The reversal of chemical reactions in contracting muscle during an applied stretch

Abstract

When a toad muscle is stretched 11 to 20% during the active phase of a twitch or a short tetanus, resisting strongly, the total heat $(H_{s})$ appearing in it up to the end of relaxation may be about equal to the total mechanical work $(W_{0})$ done during the stretch. Since all the work has disappeared, and no elastic energy is left by the end of relaxation, the net energy $(H_{s}-W_{0})$ liberated by the muscle itself is nil. It is concluded that the chemical products of the reaction provoked by the stimulus have been wholly returned to their initial state. This result depends on the amplitude and timing of the stretch. Often $H_{s}$ is rather greater than $W_{0}$, but $(H_{s}-W_{0})$ is always much less than $H_{i}$ the total heat in an isometric contraction: in these circumstances most, but not all, of the chemical products of activity have been reinstated. During a contraction with stretch the contractile component begins to lengthen as soon as the tension reaches a value slightly greater than that in an isometric tetanus. It is during this lengthening that chemical energy is re-absorbed. When work $W$, up to any moment, is done on a contracting muscle, some of it is used in producing elastic energy $E$ in the series elastic component and in the connexion to the ergometer. Only the net work $W_{n}=(W-E)$ has been taken up by the contractile component. If $H$ is the heat produced in the muscle by any moment $(H-W_{n})$ soon begins to fall when the stretch starts. In a twitch or a short tetanus $(H-W_{n})$ usually becomes substantially negative, remaining negative for a considerable period but finally returning to zero, or to a small positive value, by the end of relaxation. In a longer tetanus, with a stretch starting later and the stimulus outlasting it, $(H-W_{n})$ begins to fall directly the stretch starts, dropping sometimes to or below zero, but increasing rapidly when the stretch ends, as the stimulus continues. The fact that the net energy $(H-W_{n})$ liberated by a muscle up to any moment may reach large negative values during part of the cycle, while the total energy $(H_{s}-W_{0})$ over the whole cycle never falls below zero, is difficult to explain on any simple theory of the reversal of a chemical reaction. The difficulty is resolved by assuming that whenever the tension rises by $\Delta P$ during contraction, for whatever cause, there is a corresponding 'thermoelastic' absorption of heat $\Delta Q=0.018l_{0}\Delta P$, and conversely when the tension falls. The constant 0.018 is that observed in earlier experiments on the thermal effect of a sudden release of tension. If this assumption is correct, the real heat produced by the muscle up to any moment is greater than $H$ $\text{by}$ $Q=0.018l_{0}P$, where $P$ is the tension at that moment. Substituting $(H+Q)$ $\text{for}$ $H$, it is found that $(H+Q-W_{n})$ behaves in the same general way as $(H-W_{n})$ during and after a stretch but never becomes negative. Since $Q$ is zero at the end of relaxation, when $P=0$, the statement about total quantities stands unaltered. The results can be used to calculate the ratio of the energy absorbed in chemical resynthesis, during a stretch, to the work applied. In stretches of moderate extent, the ratio may be as high as 0.5, but in the longer and more vigorous stretches which gave complete reversal the ratio was considerably less. The thermodynamic implications are discussed.