Dependence of shortening heat on sarcomere length in frog muscle and fiber bundles.

The relation between shortening heat and sarcomere length was studied using fiber bundles from frog semitendinosus muscles as well as using whole muscles. The initial sarcomere length was varied between 2.0 and 3.66 micron. Shortening heat was estimated as the excess heat produced after a rapid isovelocity release in a 3 sec tetanus at 0 degrees C. The isometric control heat was measured in the same tetanus, before and after the period of shortening. The unstimulated whole muscles showed a large thermoelastic absorption of heat when released at sarcomere lengths longer than 2.5 micron, and the apparent shortening heat was negative at very long sarcomere lengths. The apparent shortening heat was corrected by subtracting the thermoelastic heat absorption by assuming that the thermoelastic effect was also present in releases of active muscles. The corrected shortening heat decreased linearly with increasing sarcomere length in the range 2.29-3.66 micron, intersecting the length axis at 3.73 +/- 0.21 micron. The thermoelastic heat absorption at long sarcomere lengths was substantially reduced in fiber bundles, suggesting that the parallel elasticity responsible for the thermoelastic effect is mainly present outside muscle cells. The corrected shortening heat in fiber bundles also decreased linearly with increasing sarcomere length, intersecting the length axis at 3.84 +/- 0.25 micron. Thus the results on fiber bundles, also based on correction but the extent of which is substantially smaller than in whole muscles, are in agreement with the results on whole muscles. The results are interpreted to mean that shortening heat is produced by the interaction of thick and thin filaments in contracting muscle.