Impaired relaxation despite upregulated calcium-handling protein atrial myocardium from type 2 diabetic patients with preserved ejection fraction

Abstract

BackgroundDiastolic dysfunction is a key factor in the development and pathology of cardiac dysfunction in diabetes, however the exact underlying mechanism remains unknown, especially in humans. We aimed to measure contraction, relaxation, expression of calcium-handling proteins and fibrosis in myocardium of diabetic patients with preserved systolic function.MethodsRight atrial appendages from patients with type 2 diabetes mellitus (DM, n = 20) and non-diabetic patients (non-DM, n = 36), all with preserved ejection fraction and undergoing coronary artery bypass grafting (CABG), were collected. From appendages, small cardiac muscles, trabeculae, were isolated to measure basal and β-adrenergic stimulated myocardial function. Expression levels of calcium-handling proteins, sarcoplasmic reticulum Ca2+ ATPase (SERCA2a) and phospholamban (PLB), and of β1-adrenoreceptors were determined in tissue samples by Western blot. Collagen deposition was determined by picro-sirius red staining.ResultsIn trabeculae from diabetic samples, contractile function was preserved, but relaxation was prolonged (Tau: 74 ± 13 ms vs. 93 ± 16 ms, non-DM vs. DM, p = 0.03). The expression of SERCA2a was increased in diabetic myocardial tissue (0.75 ± 0.09 vs. 1.23 ± 0.15, non-DM vs. DM, p = 0.007), whereas its endogenous inhibitor PLB was reduced (2.21 ± 0.45 vs. 0.42 ± 0.11, non-DM vs. DM, p = 0.01). Collagen deposition was increased in diabetic samples. Moreover, trabeculae from diabetic patients were unresponsive to β-adrenergic stimulation, despite no change in β1-adrenoreceptor expression levels.ConclusionsHuman type 2 diabetic atrial myocardium showed increased fibrosis without systolic dysfunction but with impaired relaxation, especially during β-adrenergic challenge. Interestingly, changes in calcium-handling protein expression suggests accelerated active calcium re-uptake, thus improved relaxation, indicating a compensatory calcium-handling mechanism in diabetes in an attempt to maintain diastolic function at rest despite impaired relaxation in the diabetic fibrotic atrial myocardium. Our study addresses important aspects of the underlying mechanisms of diabetes-associated diastolic dysfunction, which is crucial to developing new therapeutic treatments.

Chamber-specific changes in calcium-handling proteins in the type 2 diabetic human heart with preserved ejection fraction

“…Recently, we showed that right atrial (RA) muscles from diabetic patients with preserved ejection fraction (EF) and coronary artery disease (CAD) had preserved contractile function with impaired relaxation [2]. This occurred despite an unexpectedly increased sarcoplasmic reticulum calcium ATPase (SERCA2a)/phospholamban (PLB) protein ratio [2], which would promote an increased cytosolic calcium removal and hence improved relaxation.…”

“…The epicardial LV biopsies and RA appendages were collected and prepared for protein analysis, using specific antibodies against SERCA2a, PLB, PLB-S16 and PLB-T17 [2]. Differences amongst groups were compared via a t-test, chi-square or a two-way ANOVA followed by a Bonferroni post-hoc test.…”

“…The RA sample was taken prior to cardioplegia, whereas the epicardial LV biopsy was obtained from a scar-free area of the anterior wall, following cardioplegia. The local Human Ethics Committee approved the study and all patients provided informed consent.All patients had a pre-operative transthoracic echocardiogram and functional parameters were determined as previously described [2]. The epicardial LV biopsies and RA appendages were collected and prepared for protein analysis, using specific antibodies against SERCA2a, PLB, PLB-S16 and PLB-T17 [2].…”

“…All patients had a pre-operative transthoracic echocardiogram and functional parameters were determined as previously described [2]. The epicardial LV biopsies and RA appendages were collected and prepared for protein analysis, using specific antibodies against SERCA2a, PLB, PLB-S16 and PLB-T17 [2].…”

The role of CaMKII in diabetic heart dysfunction

“…release in the nonfailing myocardium of type II diabetic patients. A more recent study by Lamberts et al [88] reported reduced diastolic function in isolated trabeculae from the right atrial appendage (RAA) in type II diabetic patients, demonstrated by a slower minimum rate of relaxation, prolonged 50 % relaxation time and time constant of relaxation. Despite the reduced diastolic myocardial function in the trabeculae, the expression of SERCA2a was increased in the samples of the diabetic patients, while PLB expression was decreased.…”