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

Abstract

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 (PPPP<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.