Differential effects of length on maximum force production and myofibrillar ATPase activity in rat skinned cardiac muscle.

1. The fall of maximum Ca(2+)-activated force of cardiac myofibrils at short muscle lengths could be due to a reduction of cross-bridge cycling or to development of an opposing (restoring) force. To try to distinguish between these possibilities, we measured simultaneously myofibrillar force development and MgATPase activity (a measure of cross-bridge cycling) in rat skinned trabeculae at different muscle lengths. ATPase activity was measured photometrically from the utilization of NADH in a coupled enzyme assay. Muscle length was varied to give estimated 0.2 micron changes in sarcomere length (SL) over the range 1.4-2.4 microns. 2. Both Ca(2+)-activated force development and ATPase activity were optimal at a muscle length (Lo) where the resting SL was 2.2 microns. At Lo the maximum ATPase activity at 21 degrees C was 0.56 +/- 0.05 mM s-1 (mean +/- S.E.M., n = 6), which was equivalent to an ATP turnover per myosin S1 head of 3.3 s-1. 3. The relationship between ATPase activity and SL was curved, with rather little change in ATPase activity over the SL range 2.0-2.4 microns, but significant falls at 1.8 microns and below. At 65% of Lo (corresponding to a mean active SL of approximately 1.4 microns), the ATPase activity was only 50% of its value at 2.2 microns SL. 4. Force development decreased linearly as SL was reduced below 2.2 microns. Force fell by more than ATPase activity, particularly at SL 1.6 and 1.8 microns. 5. The fall of ATPase activity indicates that some of the decline of force production at short SL results from a fall in the net rate of cross-bridge cycling. This is probably the result of double overlap of thin filaments. However, the differential effect on force and ATPase reveals that, in the intermediate range of SL, decreased cross-bridge cycling can account for only part of the fall of force; the remainder is probably due to an increase in a restoring force, which may arise from deformation of the connective tissue in the muscle preparations used.