Boerth S R, Zimmer D B, Artman M
Department of Molecular and Cellular Pharmacology, University of South Alabama College of Medicine, Mobile 36617.
Circ Res. 1994 Feb;74(2):354-9. doi: 10.1161/01.res.74.2.354.
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.
为了在功能上补偿未成熟心肌细胞中发育不全的肌浆网，有人提出肌膜钠钙交换体可能在调节胞质钙方面发挥更主要的作用。先前使用从发育中的兔心脏制备的肌膜进行的研究表明，钠依赖的钙摄取和交换体蛋白含量在出生时最高，出生后下降。为了进一步研究钠钙交换体在正常心肌发育过程中的意义，使用从不同胎儿期和出生后各年龄段的兔和大鼠心室分离的聚腺苷酸RNA，通过Northern印迹和狭缝印迹分析对心脏钠钙交换体的稳态mRNA水平进行了定量。Northern分析使用1.35kb的豚鼠心脏钠钙交换体cDNA探针进行。通过对狭缝印迹进行光密度扫描来定量交换体mRNA水平，并用32P 5'-末端标记的寡聚（dT）重新探测相同的印迹来对结果进行标准化。在这两个物种中，交换体mRNA水平在出生时达到峰值，出生后下降。与各自物种的成年个体相比，晚期胎儿兔（妊娠第29天）的最大水平大约高六倍，新生大鼠早期（出生后第1天）高八倍。交换体mRNA和蛋白水平的平行变化表明，心脏钠钙交换体表达的发育调节涉及转录前控制机制。这些结果支持这样的概念，即在正常心脏发育过程中，钠钙交换体的表达在出生时达到最大，然后在出生后随着肌浆网对钙的调节达到功能成熟而下降。
