Work

When NCX switches sides: Experimental and computational insights into Ca 2+ regulation in the heart

Year: 2026

Type: article

Abstract: The Na + /Ca 2+ exchanger (NCX) transports Ca 2+ and Na + through the plasma membrane of cardiomyocytes. NCX dysregulation has been related to diastolic dysfunction. NCX inhibition has been identified as a potential therapeutic approach. It can accelerate the decay of the cytosolic Ca 2+ concentration ([Ca 2+ ] i ) and improve impaired cardiomyocyte relaxation. We hypothesized that this counterintuitive effect is explained by the subcellular arrangement of NCX and local ion gradients within the intracellular Ca 2+ release units. In a parallel model‐based and experimental approach, we re‐evaluated the location of NCX with regard to the dyadic cleft and its role in modulating [Ca 2+ ] i . Stimulated emission depletion imaging revealed NCX in close proximity to junctophilin (the marker for the dyadic cleft). We simulated [Ca 2+ ] dynamics in the dyadic cleft considering Ca 2+ channels, NCX molecules and local concentration gradients. Positioning NCX inside the dyadic cleft in our computational model matched its action on spark rate. In forward mode (Ca 2+ out, Na + in) NCX decreased spontaneous Ca 2+ release events (spark rate) in simulations and imaging experiments, while in reverse mode it increased them. In paced cardiomyocytes, NCX inhibition consistently increased diastolic [Ca 2+ ]. The effects of NCX inhibition on transient amplitude and peak, however, depended on extracellular [Ca 2+ ] o suggesting a role of reverse‐mode NCX activity at high [Ca 2+ ] o . NCX inhibition prolonged the early rise of [Ca 2+ ], corroborating that reverse‐mode NCX facilitates the rapid initial increase of [Ca 2+ ] i during excitation. Our combined imaging, modelling and functional data support the hypothesis that NCX resides in the dyadic cleft where it bidirectionally shapes Ca 2+ transients and spark activity. image Key points Our data suggest that Na + /Ca 2+ exchanger (NCX) is localized inside the dyadic cleft in cardiomyocytes, close to junctophilin, as shown by stimulated emission depletion imaging. Computational modelling confirmed NCX's dyadic positioning. It is critical for its effects on local Ca 2+ signalling. NCX bidirectionally modulates Ca 2+ dynamics: forward mode reduces, reverse mode increases spontaneous Ca 2+ sparks. NCX inhibition raises diastolic cytosolic Ca 2+ and alters Ca 2+ transient amplitude depending on extracellular Ca 2+ levels. Reverse‐mode NCX facilitates rapid initial Ca 2+ rise during excitation, highlighting its role in cardiac Ca 2+ regulation and potential therapy.

Source: The Journal of Physiology

DOI: 10.1113/jp290207