Expressional analysis of the cardiac Na-Ca exchanger in rat development and senescence

The cardiac Na-Ca exchanger (NCX) serves as the main calcium extrusion mechanism in heart muscle and is important in maintaining intracellular calcium homeostasis. The accumulations of NCX RNA and protein are known to be regulated in cardiac hypertrophy, by thyroid hormone and during postnatal development. In this study the temporal and spatial patterns of NCX mRNA and protein accumulations were examined, and nuclear run-on assays performed. NCX is highly expressed in late fetal and neonatal rat hearts, decreasing to adult levels by 20 days after birth for RNA (P < 0.05, fetal and 1 neonatal day old (1 ND) versus 20 day old (20 ND)). Maximal protein expression is seen in 19 embryonic day (ED) old hearts, and reaches adult levels sometime after 20 neonatal days. (P < 0.05, fetal versus adult). Spatially, NCX is homogenously expressed in early embryonic and fetal heart, followed by a decline after birth. The protein levels decline more slowly suggesting a long protein half-life. The lowest level of mRNA accumulation is seen in 6 and 18 month old animals (P < 0.05 for all time points before 10 neonatal days). In the 24 month old senescent rat, NCX transcripts are increased by almost 50% above that seen at 6 and 18 months (P < 0.05) but are not different from those at 15 neonatal days. Perinatal NCX expression is regulated transcriptionally: late fetal and neonatal hearts have high transcriptional activity but by 20 postnatal days, no detectable transcriptional activity can be demonstrated. Throughout development, at least five transcription start sites are used, and no significant difference in the 5' untranslated or 3' coding splice sites could be demonstrated, although several new cardiac splicing variants were identified. We also report the cloning of a 3.7 kb fragment containing the cardiac NCX1 promoter which is transcriptionally active in neonatal cardiomyocytes.