title: Energetics of isometric and isotonic twitches in toad sartorius.
abstract: Contractile energetics have been studied in twitches of toad sartorius muscle at 6-7 degrees C. Isometric and isotonic energy production has been measured and plotted against a wide range of developed tensions and tension-time integrals. These parameters were varied by altering the isotonic load or by changing the preset isometric length. The isometric tension-independent heat was 1.12 +/-0.18 (SD) mcal/g. The isometric heat coefficient Pl(0)/H was 12.0 +/-1.4 in muscles having twitch to tetanus ratios ranging from 0.4 to 0.6. Isometric enthalpy increased monotonically with tension or tension-time integral but the correlation between isometric heat and these parameters was poor. Isotonic enthalpy consumption was always higher than isometric enthalpy for any given tension or tension-time integral; however, isotonic heat production was consistently less than isometric heat production. The isotonic heat for the highest load (3 g) was not significantly different from the isometric tension-independent heat. Thus isotonic heat production first decreased and then increased with increasing tension or tension-time integral. In the discussion it is shown that the results conflict with all current interpretations of muscle energetics.
mesh_terms: D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D002024:Bufo marinus; D003080:Cold Temperature; D004558:Electric Stimulation; D004735:Energy Transfer; D006614:Hindlimb; D006358:Hot Temperature; D008433:Mathematics; D009119:Muscle Contraction; D013314:Stress, Mechanical; D013816:Thermodynamics