Alteration of Na,K-ATPase subunit mRNA and protein levels in hypertrophied rat heart.

To determine if an altered expression of the Na,K-ATPase alpha isoform genes is responsible for an observed increase in cardiac glycoside sensitivity in compensatory hypertrophy, we performed Northern and slot blot analyses of RNA and specific immunological detection of Na,K-ATPase isoforms in rat hearts from normal and pressure overload-treated animals induced by abdominal aortic constriction. During the early phase of hypertrophy, the only alteration is a decrease in the alpha 2 mRNA isoform. In the compensated hypertrophied heart, the levels of the predominant alpha 1 isoform (mRNA and protein) and the beta 1 subunit mRNA are unchanged. In contrast, the alpha 2 isoform (mRNA and protein) is decreased by 35% and up to 61-64% in mild (< 55%) and severe (> 55%) hypertrophy, respectively. The alpha 3 isoform (mRNA and protein), which is extremely low in adult heart, is increased up to 2-fold during hypertrophy but accounts for only approximately equal to 5% of the total alpha isoform mRNA. These findings demonstrate that, in cardiac hypertrophy, the three alpha isoforms of the Na,K-ATPase are independently regulated and that regulation occurs at a pretranslational level. The pattern of expression in hypertrophied adult heart is similar to that of the neonatal heart where the inverse regulation between the alpha 2 and alpha 3 ouabain high affinity isoforms has been reported. This suggests that distinct regulatory mechanisms controlling Na,K-ATPase isoform expression may, at least in part, be involved in the sensitivity to cardiac glycosides.