C. Lionne et al., MECHANOCHEMICAL COUPLING IN MUSCLE - ATTEMPTS TO MEASURE SIMULTANEOUSLY SHORTENING AND ATPASE RATES IN MYOFIBRILS, Biophysical journal, 70(2), 1996, pp. 887-895
We studied the ATPase of shortening myofibrils at 4 degrees C by the r
apid flow quench method. The progress curve has three phases: a P-i bu
rst, a fast linear phase k(F) of duration t(B), and a deceleration to
a slow k(S). We propose that k(F) is the ATPase of myofibrils shorteni
ng under zero external load; at t(B) shortening and ATPase rates are r
educed by passive resistance. The total ATP consumed during the rapid
shortening is ATP(c). Our purpose was to obtain information on the myo
fibrillar shortening velocity from their ATPase progress curves. We te
sted t(B) as an indicator of shortening velocity by determining the ef
fects of different probes upon it and the other ATPase parameters. The
dependence of t(B) upon the initial sarcomere length was linear, givi
ng a shortening velocity close to that of muscle fibres (V-o). The K-m
of ATP was larger for t(B) than for k(F), as found with fibers for V-
o and their ATPase. ADP and 2,3-butanedione monoxime, but not P-i, inh
ibited t(B) to the same extent as V-o. The Delta H-double dagger for t
(B) and V-o were similar. ATP, was independent of the sarcomere length
, implying that the more the myofibrils shorten, the less ATP expended
per myosin head per micron shortened. We propose that t, can be used
as an indicator for myofibrillar shortening velocities.