Arrhythmogenic calmodulin mutations disrupt intracellular cardiomyocyte Ca2+ regulation by distinct mechanisms.
Bottom Line: LQTS-CaM mutants do not consistently affect L-type Na current in heterologous cells or native cardiomyocytes, suggesting that the Na channel does not contribute to LQTS pathogenesis in the context of CaM mutations.LQTS-CaM mutants led to loss of Ca2+-transient entrainment with the rank order from greatest to least effect: CaM-D130G~CaM-D96V>CaM-F142L.CaM mutations associated with LQTS may not affect L-type Na+ current but may evoke defective Ca2+-dependent inactivation of L-type Ca2+ current.
Affiliation: Department of Physiology, University of Kentucky College of Medicine, Lexington, KY (G.Y., F.H., A.R.H., J.S.).Show MeSH
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Mentions: We investigated whether elevation of cytosolic Ca2+ could drive the loss of entrainment induced by LQTS‐CaM mutants (Figure 6). FVM expressing LQTS‐CaM mutations have significantly higher twitch amplitude than cells expressing WT CaM (Figure 8A). Notably, the mean twitch amplitude for D130G is less than that for F142L and D96V. Similarly, diastolic Ca2+ is higher in cells expressing the lower Ca2+‐affinity CaM mutations D96V and D130G (Figure 8B). CaM has multiple sites of action with respect to Ca2+ homeostasis: CaM modulation of RyR2 and Ca‐CaMKII regulation of SR Ca2+ stores. Consequently, we probed the contribution of LQTS‐CaM mutation to SR Ca2+ load by assessing caffeine‐releasable Ca2+ stores (Figure 8C). The mean SR Ca2+ appears greater in LQTS‐CaM mutant expressing cardiomyocytes compared with cells expressing WT CaM, but the differences did not reach statistical significance (Figure 8D). Examination of the decay of caffeine‐induced Ca2+ transient reveals no significant difference for any of the LQTS‐CaM mutants (Figure 8E). This suggests that Na+/Ca2+ exchange (NCX) function is not altered by LQTS‐CaM mutations. These results are consistent with the model that LQTS‐CaM mutant promoter increased trigger Ca2+ from the slowed decay of ICa,L. NCX is the main surface membrane efflux pathway, thus more LTCC Ca2+ influx paired with unchanged NCX‐based efflux could lead to elevated cytosolic Ca2+.
Affiliation: Department of Physiology, University of Kentucky College of Medicine, Lexington, KY (G.Y., F.H., A.R.H., J.S.).