Inhibiting NCX delays the early onset of Ca2+ alternans in myocytes from spontaneously hypertensive rats (SHR)

Spontaneously hypertensive rats (SHR) are more susceptible to cardiac alternans, a precursor to arrhythmias. Ca²⁺ alternans is a beat-to-beat oscillation in Ca²⁺ transient amplitude at constant stimulation frequency. We previously found that the early onset of alternans in SHR hearts is associated with prolonged sarcoplasmic reticulum (SR) Ca²⁺ release refractoriness, primarily influenced by SR Ca²⁺ load and RyR2 sensitivity. The Na⁺/Ca²⁺ exchanger (NCX) is critical for regulating intracellular Ca²⁺. In SHR myocytes, elevated intracellular Na⁺ and Ca²⁺ levels and prolonged action potential duration along with structural changes in T-tubules, where NCX is primarily located, could alter NCX function. The effect of NCX on Ca²⁺ alternans is complex: enhanced NCX activity may hasten Ca²⁺ decay, offering protection, but also reduce SR Ca²⁺ content, potentially promoting alternans. This study aimed to investigate NCX's role in alternans in SHR hearts using pharmacological and computational approaches. ORM-10962, a selective NCX inhibitor, increased Ca²⁺ transient amplitude and SR Ca²⁺ content in SHR myocytes, but had no effect on normotensive myocytes, suggesting preferential forward mode activation in SHR. The inhibitor delayed alternans onset and normalized SR Ca²⁺ release refractoriness. These findings were confirmed by the computational model. Further experiments showed that blocking of NCX's reverse mode had no impact on Ca²⁺ alternans in SHR myocytes. The results suggest that NCX hyperactivity in SHR myocytes prevents the necessary increase in SR Ca²⁺ load to overcome the prolonged refractoriness. The findings highlight NCX inhibition as a potential therapeutic strategy to prevent Ca²⁺ alternans and reduce arrhythmic risk in hypertensive conditions.