Beta-Adrenoceptor Stimulation Reveals Ca²⁺ Waves and Sarcoplasmic Reticulum Ca²⁺ Depletion in Left Ventricular Cardiomyocytes from Post-Infarction Rats with and without Heart Failure

<div><p>Abnormal cellular Ca²⁺ handling contributes to both contractile dysfunction and arrhythmias in heart failure. Reduced Ca²⁺ transient amplitude due to decreased sarcoplasmic reticulum Ca²⁺ content is a common finding in heart failure models. However, heart failure models also show increased propensity for diastolic Ca²⁺ release events which occur when sarcoplasmic reticulum Ca²⁺ content exceeds a certain threshold level. Such Ca²⁺ release events can initiate arrhythmias. In this study we aimed to investigate if both of these aspects of altered Ca²⁺ homeostasis could be found in left ventricular cardiomyocytes from rats with different states of cardiac function six weeks after myocardial infarction when compared to sham-operated controls. Video edge-detection, whole-cell Ca²⁺ imaging and confocal line-scan imaging were used to investigate cardiomyocyte contractile properties, Ca²⁺ transients and Ca²⁺ waves. In baseline conditions, i.e. without beta-adrenoceptor stimulation, cardiomyocytes from rats with large myocardial infarction, but without heart failure, did not differ from sham-operated animals in any of these aspects of cellular function. However, when exposed to beta-adrenoceptor stimulation, cardiomyocytes from both non-failing and failing rat hearts showed decreased sarcoplasmic reticulum Ca²⁺ content, decreased Ca²⁺ transient amplitude, and increased frequency of Ca²⁺ waves. These results are in line with a decreased threshold for diastolic Ca²⁺ release established by other studies. In the present study, factors that might contribute to a lower threshold for diastolic Ca²⁺ release were increased THR286 phosphorylation of Ca²⁺/calmodulin-dependent protein kinase II and increased protein phosphatase 1 abundance. In conclusion, this study demonstrates both decreased sarcoplasmic reticulum Ca²⁺ content and increased propensity for diastolic Ca²⁺ release events in ventricular cardiomyocytes from rats with heart failure after myocardial infarction, and that these phenomena are also found in rats with large myocardial infarctions without heart failure development. Importantly, beta-adrenoceptor stimulation is necessary to reveal these perturbations in Ca²⁺ handling after a myocardial infarction.</p>