Contribution of the sodium-calcium exchanger to contractions in immature rabbit ventricular myocytes

In immature cardiac myocytes, the sarcoplasmic reticulum is sparse. Thus, we hypothesized that sarcolemmal Ca2+ influx through Na(+)-Ca2+ exchange is the dominant mechanism for modulating intracellular Ca2+ during contractions in fetal and neonatal hearts. We measured Na(+)-Ca2+ exchange currents in neonatal and adult rabbit ventricular cells using a rapid solution switch into 0 mM external Na+. The current densities (mean +/- SEM) were larger in 8 neonatal cells than in 10 adult cells (5.4 +/- 1.38 versus 1.65 +/- 0.25 pA/pF). Intracellular Ca2+ transients after inhibiting the sarcoplasmic reticulum with ryanodine and thapsigargin were unchanged in 15 neonatal cells, but decreased in 15 adult cells to 78.9 +/- 5.6% of baseline. When the Ca2+ channels were also inhibited by adding nifedipine, Ca2+ transients from Na(+)-Ca2+ exchange were 30.0 +/- 3.5% of baseline in neonatal cells compared with 13.4 +/- 3.4% in adult cells. Simultaneous contractions were a larger percent of baseline in neonatal cells (85.7.6 +/- 6.4%) than in adult cells (78.9 +/- 5.6%) after inhibiting the sarcoplasmic reticulum, and were unmeasureable in many cells from both age groups after inhibiting the Ca2+ channels as well. The ratio of Na(+)-Ca2+ exchanger mRNA to sarcoplasmic reticulum Ca(2+)-ATPase mRNA levels decreased from 1.0 +/- 0.13 to 0.4 +/- 0.03 to 0.26 +/- 0.02 in fetal, neonatal and adult ventricles, respectively. These measurements were consistent with a dominant role for the Na(+)-Ca2+ exchanger in the immature heart.

Effect of glutamate-aspartate reperfusion on postischemic neonatal myocardium

OBJECTIVE

We postulated that L-glutamate- and L-aspartate-enriched perfusate would improve functional recovery of postischemic neonatal rabbit hearts.

METHODS

Isolated working neonatal rabbit hearts were perfused with Krebs-Henseleit buffer and then subjected to 1 hour of hypothermic cardioplegic arrest with St. Thomas' Hospital solution. Hearts were then reperfused with L-glutamate- and L-aspartate-enriched (20 mmol/L) Krebs-Henseleit buffer (AA-enriched Krebs-Henseleit buffer). Hearts reperfused with Krebs-Henseleit buffer alone acted as controls (experiment A). Another group of hearts underwent a similar protocol but were reperfused with the AA-enriched Krebs-Henseleit buffer with correction of the sodium content (experiment B).

RESULTS

Hearts reperfused with AA-enriched Krebs-Henseleit buffer showed a significant decrease in aortic flow at both 15 (p = 0.04) and 30 (p = 0.025) minutes compared with controls. Arrhythmias were frequent. Sodium content of the AA-enriched Krebs-Henseleit buffer was 174 +/- 0.5 mmol/L. In experiment B, hearts reperfused with the AA-enriched Krebs-Henseleit buffer with correction of the sodium content exhibited no difference in aortic flow and cardiac output at either 15 or 30 minutes (p = 0.95 and 0.5 and 0.48 and 0.78, respectively) compared with controls. No arrhythmias were observed. The sodium content of the AA-enriched Krebs-Henseleit buffer was 146 +/- 0.7 mmol/L.

CONCLUSIONS

A beneficial effect on functional recovery of neonatal hearts reperfused with AA-enriched Krebs-Henseleit buffer was not demonstrated.

Sodium-calcium exchange: recent advances

Na-Ca exchange proteins are involved in Ca homeostasis in a wide variety of tissues. Unique Na-Ca exchangers have been identified by molecular biological approaches and it appears that these may represent a superfamily of ion transporters, similar to that identified for ion channels. Major advances in our understanding of these transporters have occurred in the past decade by combining molecular approaches with electrophysiological analyses. The regulatory and transport properties of Na-Ca exchangers are beginning to become understood in molecular detail. It also appears that the physiological roles of Na-Ca exchange may be quite complex. This brief review highlights some recent advances in Na-Ca exchange research obtained through the combination of molecular biological and electrophysiological approaches.

Calcium homeostasis and control of contractility in the developing heart

The Ca2+ concentration within the myocyte is an important determinant of myocardial contractility. Substantial changes in the cellular processes responsible for transport of Ca2+ ions across the sarcolemmal and sarcoplasmic reticulum membranes occur during maturation of the heart. In this article, the mechanisms underlying these changes and their impact on myocardial performance are discussed in detail.

Distribution of the Na+/Ca2+ exchange protein in developing rabbit myocytes

The Na+/Ca2+ exchanger is a major pathway for transmembrane flux of Ca2+ in cardiac cells. Immunolabeling in adult rabbit myocytes showed localization of the Na+/Ca2+ exchanger to the peripheral sarcolemma and especially in the T tubules. Previous studies have also demonstrated higher Na+/Ca2+ exchanger activity in fetal and newborn rabbit hearts in which the T tubular system is not completely developed. Indirect immunofluorescent studies were performed to localize the Na+/Ca2+ exchanger in isolated myocytes from immature (5, 11, 17, and 30 days) and adult rabbits. Cells were incubated with a monoclonal antibody to the exchanger followed by fluorescein-labeled goat anti-mouse antibody. It is found that at 5 days of age the immunofluorescent labeling was very intense and confined to the peripheral sarcolemma. After 11 days of age, localization of labeling followed the development of the T tubules. The exchanger appeared in the T tubules as soon as they were formed. The Na+/Ca2+ exchange protein is abundantly localized to the peripheral sarcolemma before and during the development of T tubule system.

Steady-state mRNA levels of the sarcolemmal Na(+)-Ca2+ exchanger peak near birth in developing rabbit and rat hearts

To functionally compensate for an underdeveloped sarcoplasmic reticulum in immature cardiomyocytes, it has been proposed that the sarcolemmal Na(+)-Ca2+ exchanger may assume a more predominant role for regulating cytosolic Ca2+. Previous studies using sarcolemma prepared from developing rabbit hearts demonstrated that Na(+)-dependent Ca2+ uptake and exchanger protein content were highest at birth and declined postnatally. To further investigate the significance of the Na(+)-Ca2+ exchanger during normal myocardial development, steady-state mRNA levels of the cardiac Na(+)-Ca2+ exchanger were quantitated by Northern blot and slot-blot analyses using poly(A+) RNA isolated from rabbit and rat ventricles at various fetal and postnatal ages. Northern analyses were performed with a 1.35-kb guinea pig cardiac Na(+)-Ca2+ exchanger cDNA probe. Exchanger mRNA levels were quantitated by densitometric scans of the slot blots, and results were normalized by reprobing the same blots with 32P 5'-end-labeled oligo(dT). In both species, exchanger mRNA levels peaked near birth and declined postnatally. Maximal levels were approximately sixfold greater in the late fetal rabbit (gestational day 29) and eightfold greater in the early newborn rat (postnatal day 1) compared with adults of the respective species. The parallel changes in exchanger mRNA and protein levels suggest that developmental regulation of cardiac Na(+)-Ca2+ exchanger expression involves pretranslational control mechanisms. These results support the concept that during normal cardiac development, Na(+)-Ca2+ exchanger expression is maximal near the time of birth and then declines postnatally as Ca2+ regulation by the sarcoplasmic reticulum reaches functional maturity.