Tension transients during steady shortening of frog muscle fibres

Single intact fibres from frog muscle at 0-1 degrees C were stimulated to produce isometric tetani at a sarcomere length of about 2.25 micron, using a spot-follower apparatus to control the length of the central part of a fibre. When the plateau of the tetanus was reached the fibre was forced to shorten by applying a step and ramp length change in an approximation to an isotonic release. When tension had reached a steady level, Ti, during shortening, tension transients were elicited by applying step changes of length, complete within 0.2 ms, ranging from a stretch of 1.5 nm per half-sarcomere to a release of 6 nm per half-sarcomere. The tension transients recorded during shortening were qualitatively similar to those previously recorded in isometric tetani. There were four phases: phase 1, the change of tension during the step; phase 2, a rapid partial recovery of tension; phase 3, a delay or reversal of recovery; phase 4, a slower recovery of tension to the level before the step was applied. Measurements were made of the extreme tension, T1, attained during a step, and the level, T2, to which tension recovers in phase 2. The excursion of tension, [T1-Ti], during a small step of given size, fell with increase of shortening velocity, reaching about 40% of the isometric value near the maximum velocity of shortening. T2 fell as shortening velocity was increased and the fraction of steady tension recovered, T2/Ti, also decreased, so that the proportion of tension recovery in phase 4 increased. All the recovery phases became progressively more rapid with increase of shortening velocity. The early tension response was matched with a delay-line simulator so as to estimate the value of the instantaneous stiffness. Stiffness during shortening was found to decrease approximately linearly with tension, reaching about 35% of the isometric value as tension approached zero. It was impossible to match the early tension response in a rapidly shortening fibre without assuming decreased stiffness. The decline of stiffness is interpreted as due largely to reduced number of attached cross-bridges, but quantitative estimates would be affected by possible filament compliance and non-linearity of cross-bridge stiffness. The decrease in T2 also suggests fewer cross-bridges are attached as shortening velocity increases, but uncertainties about the processes determining phase 2 during shortening do not permit a precise estimate of stiffness to be made.(ABSTRACT TRUNCATED AT 400 WORDS)