Differential effects of phosphorylation of regions of troponin I in modifying cooperative activation of cardiac thin filaments

Ischemia and heart failure are associated with protein kinase C (PKC) dependent phosphorylation of cardiac troponin I (cTnI). We investigated the effect of phosphorylation of cTnI PKC sites S43, S45 and T144 under normal (pH 7.0) and acidic (pH 6.5) conditions on tension in skinned fiber bundles from a mouse heart. To mimic the PKC phosphorylation, we exchanged troponin (cTn) in these fiber bundles with cTn complex containing either cTnI-(S43E/S45E) or cTnI-(T144E). We determined how pseudo-phosphorylation and acidic pH affect activation of thin filaments by strongly bound crossbridges by use of n-ethyl maleimide (NEM-S1) to mimic rigor. We hypothesized that PKC phosphorylation of cTnI amplifies the effect of ischemic/hypoxic conditions to depress myofilament force and Ca2+-responsiveness by reducing the ability of rigor crossbridge to activate force. Pseudo-phosphorylation of cTnI at S43/S45 exacerbated the effect of acidic pH to induce a rightward shift in the Ca2+-tension relation. Under acidic conditions, fibers regulated by cTnI-(S43E/S45E) demonstrated a significant reduction in the ability of NEM-S1 to recruit cycling crossbridges, when compared to controls regulated by cTnI. Similar effects of pseudo-phosphorylation of cTnI-(T144) occurred, but to a lesser extent that those of pseudo-phosphorylation of S43/S45. We conclude that under acidic conditions PKC phosphorylation of cTnI residues at S43/S45 and at T144 is likely to have differential, but significant effects in depressing the ability of both Ca2+ and rigor crossbridges to activate force generation. Although these effects of PKC dependent phosphorylation may be maladaptive in heart failure, they may also spare ATP consumption and be cardio-protective in ischemia.