Post-transcriptional regulation of the Na+/Ca2+ exchanger in aging rat heart.

Altered calcium homeostasis in the senescent heart appears to be the result, at least in part, of decreased Na+/Ca2+ exchange activity. To further investigate the basis of the decrease in Na+/Ca2+ exchange activity, Na+/Ca2+ exchanger gene expression in the heart was compared in 3 and 24 month old male Fischer 344 rats. Sarcolemmal vesicles prepared from left ventricle and septum showed reduced Na(+)-dependent Ca2+ uptake in 24 month old animals when compared to 3 month old animals (0.156 +/- 0.005 and 0.135 +/- 0.008 nmol Ca2+/mg/10 s; mean +/- S.E. for 3 month and 24 month old animals, respectively). Western analysis showed immunodetectable Na+/Ca2+ exchanger protein levels were decreased by 19% in 24 month old animals when compared to 3 month old animals. Poly(A+) RNA was purified from left and right ventricle and left and right atria and subjected to Northern analysis using digoxin labeled cDNA probes for the Na+/Ca2+ exchanger and actin. The Na+/Ca2+ exchanger probe labeled a 7 kb message in both ventricle and atria, while the actin probe labeled both beta-actin (2.2 kb) and alpha-actin (1.4 kb). The steady state level of expression of Na+/Ca2+ exchanger Poly(A+) RNA when normalized to beta-actin, was similar when ventricle and atria were compared. There were no observable differences in Na+/Ca2+ exchanger or alpha-actin Poly(A+) RNA steady state levels when comparing 3 and 24 month old animals. The results suggest that reduced Na+/Ca2+ exchange activity in the left ventricle of 24 month old animals was most likely the result of post-transcriptional modification of the protein that was detectable by Western analysis.