Role of Na+/Ca2+ exchange in contraction and relaxation in immature ventricular myocytes

Transient changes in Ca2+ activity within the cytosol are important determinants of myocardial contraction and relaxation. The cellular processes responsible for regulation of intracellular Ca2+ concentration undergo major changes during maturation of the heart. The immature rabbit heart relies on transsarcolemmal Ca2+ entry and efflux via the Na+/Ca2+ exchanger for contraction and relaxation. Exchanger activity and amounts of immunoreactive protein and mRNA are increased in immature rabbit hearts compared to adults. Further support for the importance of Na+/Ca2+ exchange in the immature heart comes from observations that exchanger current density in newborn is about four fold higher than adult rabbit myocytes and exchanger activity is sufficient for normal contraction and relaxation in neonatal myocytes. Thus, in contrast to the well described role of the Ne+/Ca2+ exchanger in mature myocardium, the Na+/Ca2+ exchanger appears to be the predominate pathway for calcium transport to and removal from the contractile elements in immature ventricular myocytes.