SARCOMERE-LENGTH DEPENDENCE OF THE RATE OF TENSION REDEVELOPMENT AND SUBMAXIMAL TENSION IN RAT AND RABBIT SKINNED SKELETAL-MUSCLE FIBERS

1. We examined the hypothesis that in skeletal muscle the steep relati onship between twitch tension and sarcomere length (SL) within the ran ge 2.30 to 1.85 mu m involves SL-dependent alterations in the rate of tension development. 2. In skinned preparations of both rat slow-twitc h and rabbit fast-twitch skeletal muscle fibres the rate of tension re development (k(tr)) at 15 degrees C was reduced at short SL (similar t o 2.00 mu m) compared with a longer SL (similar to 2.30 mu m). In subm aximally activated fibres, the decrease in k(tr), over this range of l engths was greater in fast-twitch fibres (38% reduction) than in slow- twitch fibres (l4% reduction). 3. Ca2+ sensitivity of tension, as asse ssed as the pCa (-log[Ca2+] for half-maximal activation, or pCa(50), d ecreased to a greater extent in rabbit fast-twitch skeletal muscle fib res than in slow-twitch fibres from both rabbit and rat when SL was re duced from similar to 2.30 to similar to 1.85 mu m. The Delta pCa(50) over this SL range was 0.24 +/- 0.07 pCa units in fast-twitch fibres f rom rabbit psoas muscle. The Delta pCa(30) for slow-twitch fibres from rabbit and rat soleus muscle was 0.08 +/- 0.02 and 0.10 +/- 0.04 pCa units, respectively. 4. Osmotic compression of both slow-twitch and fa st-twitch fibres at a SL of 2.00 mu m increased k(tr) to values simila r to those obtained at a SL of 2.30 mu m in the absence of dextran. Th is result indicates that the slower rate of tension redevelopment at s hort SL is due in large part to the increase in interfilament lattice spacing associated with shorter SL. 5. Taken together, these results s uggest that length dependence of twitch tension is, in part, due to le ngth dependence of isometric cross-bridge interaction kinetics, an eff ect that is mediated by length-dependent changes in interfilament latt ice spacing.