Increased calcium leak associated with reduced calsequestrin expression in hyperthyroid cardiomyocytes

- Abnormal calcium leak and arrhythmogenic activity is observed in cardiomyocytes of hyperthyroid rats.
- Abnormal cardiomyocyte calcium leak in hyperthyroidism is related to a reduction in calsequestrin-2 expression rather than SR Ca2+ overload.
- The reduction in calsequestrin-2 expression along with increased SERCA pump activity promoted a gain-of-function in RyRs without changes in SR Ca2+ load.

IntroductionCalcium (Ca2+) leak during cardiac diastole is chiefly mediated by intracellular Ca2+ channel/Ryanodine Receptors. Increased diastolic Ca2+ leak has been proposed as the mechanism underlying the appearance of hereditary arrhythmias. However, little is known about alterations in diastolic Ca2+ leak and the specific roles played by key intracellular Ca2+-handling proteins in hyperthyroidism, a known arrhythmogenic condition.AimWe sought to determine whether there were modifications in diastolic Ca2+ leak, based on the recording of Ca2+ sparks and Ca2+ waves; we also investigated changes in the expression and activity of key Ca2+ handling proteins, including ryanodine receptors, Sarco-Endoplasmic Reticulum Ca2+ ATPase pump and calsequestrin in isolated left-ventricular cardiomyocytes isolated from hyperthyroid rats.Materials and methodsElectrocardiography (ECG) recordings were performed in control and hyperthyroid rats. Ca2+ sparks, Ca2+ waves, and electrically-stimulated Ca2+ transients were recorded in Fluo-3-loaded cardiomyocytes from both experimental groups using confocal microscopy. In addition, left-ventricular homogenates and Ryanodine Receptor-enriched membrane fractions were prepared for assessing [3H]-ryanodine binding, hydrolytic ATPase activity of SERCA pump and expression levels of key proteins by Western blot, and cDNA for real-time qPCR.Results and conclusionsExtrasystoles were observed in hearts of hyperthyroid rats by ECG recordings. Arrhythmogenic activity, high incidence of Ca2+ waves, and de novo Ca2+ wavelets −in the absence of sarcoplasmic reticulum Ca2+ overload- were recorded in these cardiomyocytes. The exacerbated diastolic Ca2+ leak and arrhythmogenic activities were related to a diminished expression of calsequestrin along with increased SERCA pump activity, which, in effect, promoted a gain-of-function in RyRs without alterations in SR Ca2+ load, RyR expression or its Ca2+ sensitivity.

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