Abstract
Observations made in temperature studies on mammalian muscle during force development, shortening, and lengthening, are re-examined. The isometric force in active muscle goes up substantially on warming from less than 10 °C to temperatures closer to physiological (>30 °C), and the sigmoidal temperature dependence of this force has a half-maximum at ~10 °C. During steady shortening, when force is decreased to a steady level, the sigmoidal curve is more pronounced and shifted to higher temperatures, whereas, in lengthening muscle, the curve is shifted to lower temperatures, and there is a less marked increase with temperature. Even with a small rapid temperature-jump (T-jump), force in active muscle rises in a definitive way. The rate of tension rise is slower with adenosine diphosphate (ADP) and faster with increased phosphate. Analysis showed that a T-jump enhances an early, pre-phosphate release step in the acto-myosin (crossbridge) ATPase cycle, thus inducing a force-rise. The sigmoidal dependence of steady force on temperature is due to this endothermic nature of crossbridge force generation. During shortening, the force-generating step and the ATPase cycle are accelerated, whereas during lengthening, they are inhibited. The endothermic force generation is seen in different muscle types (fast, slow, and cardiac). The underlying mechanism may involve a structural change in attached myosin heads and/or their attachments on heat absorption.
Highlights
As first proposed in the mid-1950s, and referred to as the sliding filament theory, active muscle contraction involves the relative sliding between two sets of filaments, the thin (A = actin) filaments and the thick (M = myosin) filaments in a sarcomere ([1,2] refs therein)
The driving mechanical process is a repetitive interaction of myosin heads on actin filaments; a crossbridge attaches to actin, undergoes a conformational change generating muscle force, and power, and detaches
This mechanics cycle is coupled to an enzymic reaction, hydrolysis of ATP by acto-myosin ATPase [3], so that energy liberated during release of the products of ATP hydrolysis is converted into work; an active muscle is a machine converting chemical to mechanical energy
Summary
The driving mechanical process is a repetitive interaction of myosin heads (crossbridges) on actin filaments; a crossbridge attaches to actin, undergoes a conformational change generating muscle force, and power, and detaches. This mechanics cycle is coupled to an enzymic reaction, hydrolysis of ATP by acto-myosin ATPase [3], so that energy liberated during release of the products of ATP hydrolysis (phosphate = Pi, and adenosine diphosphate = ADP) is converted into work (and heat); an active muscle is a machine converting chemical to mechanical energy. The relationship of the results from temperature studies on muscle to findings from other studies, listed earlier, remains unclear
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