Is the chemomechanical energy transformation reversible?

In glycerol-extracted insect fibrillar muscle suspended in ATP salt solution the incorporation of 32Pi into ATP was studied during the performance of positive or negative oscillatory work and under a variety of mechanical and ionic conditions. An increase in calcium ion concentration from 10(-8)--10(-5) M increased the incorporation rate in proportion to the increase in ATPase activity, mean tension and immediate stiffness, which is a measure of the extent of actin-myosin interaction. Sinusoidal stretches (at 1% Lo) performed at 5 Hz induced the fibres to perform optimal positive oscillatory work and it caused a doubling of the incorporation rate (ant ATPase activity). A decrease or increase of the frequency below or above the optimum of 5 Hz always decreased the power output as well as the incorporation rate which, however, was still noticeable even under conditions where work was done on the fibres. A similar frequency dependence was found when square-wave rather than sinusoidal stretches were applied and this effect could be related to the finding that the rate of stretch-induced incorporation was highest shortly after stretching and then declined to low values (after about 100 ms). These results suggest the formation of an energy-rich intermediate (actomyosin-ADP?) during the contraction process induced by stretching and this intermediate must be assumed to accumulate transiently after stretching.