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. High-energy phosphate metabolism and energy liberated as heat and work were measured in 3 sec tetani of frog sartorius muscles at 0 OC.2. Three contraction periods were studied: (a) shortening at near-maximum velocity for 0-3 sec from sarcomere length 2-6 to 18 sum, beginning after 2 sec of isometric stimulation, (b) the 0 7 sec isometric period immediately following such rapid shortening, (c) the period from 2 to 3 sec in an isometric tetanus at sarcomere length 1P8 um.3. There were no significant changes in levels of ATP, ADP or AMP in any contraction period. The observed changes in inorganic phosphate and creatine levels indicated that the only significant reaction occurring was phosphocreatine splitting.4. The mean rate of high-energy phosphate splitting during rapid shortening, 0-48 + 0-24 ,imole/g . sec (mean + S.E. of mean, n = 29; 'g' refers to blotted muscle weight), was not significantly different from that in the 1 sec period in the isometric tetanus, 0-32 +011 smole/g. sec (n = 17). The mean rate in the post-shortening period, 0 71 +0 10 ,umole/g . sec (n = 22), was greater than that in the 1 sec period in the isometric tetanus, and this difference is significant (P < 0-02, t test).5. A large quantity of heat plus work was produced during the rapid shortening period, but less than half of this could be accounted for by simultaneous chemical reactions. The unexplained enthalpy production was 6-5 + 26 mJ/g (mean +S.E. of mean). No significant unexplained enthalpy was produced in the 1 sec period in the isometric tetanus.6. In the post-shortening period the observed enthalpy was less, by 6-2 + 2-6 mJ/g, than that expected from the simultaneous chemical reactions.7. The results are interpreted in terms of an exothermic shift in the population of cross-bridge states during rapid shortening. It is suggested that a relatively slow subsequent step prevents many of these cross-bridges from completing the cycle and splitting ATP until after the end of shortening.

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 B S T R A C T Measurements of the time course of high energy phosphate splittingand energy liberation were performed on rapidly shortening Rana pipiens skeletal muscles. In muscles contracting 30 times against small loads (<0.02 Po), the ratio of explained heat + work (H + W) (calculated from the measured high energy phosphate splitting) to observed H + W (from myothermal and mechanical measurements) was 0.68 ~ 0.08 and is in agreement with results obtained in isometric tetani of R. pipiens skeletal muscle. In lightly afterloaded muscles which were tetanized for 0.6 s and whose metabolism was arrested at 3.0 s after the beginning of stimulation, a similar ratio of explained H + W to observed H + W was obtained. However, in identical contractions in which metabolism was arrested at 0.5-0.75 s after the beginning of stimulation, the ratio of explained H + W to observed H + W declined significantly to values ranging from 0.15 to 0.40. These results suggest that rapid shortening at the beginning of contraction induces a delay between energy production and measurable high energy phosphate splitting. This interpretation was tested and confirmed in experiments in which one muscle of a pair contracted isometrically while the other contracted against a small afterioad. The afterload and stimulus pattern were arranged so that at the time metabolism was arrested, 0.5 s after the beginning of stimulation, the total energy production by both muscles was the same. Chemical analysis revealed that the isotonically contracting muscle split only 25% as much high energy phosphate as did the isometrically contracting muscle.Identification of the chemical sources of the energy liberated during contraction is important in understanding the molecular mechanism of muscle contraction. In frog muscle contracting at 0°C, the primary energy-yielding reaction is the splitting of phosphocreatine (PC), which occurs subsequent to ATP hydrolysis (Carlson et al., 1963(Carlson et al., , 1967Marechal and Mommaerts, 1963;Wilkie, 1968 duced in proportion to PC hydrolysis and that the ratio of H + W to APC is approximately -11 kcal/mol (Carlson et al., 1963(Carlson et al., , 1967Wilkie, 1968). This result is independent of the type of contraction-relaxation cycle. Isometric and isotonic twitches and tetani lead to the same conclusion. Since the molar enthalpy change, M-/, for PC splitting is -8.1 kcal/mol (Woledge, 1972), the above experiments suggest that only a fraction, approximately 0.74, of the observed H + W during contraction can be explained by PC splitting. This fraction is called the explained enthalpy fraction.To further elucidate the relation of metabolism to energy production it is necessary to extend the above experiments to the time course of single isometric and isotonic contractions. As a muscle presumably returns to its precontraction state after a contraction-relaxation cycle, the above results do not imply that during a single cycle the same relation of energy liberation to metabolism will be observed. Homsher et al. (1975) have...

A B S X R A C T Unpoisoned sartorius muscles of Rana temporaria were stimulated tetanically in isometric contractions lasting up to 20 s at 0~ The observed enthalpy (heat + work) production and the chemical changes in these contractions were measured, and a comparison was made between the observed enthalpy and the enthalpy that could be explained by the chemical changes. Like earlier workers, we found that the only net known reaction of energetic significance that occurred was dephosphorylation of n-phosphoryl creatine (PC), and we found a significant evolution of unexplained enthalpy (UE), a portion of the observed enthaipy which could not be explained by the extent of PC dephosphorylation. We measured the total quantity and the rate of production of the UE, and we found that its rate of evolution, which was most rapid during the first 750 ms of contraction, fell progressively to zero by the 8th s of contraction; i.e., after 8 s of contraction, all the observed enthalpy is adequately explained by PC dephosphorylation. The timecourse of evolution of the UE was slower than that of the labile enthalpy (a component of the enthalpy evolved in isometric contraction whose rate of production declines exponentially at -1 s-~). We conclude that, although the magnitudes of these enthalpy quantities may be similar, they are not derived from the same chemical reaction in muscle.