2026
The Journal of Physiology

When NCX switches sides: Experimental and computational insights into Ca2+ regulation in the heart

Authors:

Wilhelm Neubert, Judith Hüttemeister, Paulina Sander, Nagaiah Chamakuri, Felix Hohendanner, Martin Falcke, Frank R. Heinzel

Keywords:

Na+/Ca2+ exchanger; NCX; cardiomyocytes; dyadic cleft; cardiac myocytes; diastolic dysfunction; excitation-contraction coupling

Abstract:

The Na⁺/Ca²⁺ exchanger (NCX) transports Ca²⁺ 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 Ca²⁺ concentration ([Ca²⁺]_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 Ca²⁺ 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 [Ca²⁺]_i. Stimulated emission depletion imaging revealed NCX in close proximity to junctophilin (the marker for the dyadic cleft). We simulated [Ca²⁺] dynamics in the dyadic cleft considering Ca²⁺ 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 (Ca²⁺ out, Na⁺ in) NCX decreased spontaneous Ca²⁺ release events (spark rate) in simulations and imaging experiments, while in reverse mode it increased them. In paced cardiomyocytes, NCX inhibition consistently increased diastolic [Ca²⁺]. The effects of NCX inhibition on transient amplitude and peak, however, depended on extracellular [Ca²⁺]_o suggesting a role of reverse-mode NCX activity at high [Ca²⁺]_o. NCX inhibition prolonged the early rise of [Ca²⁺], corroborating that reverse-mode NCX facilitates the rapid initial increase of [Ca²⁺]_i during excitation. Our combined imaging, modelling and functional data support the hypothesis that NCX resides in the dyadic cleft where it bidirectionally shapes Ca²⁺ transients and spark activity.