The heat production associated with the maintenance of a prolonged contraction and the extra heat produced during large shortening

Abstract

In a recent paper (Hill, 1949 a), it was shown that the heat produced in a single twitch is made up of two parts: (a) the heat of shortening which is simultaneous with and proportional to the shortening, and (b) the heat of activation. The latter starts at its maximum speed shortly after a shock, falling in rate from then on and finishing by the time that relaxation begins. In another paper (Hill, 1949c) the use of a quick stretch applied shortly after a shock showed that the full strength of a contraction (equal to that in a maximal tetanus) is developed very rapidly after the end of the latent period, remaining on a plateau for a time, then gradually disappearing in relaxation.These facts suggested that each of the successive shocks required to maintain a prolonged contraction merely restored the full strength of the contraction from the level to which it had relaxed in the interval between shocks, and that the heat produced in a maintained contraction was no more than the summated effect of the heats of activation of the successive responses. If this was so, we should expect the maintenance heat, like the activation heat, to be little affected by muscle length. The first object of the present investigation was to test this conclusion.If the maintenance heat is the summated accompaniment of the succession of re-activations by which the onset of relaxation is deferred, it should be greater or less according as relaxation is faster or slower. In two respects this is known already to be the case: (a) a rise of temperature increases the rate of relaxation and of maintenance heat production (Hartree & Hill, 1921), and (b) previous activity (Bronk, 1930) and the presence of C02 (Bozler, 1930) decrease both. The second object of the present experiments was to find out, in a single maintained contraction, whether the two effects run parallel.In an isometric contraction considerable shortening of the contractile material, with consequent stretching of the series elastic material, occurs in the

The mechanical and thermal properties of frog slow muscle fibres

“…The time taken for the tension to fall by 20 % from its maximum, TO-2R, is variable. Abbott (1951) has shown that the change with time of heat rate during an isometric tetanus of twitch fibres K. FLOYD AND I. C. H. SMITH is accompanied by changes in tension relaxation rate. This may be because both parameters measure the rate of cross-bridge turnover.…”

Transient Phases of the Isometric Tetanus in Frog's Striated Muscle

“…During prolonged repetitive stimulation [Kt] will increase to a steady-state value which should be attained more rapidly, the higher the frequency of stimulation. Abbott (1951) found that Q decreased to a steady value more rapidly at a higher frequency; the steady value was slightly affected by the increase in frequency.…”

“…The rate constant for relaxation, r, depends on the duration of stimulation during the tetanus, D, as well as on L and T. The results of Abbott (1951) and Aubert (1956) and F, = 10.6 kcal/mole. The data were not sufficient in number or precision to estimate 4)(s, T), although clearly increased with temperature at all sarcomere lengths.…”

“…Although evaluation of r is more laborious than the methods Hartree and Hill ( 1921) and Abbott ( 1951) used to quantify relaxation, and more laborious than measurement of the half-time of tension fall (Jewell and Wilkie, 1960), r has two merits as an index of relaxation. r may be measured in both twitches and tetani, providing a means for comparing relaxation in these processes.…”

“…First, a decline in the rate of heat production, 0, during the course of an isometric tetanus roughly parallels a decline with increasing D in an index for the rate of relaxation (Abbott, 1951;Aubert, 1956). Second, in the rectus femoris of the tortoise neither Q nor the time course of relaxation changes during a prolonged isometric tetanus (Woledge, 1968).…”