The Effect of Shortening on Energy Liberation and High Energy Phosphate Hydrolysis in Frog Skeletal Muscle

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Abstract
It is generally assumed that the increased rate of energy liberation (as heat and work, h+w) accompanying shortening stems from an increased rate of crossbridge cycling and ATP hydrolysis. Experiments were performed to test two premises of this assumption: first, is the increased rate of heat production accompanying shortening derived from crossbridge activity? This question was answered by measuring the amount of shortening heat produced by a fixed displacement of 0.3 μm/sarcomere in the sarcomere length range of 2.25–3.75 μm. Shortening heat declines linearly with decreasing amounts of thick and thin filament overlap and becomes zero at a sarcomere spacing of ca. 3.70 μm. Secondly, the extent to which the measured consumption of high energy phosphate accounts for the measured tetanic (h+w) production during and after shortening for 300 ms at a velocity of Vmax or ½Vmax was examined. The results of these experiments showed that within 700 ms of the end of shortening at both velocities, all the (h+w) could be explained by the hydrolysis of ATP. At Vmax all the (h+w) produced by the end of shortening could be explained by the measured ATP hydrolysis. However, at Vmax less than half of the (h+w) produced by the end of shortening could be explained by the measured ATP splitting and there was a high rate of ATP splitting after the end of shortening. These results suggest that while shortening at velocities ≦Vmax the energy liberation is indeed derived from an increased rate of ATP hydrolysis by crossbridges, at Vmax the crossbridge ATPase cycle differs somewhat from that at lower shortening velocities.

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