Substitution of cardiac troponin C into rabbit muscle does not alter the length dependence of Ca2+ sensitivity of tension.

1. The isometric length-tension relationship for cardiac muscle is generally steeper than for skeletal muscle in the physiological range of sarcomere lengths. Recent studies suggest that cardiac troponin C (cTnC) may have intrinsic properties that confer greater length-dependent changes in Ca2+ sensitivity of tension than for skeletal troponin C (sTnC). We tested this hypothesis by characterizing tension-pCa (pCa is -log[Ca2+]) relationships in rabbit skinned psoas muscle fibres at mean sarcomere lengths of 2.32 and 1.87 microns both before and after partial replacement of endogenous sTnC with cTnC. 2. In untreated control fibres, the mid-point (pCa50) of the tension-pCa relationship shifted to lower pCa by 0.15 +/- 0.02 pCa units, i.e. became less sensitive to Ca2+, when sarcomere length was reduced, and the relationship became steeper. 3. Partial extraction of endogenous sTnC and reconstitution with cTnC resulted in no change in the length-dependent shift of pCa50 when reconstitution with cTnC was more than 95% complete; however, when reconstitution was less than 95% complete, there were significant increases in the length-dependent shift in pCa50. 4. An increase in the length-dependent shift of pCa50 was also observed in fibres from which sTnC was partially extracted, but no cTnC was subsequently re-added. 5. We conclude that differences in type of TnC alone are not sufficient to explain differences between skeletal and cardiac muscles in the length dependence of Ca2+ sensitivity of tension.