Changes in calcium cycling precede cardiac dysfunction during autoimmune myocarditis in mice

Myocardial inflammation contributes to the development of dilated cardiomyopathy, as well as other cardiac diseases. We have previously shown decreased left ventricular function in mice with autoimmune myocarditis. To test the hypothesis that decreased function is mediated by changes in contractility and/or Ca2+ cycling, we isolated cardiac myocytes from mice with myocarditis and age-matched controls at two time points: day 18 (prior to cardiac dysfunction) and day 35 (during cardiac dysfunction). We measured cell shortening and the Ca2+ transient simultaneously at 28 degrees C and 0.3 Hz. We also quantified proteins which regulate contractility and [Ca2+](i), using Western blot analysis. Results showed no change in cell shortening or systolic Ca2+ on day 18, despite a significant reduction in diastolic Ca2+. By day 35, the decrease in diastolic Ca2+ was accompanied by significantly reduced cell shortening and a decrease in the systolic Ca2+ transient. Protein levels of the sarcoplasmic reticulum Ca2+ ATPase were unchanged at both time points, while phospholamban and the sodium/calcium exchanger were significantly reduced in myosin-immunized mice at both time points. Calsequestrin was unchanged at day 18, but was significantly reduced in the myosin-immunized mice on day 35. Results of this study suggest that decreased diastolic Ca2+, as well as protein levels of phospholamban and the sodium/calcium exchanger, may actually contribute to disease progression in autoimmune myocarditis, while changes in calsequestrin may be related to systolic dysfunction in this model.