Molecular basis of depression of Ca2+ sensitivity of tension by acid pH in cardiac muscles of the mouse and the rat*

Abstract Background: Acid pH decreases the Ca 2+ sensitivity of myocardial tension generation, and recent studies have suggested that regulatory proteins are involved. The current study defines the molecular basis of this effect on troponin C (TnC) and troponin I (TnI) and also addresses previous differences between the rat and mouse. Methods and Results: Endogenous cardiac TnC and cardiac TnI in isolated trabeculae frommice and rats were exchanged with their fast-twitch skeletal muscle counterparts. A cardiac-skeletal TnC chimera was used to define the target region for proton action on cardiac TnC. Finally; cardiac TnC and skeletal TnC were genetically modified by insertion of a tryptophan for phenylalanine-26 to probe the pH effects with fluorescence spectroscopy. The pH 6.2 effects on Ca 2+ sensitivity of force development in mouse and rat cardiotrabeculae are largely accounted for by the proton influences on TnC (23%) and TnI (53%). In cardiac TnC, residues 1 to 41 provide the target region. Comparison of the Ca 2+ -induced fluorescence in isolated cardiac TnC and skeletal TnC also indicated a greater pH effect in the cardiac isoform. Conclusions: The studies provide firm evidence that both TnC and TnI moieties are involved inthe mechanism of acidosis causing reduction in the Ca sensitivity of force development in the myocardium. The findings rule out the possibility of interspecies variations in the underlying mechanisms. The genetically designed TnCs and a chimera demonstrate that the observed TnC-mediated difference in the pH effects on Ca 2+ + sensitivity of tension between cardiac and skeletal muscles is preserved in these isolated proteins. The N-terminal amino acid residues 1 to 41 in cardiac TnC are established as the pH sensor of this protein in the mouse as in the rat.