SUMMARY1. The length dependence of Ca2+ -activated tension within the ascending limb of the length-tension relationship, corresponding to sarcomere lengths below about 2-25 ptm, was investigated in skinned fibres from rabbit psoas muscle. At high [Ca2+] a shallow phase and then a steep phase of tension decline were observed as sarcomere length was reduced, while at low [Ca2+] tension decreased monotonically with decreases in sarcomere length. The sarcomere length at which the ascending limb intersected zero tension was greater for lower concentrations of Ca2+.2. The length-tension relationship from maximally activated fibres changed when filament lattice spacing was reduced by osmotic compression. Relationships obtained in the presence of 5% (w/v) dextran T500 more distinctly demonstrated both the shallow and steep portions of the ascending limb than did relationships fr'om untreated fibres.3. As striation spacing was decreased a progressive decline in the Ca2+ sensitivity of tension development was observed. Tension-pCa relationships from both control and dextran-treated fibres underwent a rightward shift (i.e. to a higher [Ca2+]) by 023 pCa units as sarcomere length was reduced between 2-46 and 1'54 ,um.4. Fibre stiffness was studied by applying a 3-3 kHz sinusoidal length change at one end of the fibre and measuring the resultant tension change. At submaximal activation (pCa 5-8), stiffness increased relative to tension as sarcomere length was decreased below -2-4 /tm, suggesting that there is an activation-related internal load at low [Ca2+]. At maximal activation, a significant increase in this ratio occurred only at sarcomere lengths less than -1-8 ,um, and presumably involved collision of the thick filaments with the Z-lines.5. Length-dependent changes in the Ca2+ sensitivity of tension development do not appear to be the result of shortening-induced dissociation of Ca2+ from troponin-C, the Ca2+ binding subunit of troponin. Fibres activated in the absence of Ca2+, by the partial removal of whole troponin complexes, produced length-tension relationships similar to those observed in the same fibres before troponin removal at a submaximal [Ca2+] yielding similar active tensions.

Partial extraction of troponin C (TnC) decreases the Ca" sensitivity of tension development in mammalian skinned muscle fibers (Moss, R . L ., G . G . Giulian, and M . L . Greaser . 1985 . journa l of General Physiology . 86 :585), which suggests that Ca"-activated tension development involves molecular cooperativity within the thin filament . This idea has been investigated further in the present study, in which Ca' --insensitive activation of skinned fibers from rabbit psoas muscles was achieved by removing a small proportion of total troponin (Tn) complexes . Ca"-activated isometric tension was measured at pCa values (i .e ., -log[Ca 2+ ]) between 6 .7 and 4 .5 : (a) in control fiber segments, (b) in the same fibers after partial removal of Tn, and (c) after recombination of Tn . Tn removal was accomplished using contaminant protease activity found in preparations of LC from rabbit soleus muscle, and was quantitated using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and scanning densitometry . Partial Tn removal resulted in the development of a Ca 2 '-insensitive active tension, which varied in amount depending on the duration of the extraction, and concomitant decreases in maximal Ca2+ -activated tensions. In addition, the tension-pCa relation was shifted to higher pCa values by as much as 0 .3 pCa unit after Tn extraction . Readdition of Tn to the fiber segments resulted in the reduction of tension in the relaxing solution to control values and in the return of the tension-pCa relation to its original position . Thus, continuous Ca 2+ -insensitive activation of randomly spaced functional groups increased the Ca 21 sensitivity of tension development in the remaining functional groups along the thin filament . In addition, the variation in Ca 2 '-insensitive active tension as a function of Tn content after extraction

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