestion which tolerate external [Ca2+] in the millimolar range. However, a persistent problem with these preparations is that, unlike in situ adult ventricular fibres, the isolated fibres usually beat spontaneously. This spontaneity suggests persistent ionic leakage not present in situ. A preferable preparation for mechanical and electrical studies would be one which is quiescent but excitable in response to electrical stimulation and which does not undergo contracture with repeated stimulation. We report here a modified method of cardiac fibre isolation and perfusion which leaves the fibre membrane electrically excitable and moderately resistant to mechanical stress so that the attachment of suction micropipettes to the fibre is possible for force measurement and length control. Force generation in single isolated adult rat heart fibres is consistent with in situ contractile force. The negative staircase effect (treppe) characteristic of adult not heart tissue is present with increased frequency of stimulation. Isometric developed tension increases with fibre length as in in situ ventricular tissue.