Na,K-ATPase mRNA(beta 1) expression in rat myocardium - effect of thyroid status

The abundance of Na,K-ATPase and its alpha and beta subunit mRNAs is upregu lated in cardiac and other target tissue by thyroid hormone (T-3) Multiple Na,K-ATPase mRNA(beta 1) species encoding an identical beta 1 polypeptide a re expressed in the heart. The different mRNA(beta 1) species result from u tilization of two transcription start-sites in the first exon and multiple (five) poly(A) signals in the terminal exon of the beta 1 gene. In the pres ent study we identify the mRNA(beta 1) species that are expressed in rat ve ntricular myocardium under basal conditions, and determine whether they are differentially regulated by T-3 mRNA(beta 1) species were identified by 3' -RACE followed by DNA sequencing, and by Northern blotting using probes der ived from different regions of rat cDNA(beta 1). Five mRNA(beta 1) species are expressed in rat heart: mRNA(beta 1) species that are initiated at the first transcription start-site and end at the first, second and fifth poly( A) sites (resulting in mRNAs of 1630, 1810, and 2780 nucleotides), and mRNA (beta 1) species initiated at the second transcription start-site and endin g at the second and fifth poly(A) sites (resulting in mRNAs of 1500 and 249 0 nucleotides); in order of increasing length, the five mRNAs constitute 0. 04, 0.15, 0.38, 0.11 and 0.32 of total mRNA(beta 1) content. In hypothyroid rats (induced by addition of propyl-thiouracil to the drinking water for 3 weeks), total mRNA(beta 1) content decreased to 0.18 euthyroid levels, whi ch was associated with a disproportionate 7.5-fold decrease in the abundanc e of the longest transcript (P < 0.05); transcripts initiating at the first transcription stare-site and ending at the second poly(A) signal in hypoth yroid hearts were 0.26 euthyroid levels (P < 0.05). Hyperthyroidism induced by injection of normal rats with three doses of 100 mu g T-3/100 g body we ight every 48 h resulted in an overall approximate to 2-fold increase in mR NA(beta 1) content with no change in the fractional contribution of any of the mRNA(beta 1) species. The results indicate a complex heterogeneity in t he expression of mRNA(beta 1) in myocardium.