Effect of inorganic phosphate and ADP on the myofilament sliding induced by laser flash photolysis of caged ATP.

Using the technique of laser flash photolysis of caged ATP, we have suggested that, under nearly isometric conditions, the unitary distance of myofilament sliding per ATP molecule (myosin head powerstroke) is about 10 nm. To give further information about the mechanism of myofilament sliding, we studied the effect of inorganic phosphate (Pi) and ADP on the photoreleased ATP-induced shortening of single glycerinated muscle fibers under very small external loads. Both the velocity and the distance of the myofilament sliding induced by 150 microM ATP increased by Pi (20 mM), and decreased by ADP (0.4 mM). On the other hand, Pi and ADP showed no significant effect on the myofilament sliding induced by 100 and 75 microM ATP. The potentiating effect of Pi on the myofilament sliding with 150 microM ATP can be explained as being due to the increase in population of AM.ADP.Pi with corresponding decrease in population of AM.ADP, and also the increase in population of M.ADP.Pi and M.ATP. Meanwhile, the inhibitory effect of ADP can be simply accounted for to be due to an accumulation of AM.ADP that already finished their force generating process. The ineffectiveness of Pi and ADP on the myofilament sliding with 100 and 75 microM ATP is consistent with the view that it is caused by almost synchronized single myosin head powerstrokes.