ATPase activity in rapidly activated skinned muscle fibres

The ATPase activity of single mechanically skinned fibres from semitendinosus muscle of Rana esculenta was measured. The method was based on the fluorimetrically detected consumption of NADH inside the fibre. NADH breakdown is caused by the reaction of an ATP regenerating system coupled to an NADH oxidising system. The time resolution was about 5 s. The initial rise in ATPase activity was delayed when a fibre was activated by Ca, and ATPase activity reached its plateau at the same time as fibre tension. An activated fibre that was isolated from the incubation solution by immersing it into oil showed a temporary decrease in tension without corresponding decrease in the ATPase activity.