Modification of temperature dependence of myofilament Ca sensitivity by troponin C replacement

The Ca sensitivity of chemically skinned right ventricular trabeculae from the rat heart was determined at 22 and 8°C. Endogenous troponin C (TnC) was then extracted with EDTA and replaced with either bovine cardiac TnC or rabbit fast-twitch skeletal TnC. The temperature dependence of myofilament Ca sensitivity was then reevaluated. Cooling native cardiac tissue from 22 to 8°C reduced the pCa (-log₁₀ [Ca²⁺]), generating half-maximal tension (K( 1/2 )) from 5.20 ± 0.07 to 4.89 ± 0.08 (SD, n = 14), and also reduced maximum Ca-activated force to 33 ± 6% of its value at 22°C. After extraction of endogenous TnC and reconstitution with cardiac TnC, cooling from 22 to 8°C caused a similar shift in mean K( 1/2 ) from 4.93 ± 0.08 to 4.69 ± 0.06 (n = 7). When skeletal TnC was reconstituted into TnC-extracted ventricular fibers, cooling from 22 to 8°C led to a much smaller mean shift in K( 1/2 ) from 4.88 ± 0.07 to 4.78 ± 0.04 (n = 7). The results show that the magnitude of the cooling-induced shift in myofilament Ca sensitivity observed in the native state (or after reconstitution with cardiac TnC) is significantly reduced if the fiber is reconstituted with skeletal TnC (P < 0.001). This indicates that the temperature dependence of myofilament Ca sensitivity of cardiac muscle can be modified by incorporation of skeletal TnC. Thus Ca binding to TnC plays an important role in determining the temperature dependence of myofilament Ca sensitivity.