In rabbit skinned muscle fibres, decreasing sarcomere length resulted in a progressive decline in the Ca2+ sensitivity of tension development, shifting the tension-pCa relationship rightward.
The study demonstrates that length-dependent changes in Ca2+ sensitivity of tension development in rabbit skeletal muscle fibers are not due to shortening-induced dissociation of Ca2+ from troponin-C.
The length dependence of Ca2+-activated tension within the ascending limb of the length-tension relationship, corresponding to sarcomere lengths below about 2.25 micron, 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 from 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 0.23 pCa units as sarcomere length was reduced between 2.46 and 1.54 microns. 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 approximately 2.4 microns, 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 approximately 1.8 microns, 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.
Allen et al. (Tue,) reported a other. Changes in sarcomere length and Ca2+ concentration was evaluated on Length dependence of Ca2+-activated tension. In rabbit skinned muscle fibres, decreasing sarcomere length resulted in a progressive decline in the Ca2+ sensitivity of tension development, shifting the tension-pCa relationship rightward.