Contribution of sodium-calcium exchange to contraction and relaxation in developing human cardiac myocytes

We have shown that the sarcoplasmic reticulum develops during the 2nd trimester in human fetal cardiac myocytes. We measured the contribution of Na-Ca exchange to contractions and relaxation in myocyte clusters and ventricular myocytes from 1st and 2nd trimester human fetal hearts, respectively. Studies were performed under the guidelines of IRB protocol # 5754-96. Myocytes were bathed in normal solution at 30° and field-stimulated at 0.25 Hz. Contractions were measured with an edge detector. During the 1st trimester, rapid application of Na-free external solution increased peak contractions and prolonged relaxation compared to baseline. During the 2nd trimester, there was a larger increase in contractions, and smaller change in relaxation. Cardiac Na-Ca exchanger mRNA levels (RT-PCR), increased between 1st and 2nd trimesters: did not change between the 2nd trimester, 3rd trimester and newborn; and decreased post-natally. The Na-Ca exchanger contributes to contractions in human fetal hearts. Inhibition of Na-Ca exchange during the 2nd trimester may have less effect on relaxation because the sarcoplasmic reticulum is functional.