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 slotblot 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 F etal and newborn hearts from a variety of species exhibit profound differences in contraction and relaxation processes compared with adults.1-3 Many developmental changes in cardiac contractile function have been attributed to alterations in Ca2+ regulation by the cardiomyocyte.1'2 Ultrastructural and biochemical analyses indicate that Ca2' regulation by the sarcoplasmic reticulum (SR) of the immature heart is deficient compared with that of the adult heart.2,4,5 To compensate for an underdeveloped SR, immature cardiomyocytes appear to be relatively more dependent on transsarcolemmal Ca21 fluxes for directly regulating Ca21 concentrations at the contractile proteins during excitation-contraction coupling.1'6-8 The major Ca2+-regulatory proteins of the sarcolemma (SL) include the voltage-sensitive Ca21 channels and the Na+-Ca2' exchanger. Several studies indicate that voltage-sensitive Ca21 channels are functionally diminished in early development of the chicken and rabbit heart, despite a relatively greater dependence on transsarcolemmal Ca21 influx for contraction.9-12 It has been hypothesized that the Na+-Ca2' exchanger may provide an alternative mechanism for Ca21 influx in the newborn rabbit heart.1"'13 In contrast to studies of the developing chicken and rabbit, Cohen and Lederer14 demonstrated that voltage-sensitive Ca 2+ channel curReceived July 6, 1993; accepted November 12, 1993. From the Department of Molecular and Cellular Pharmacology (S.R.B., D.B.Z., M.A.) and the Department of Pediatrics (M.A.), University of South Alabama College of Medicine, Mobile. Correspondence to Michael Artman, MD, Department of Pediatrics, University of South Alabama Medical Center, 2451 Fillingim St, Mobile, AL 36617. were normalized by reprobing the same blots with 32P 5'-endlabeled 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+Ca 2+ exchanger expression involves pretranslational control mechanisms. These results support the concept that during normal cardiac development, Na+-Ca 2+ exchanger expression is maximal near the time of birth and then declines postnatally as Ca 2+ regulation by the sarcoplasmic reticulum reaches functional maturity. (Circ Res. 1994;74:354-359.)