Changes in ryanodine receptor (RyR) function and synchronization of Ca2+ release from the sarcoplasmic reticulum (SR) of cardiomyocytes in ischemic cardiomyopathy

Abstract

SUMMARY Heart failure (HF) remains an important health problem and economic burden in the current society (1). Improvements in acute myocardial infarction (MI) treatment, better drug therapy and secondary prevention lead to more patients surviving the initial cardiac damage and developing HF at a later timepoint. The focus is now on the underlying mechanisms of evolution to HF. There has been significantevidence that disturbance of the intracellular Ca2+ homeostasis in cardiomyocytes plays an important role in the pathophysiology of HF. A hallmark feature of HF pathogenesis is impaired sarcoplasmic reticulum (SR) function, with reduced SR Ca2+ load. In addition to defective SR Ca2+ resequestration caused by decreased sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) activity, several lines of evidence revealed the existence of a diastolic Ca2+ leak mediated by defective regulation of ryanodine receptors (RyRs). The overall aim of this thesis was to investigate the roleof changes in RyR function in regional contractile dysfunction in ischemic cardiomyopathy (ICM). We investigated the effect of proximity of T-tubules (TTs) on intrinsic RyR function. In cardiomyocytes isolated from healthy control pigs, we found the existence of a functional link between Ca2+ spark duration and TT proximity. Our results shown that the Na+/Ca2+ exchanger (NCX) plays an important role modulating diastolic Ca2+ release. NCX in the TTs was able to shorten the duration of spontaneous diastolic Ca2+ release through RyR. In a pig model of ischemic cardiomyopathy, this regulatory mechanism was lost after cellular remodeling, and a reduction of TTs and NCX prolonged spark duration close to themembrane, which might increase the propensity for generation of spontaneous Ca2+ waves. We next evaluated regional cellular remodelingand Ca2+ handling in the area remote to a moderate size MI in a pig model of ischemic cardiomyopathy without overt HF. We compared these findings to our earlier data on changes in the MI adjacent area. Our data indicate that, opposite to loss of TTs, dyssynchrony and slowing of Ca2+ release in the area adjacent to the MI, the area remote to the MI shows maintained TT density and even increased efficiency of excitation-contraction coupling. While RyR function and spark-mediated Ca2+ leak was potentiated in both substrates, differences in spark frequency suggest differences in the underlying mechanisms. Different triggers for remodeling -ischemia vs. stretch- are probably important, and different signaling pathways are expected to be involved in remodeling in the different areas of the heart. In the last part of our study, we investigated whether RyR function and Ca2+ handling could be modulated during post-MI remodeling. In a mouse model of ischemic cardiomyopathy, we investigated theeffect of voluntary exercise early after MI on post-MI remodeling. Early voluntary exercise training after MI restored cell contraction to normal values, predominantly because of changes in myofilament Ca2+ response, and had a beneficial effect on diastolic Ca2+ handling. However, the beneficial effect was not a complete reversal of remodeling as hypertrophy and loss of repolarizing K+ currents were not affected. Secondly, we investigated the effect of cardio-specific FK506-binding protein (12.6 kDa, FKBP12.6) overexpression on RyR function in a transgenic mouse model,with and without MI. At baseline, FKPB12.6 overexpression could effectively reduce RyR open probability (Po) and fractional Ca2+ release from the SR, with maintained [Ca2+]i transient (and contraction) by an increased SR Ca2+ content. However, reduction of RyR Po by this approach appeared insufficient to prevent left ventricular hypertrophy and reduce post-MI remodeling, which is even exaggerated in transgenic mice.To conclude, this work supports a role for TTs in modulation of RyR function in healthy cardiomyocytes. During remodeling in ICM, RyR function can be modulated because of alterations in TT density and/or because of intrinsic changes in RyR, depending on the location in the heart and the nature of the stimulus for remodeling. RyR function can be modulated by different therapeutic approaches. However, the exact role of such an approach is still undetermined and it is important to remember that post-MI remodeling includes more than only changes in RyR function.status: publishe