CONTRIBUTIONS OF TROPONIN-I AND TROPONIN-C TO THE ACIDIC PH-INDUCED DEPRESSION OF CONTRACTILE CA2+ SENSITIVITY IN CARDIOTRABECULAE

Acid pH diminishes the Ca2+ sensitivity for force generation in both cardiac and skeletal muscles, but the mechanisms for these remain undetermined. In permeabilized (skinned) single myofibers of fast-twitch skeletal muscle of the rat, we find that pCa(50) of the pCa-force relationship was 5.73 in pH 7 and 5.02 in pH 6.2 (Delta PKskeletal = pCa(50) in pH 7 - pCa(50) in pH 6.2 = 0.71 pCa unit); on the other hand, in skinned cardiotrabeculae, the pCa(50) was 5.79 in pH 7 decreasing to 4.14 in pH 6.2 (Delta pK(cardiac) = 1.65 pCa units). We have used this large differential between cardiac/skeletal Delta pKs to probe the mechanisms of the pH effects. Since troponin C (TnC) and troponin I (TnI) each have a central role in the Ca2+ switch, we exchanged these proteins in cardiac muscle with their skeletal counterparts and reinvestigated the pH effects. Firstly, with fast-twitch skeletal muscle (sTnC) substituting for 80% of the endogenous cardiac TnC (cTnC), the cardiac pH effect was decreased marginally (modified Delta pK = 1.39 pCa units). This TnC-mediated change was further probed with two distinct cardiac-skeletal TnC chimeras, c1/s and CBc1/s (the Ca2+-binding c1/s), in which a majority of the N-terminal 41 amino acid residues was made cardiac and the rest skeletal [Gulati, J., and Rao, V. G. (1994) Biochemistry 33, 9052-9056]. The phenotype shift following sTnC/cTnC exchange in the trabeculae was blocked when c1/s was used in lieu of sTnC; on the other hand, interestingly, CBc1/s exactly mimicked sTnC. We conclude that cardiac 1-41 residues contain the effective pH sensor of cTnC. Next, to monitor the TnI effect, both cTnI and cTnC were exchanged with an sTnI + sTnC complex. This obliterated the cardiac/skeletal pH differential, suggesting that cTnI was dominant in specifying the cardiac type pH effect. The results of a hybrid complex (cTnI + sTnC) were also consistent with this. After accounting for the residual components following TnC and TnI exchanges, we estimated that 31% of the cardiac/skeletal pH differential was caused by TnC and a dominant 66% by TnI.