Temperature and amplitude dependence of tension transients in glycerinated skeletal and insect fibrillar muscle.

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Abstract
Quick stretches and releases were applied to small bundles of glycerinated fibers of rabbit psoas and insect fibrillar flight muscle. The resulting tension changes were recorded at various temperatures and amplitudes of length change. The results from the 2 preparations had many features in common. At temperatures near 0.degree. C the asymmetry of the initial tension recovery after stretch and release was very obvious. The complete tension course was described as an elastic change occurring simultaneously with the length change followed by recovery consisting of the sum of a number of exponential terms. These terms usually corresponded to the phases discernible without curve fitting, but in some cases a monotonic rise or fall of tension was seen to consist of 2 components only after curve fitting. After either stretch or release there was a phase of rapid tension recovery toward the value before the length change. The rate constant of this phase increased as the amplitude of stretch or release was increased to about 2 nm/half sarcomere. At higher amplitudes it remained nearly constant. At temperatures near 0.degree. C there was a 2nd and much slower continuation of the recovery after stretch. The rate constant of this 2nd phase was much more sensitive to temperature than that of the 1st phase and it became slower with increasing amplitude of stretch. As the temperature was raised the speed of the 2nd phase approached the speed of the 1st phase so that at room temperatures the initial tension recovery after stretch and release was nearly symmetrical. Under many conditions these processes were followed by a change in the opposite direction, the delayed tension described by earlier workers. This 3rd phase of tension change had about the same temperature sensitivity as the 2nd phase of the recovery seen after stretch. The tension due to stretch activation was not maintained in rabbit muscle, resulting in a 4th possible phase, a recovery of tension towards the value before the length change. This was absent or of low amplitude in insect flight muscle. These tension changes were interpreted on the basis of an extension of a non-linear model. The elastic tension change and the initial fast recovery seemed both properties of the attached cross-bridges, while the slower recovery may be due to the detachment of cross-bridges which happened to be attached at the instant the length change was applied. The delayed tension reflected the approach to equilibrium of the number of attached bridges, changed by an effect of muscle length on the attachment rate. That the delayed tension was not maintained in rabbit psoas muscle may be due to the effect of length on attachment rate being transitory.