ATPASE ACTIVITY AND FORCE PRODUCTION IN SKINNED RAT CARDIAC-MUSCLE UNDER ISOMETRIC AND DYNAMIC CONDITIONS

ATPase activity and force were determined in maximally activated skinn ed cardiac trabeculae from rat at 20 degrees C, 2.2 mu m sarcomere len gth and pH 7.0. Measurements were performed during isometric contracti on, and when square-wave shaped length changes with an amplitude of 0. 5-5% of the resting length (L(0)) and a repetition frequency of 2-167 Hz were imposed. Isometric ATPase activity amounted to 0.48 +/- 0.04 m M/s, and isometric force was 53 +/- 3 kN/m(2). During length changes, an increase in ATP consumption was observed, reminiscent of the Fenn e ffect, to a maximal value of 1.7 times the isometric value, while the average force decreased. The frequency dependent increase in ATPase ac tivity during length changes is compatible with a mono exponential rec overy process, with a rate constant of 32 +/- 7/s. In a two-state sche me of crossbridge cycling, an apparent attachment rate of 28.6/s and a n apparent detachment rate of 3.4/s were determined. To relate these o bservations with crossbridge kinetics during isotonic shortening, ramp -shaped length changes (2.5% L(0), 23 Hz) were imposed. ATPase activit y and average force during ramp shortenings were similar to the values obtained during square-wave shaped length changes with the same ampli tude and frequency. During lengthening ramps, the increase in ATPase a ctivity and the decrease in average force were significantly less than during the square-wave protocol. These results indicate that the ener getic effect of stretches is relatively small. (C) 1996 Academic Press Limited