Characteristics and Mechanism(s) of Force Generation by Increase of Temperature in Active Muscle
K. W. Ranatunga
School of Physiology & Pharmacology, University of Bristol, Medical Sciences Building, Bristol BS8 1TD, England, UK
This chapter reviews the basic observations made in temperature- perturbation experiments on active mammalian skeletal muscle. In the isometric muscle, a small rapid temperature-jump (T-jump) induces a characteristic biphasic tension rise, where the initial fast phase represents force generation in attached crossbridges; thus, crossbridge force rises when heat is absorbed, endothermic. Evidence suggests that a T-jump enhances a pre-phosphate release step in the acto-myosin (crossbridge) ATPase cycle. During steady shortening, the T-jump force generation is enhanced, whereas during steady lengthening, the T-jump force generation is depressed. The sigmoidal temperature dependence of steady
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active force may be largely due to the endothermic nature of force generation. Compared to the isometric curve, the force versus temperature curve is sharper and shifted to higher temperatures with steady shortening and to lower temperatures with steady lengthening in a velocity dependent manner. Overall, the results would be compatible with the notion that during steady lengthening the force generation is depressed and ATPase cycle is slowed, whereas during shortening this step and the ATPase cycle are accelerated. What structural mechanism underlies this end othermic crossbridge force generation still remains unclear.