Influence of a constitutive increase in myofilament Ca2+-sensitivity on Ca2+-fluxes and contraction of mouse heart ventricular myocytes

Abstract

Chronic increases in myofilament Ca2+-sensitivity in the heart are known to alter gene expression potentially modifying Ca2+-homeostasis and inducing arrhythmias. We tested age-dependent effects of a chronic increase in myofilament Ca2+-sensitivity on induction of altered alter gene expression and activity of Ca2+ transport systems in cardiac myocytes. Our approach was to determine the relative contributions of the major mechanisms responsible for restoring Ca2+ to basal levels in field stimulated ventricular myocytes. Comparisons were made from ventricular myocytes isolated from non-transgenic (NTG) controls and transgenic mice expressing the fetal, slow skeletal troponin I (TG-ssTnI) in place of cardiac TnI (cTnI). Replacement of cTnI by ssTnI induces an increase in myofilament Ca2+-sensitivity. Comparisons included myocytes from relatively young (5–7 months) and older mice (11–13 months). Employing application of caffeine in normal Tyrode and in 0Na+ 0Ca2+ solution, we were able to dissect the contribution of the sarcoplasmic reticulum Ca2+ pump (SR Ca2+-ATPase), the Na+/Ca2+ exchanger (NCX), and “slow mechanisms” representing the activity of the sarcolemmal Ca2+ pump and the mitochondrial Ca2+ uniporter. The relative contribution of the SR Ca2+-ATPase to restoration of basal Ca2+levels in younger TG-ssTnI myocytes was lower than in NTG (81.12 ± 2.8% vs 92.70 ± 1.02%), but the same in the older myocytes. Younger and older NTG myocytes demonstrated similar contributions from the SR Ca2+-ATPase and NCX to restoration of basal Ca2+. However, the slow mechanisms for Ca2+ removal were increased in the older NTG (3.4 ± 0.3%) vs the younger NTG myocytes (1.4 ± 0.1%). Compared to NTG, younger TG-ssTnI myocytes demonstrated a significantly bigger contribution of the NCX (16 ± 2.7% in TG vs 6.9 ± 0.9% in NTG) and slow mechanisms (3.3 ± 0.4% in TG vs 1.4 ± 0.1% in NTG). In older TG-ssTnI myocytes the contributions were not significantly different from NTG (NCX: 4.9 ± 0.6% in TG vs 5.5±0.7% in NTG; slow mechanisms: 2.5 ± 0.3% in TG vs 3.4 ± 0.3% in NTG). Our data indicate that constitutive increases in myofilament Ca2+-sensitivity alter the relative significance of the NCX transport system involved in Ca2+-homeostasis only in a younger group of mice. This modification may be of significance in early changes in altered gene expression and electrical stability hearts with increased myofilament Ca-sensitivity.

Comparable calcium handling of human iPSC-derived cardiomyocytes generated by multiple laboratories

“…Furthermore, contribution of SERCA (61%) and NCX (31%) in hiPSC-CMs was not significantly different from adult rabbit-CMs (SERCA 60%, NCX 32%). As expected [29, 30], SERCA (89%) dominated in mouse-CMs. On the other hand, the relative contribution of non-NCX removal pathways was significantly larger in hiPSC-CMs (8-9%) compared to rabbit-CMs (4%) and mouse-CMs (2%).…”

“…Cytoplasmic Ca removal is mediated by three major pathways: (1) Ca uptake into the SR by the sarcoendoplasmic reticulum CaATPase (SERCA), (2) Ca efflux from the cell via the NCX, and (3), non-NCX non-SR mediated pathways such as plasma lemma Ca ATPase and mitochondria [25, 29]. The contribution of the individual Ca removal mechanisms has been studied extensively in non-human CMs [30, 31] but not yet in hiPSC-CMs. SR-independent Ca removal was tested by rapid caffeine application and was significantly slower in hiPSC-CMs compared to rabbit and mouse CMs (Fig.…”

The naked mole-rat exhibits an unusual cardiac myofilament protein profile providing new insights into heart function of this naturally subterranean rodent

“…Interestingly, it was shown that the expression of ssTnI in the mouse heart results in a leftward shift of the force-pCa relationship, increasing the myofilament calcium sensitivity [41, 55]. Conversely, reduced phosphorylation of MLC-2 results in a rightward shift of the force-pCa relationship [11,20].…”

Calcium Signaling and Cardiac Arrhythmias

“…183 In this model, constitutive increase in Ca 2+ sensitivity is associated with increased expression of NCX which might result in increased I NCX current to maintain Ca 2+ homeostasis during diastole when SERCA2a is also reduced. 184 Interestingly, this observation was noted in younger but not in older mice, reflecting the early age of onset of arrhythmias in TNNT2 -positive subjects. The reentry hypothesis is supported by evidence that TNNI3 mutations can increase spatial dispersion of activation times across the myocardium, thereby promoting reentry.…”