Differential pH effect on calcium-induced conformational changes of cardiac troponin C complexed with cardiac and fast skeletal isoforms of troponin I and troponin T.

The aim of this study is to investigate the molecular events associated with the deleterious effects of acidosis on the contractile properties of cardiac muscle as in the ischemia of heart failure. We have conducted a study of the effects of increasing acidity on the Ca(2+) induced conformational changes of pyrene labelled cardiac troponin C (PIA-cTnC) in isolation and in complex with porcine cardiac or chicken pectoral skeletal muscle TnI and/or TnT. The pyrene label has been shown to serve as a useful fluorescence reporter group for conformational and interaction events of the N-terminal regulatory domain of TnC with only minimal fluorescence changes associated with C-terminal domain. Results obtained show that the significant decreases at pH 6.0 of site II Ca(2+) affinity of PIA-cTnC when complexed as a binary complex with either cTnI or cTnT are significantly reduced when cTnI is replaced with sTnI or cTnT with sTnT. However, this effect is appreciably diminished when the cTnI and cTnT in the ternary complex are replaced by sTnI and sTnT. The smaller effects in the ternary complex of replacing both cTnI and cTnT by their skeletal counterparts on depressing the Ca(2+) affinity from pH 7.0 to 6.0 arise from TnI replacement. Thus, changes in TnC conformation resulting from isoform-specific interactions with TnI and TnT could be an integral part of the effect of pH on myofilament Ca(2+)sensitivity.