Measurement Of ATPase Activity During Ramped Stretches In Contracting Skeletal Muscle Fibers Of The Rabbit

Abstract

Skeletal muscle force response to small amplitude and low velocity ramp stretches is biphasic. An initial fast increase in force, where myosin heads are forcibly detached is followed by a slower increase in force with net re-attachment of myosin heads to actin. The average ATPase rate is very low during stretch (Curtin & Davies,1973), but due to lack of time resolution the two phases to match force changes have never been resolved. We therefore examined and modeled tension and ATPase responses to ramp stretches (5% and 1% of fiber length, Lo) at low velocities (0.1 and 0.5 Lo/s) in permeabilised fiber bundles of rabbit psoas at 12 and 20°C. We show that ATPase activity drops to near zero during the initial fast phase of the stretch and increases slightly but still remains lower than isometric during the second part of the stretch phase, returning to normal post-stretch. The response was not as marked at 12 as at 20°C, although ATPase rate was still reduced in both the fast initial and slow secondary phase of the force response. During the initial phase the myosin heads are forcibly removed from actin, the cross-bridge cycle is not complete and release of hydrolysis products is interrupted. In the second phase, myosin heads re-attach whilst the muscle is still lengthened and the cross-bridge cycle is truncated for a fraction of attached heads, leading to slower Pi release than during isometric conditions. These effects are less marked at 12 than at 20°C because the fraction of strongly bound cross-bridges is reduced at the lower temperature.Curtin, N.A. & Davies, R.E. (1973). Cold Spring Harbor Symp. on Q. Biol., 37, 619-626.