Calcium‐dependent protein kinase C alpha and the frequency‐dependent increase in phosphorylation of troponin I in failing hearts

Abstract

In healthy myocardium the force‐frequency relation, one of the key factors modulating cardiac performance, is positive, while in failing myocardium a negative relation is found. This difference has been ascribed to changes in calcium homeostasis and, more recently, to changes in calcium sensitivity through increased phosphorylation of troponin I (TnI). Our objective was to determine whether altered activation of calcium‐dependent protein kinase C alpha (PKCα) determines the increase in phosphorylation of TnI with frequency in failing hearts.Right ventricular heart failure was induced by monocrotaline (MCT; 80mg/kg). After 4 weeks isovolumic pressure–frequency relations were determined in Langendorff‐perfused hearts in a control (n=19) and MCT‐treated group (n=19). Thereafter, hearts were kept quiescent or paced at 9 Hz for 60 min and flash frozen to determine phosphorylation of TnI and PKCα by 1D‐gel electrophoresis.The pressure–frequency relation was constant in control and negative in MCT group. Phosphorylation of TnI did not depend on frequency in control but increased in MCT group. Protein expression and phosphorylation levels of PKCα were both increased with frequency in MCT group.These results indicate that frequency‐dependent changes in PKC‐phosphorylatable sites of TnI in failing hearts originate from an altered balance in calcium‐dependent kinase/phosphatase activity.