Na,K‐ATPase mRNAβ1 expression in rat myocardium – effect of thyroid status

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Abstract
The abundance of Na,K‐ATPase and its α and β subunit mRNAs is upregulated in cardiac and other target tissue by thyroid hormone (T3). Multiple Na,K‐ATPase mRNAβ1 species encoding an identical β1 polypeptide are expressed in the heart. The different mRNAβ1 species result from utilization of two transcription start‐sites in the first exon and multiple (five) poly(A) signals in the terminal exon of the β1 gene. In the present study we identify the mRNAβ1 species that are expressed in rat ventricular myocardium under basal conditions, and determine whether they are differentially regulated by T3. mRNAβ1 species were identified by 3′‐RACE followed by DNA sequencing, and by Northern blotting using probes derived from different regions of rat cDNAβ1. Five mRNAβ1 species are expressed in rat heart: mRNAβ1 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 mRNAβ1 species initiated at the second transcription start‐site and ending at the second and fifth poly(A) sites (resulting in mRNAs of 1500 and 2490 nucleotides); in order of increasing length, the five mRNAs constitute 0.04, 0.15, 0.38, 0.11 and 0.32 of total mRNAβ1 content. In hypothyroid rats (induced by addition of propyl‐thiouracil to the drinking water for 3 weeks), total mRNAβ1 content decreased to 0.18 euthyroid levels, which was associated with a disproportionate 7.5‐fold decrease in the abundance of the longest transcript (P < 0.05); transcripts initiating at the first transcription start‐site and ending at the second poly(A) signal in hypothyroid hearts were 0.26 euthyroid levels (P < 0.05). Hyperthyroidism induced by injection of normal rats with three doses of 100 µg T3/100 g body weight every 48 h resulted in an overall ≈ 2‐fold increase in mRNAβ1 content with no change in the fractional contribution of any of the mRNAβ1 species. The results indicate a complex heterogeneity in the expression of mRNAβ1 in myocardium.