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

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.