Role of Na⁺-Ca²⁺ exchange in neonatal and adult ventricular cells

The present paper expanded an integrated mathematical model, which successfully simulated developmental changes in electrophysiological activities of rodent ventricular cells, to contrive models that reflect one of the hypothesis on the role of Na⁺-Ca"⁺ exchange in neonatal ventricular cell: the Na⁺-Ca⁺ exchange assumes a relatively greater role in neonatal cells with a morphologically sparse SR. Our simulation with the expanded model showed that neonatal ventricular cell consumed larger amount of adenosine triphosphate (ATP) than adult ventricular cell did, owing to long relaxation time. The amount of ATP consumed per beat was reduced to the amount equivalent to adult ventricular cell, as increasing the relative current density of Na⁺-Ca²⁺ exchange (F _NCx) by 4-fold, which is consistent with the observed current density in neonatal rabbit ventricular cell. To further validate the hypothesis, the relative functional activity of SR (F_SR) was incrementally increased from neonatal to adult levels and subsequently increased FNCX, and found that increasing F_Ncx had negative impact on sarcomere relaxation velocity in ventricular cell model with large F_SR. All the results thus theoretically supported the hypothesis that the Na⁺-Ca²⁺ exchange assumes a relatively greater role in neonatal cells with a morphologically sparse SR.