Altered expression of Na+ transporters at the mRNA level in rat normal and hypertrophic myocardium

Abstract

Intracellular Na(+) ([Na(+)](i)) regulation plays a crucial role in the structural, mechanical, and electrical properties of myocardium. It is assumed that the [Na(+)](i) handling system may differ not only between normal and diseased hearts but also regionally within a heart. To gain new insight concerning disease- and region-dependent differences in the [Na(+)](i)-regulatory system, we investigated mRNA expression of Na+ transporters, the principal determinants of [Na(+)](i). Nonischemic pressure-overloaded hypertrophy was created by suprarenal abdominal aortic constriction of 50% for 7 weeks. mRNA abundances of Na(+)-Ca(2+) exchanger (NCX1), Na(+)-H(+) exchanger (NHE1), Na(+)-K(+)-2Cl(-) exchanger (NKCC1) and Na(+), K(+)-ATPase multigene family(alpha(1), alpha(2), alpha(3), and beta(1) isoforms) were measured by the real-time quantitative polymerase chain reaction method. mRNA abundance of all transporters mediating Na(+) influx (NCX1, NHE1, and NKCC1) was significantly upregulated as compared to normal. In contrast, Na(+)-efflux-mediating transporter (Na(+), K(+)-ATPase) mRNA expression was unaltered between normal and hypertrophic hearts. Losartan, an angiotensin II AT1 receptor antagonist, significantly attenuated upregulation of Na(+)-influx-mediating transporters induced by aortic constriction. The onset of Na(+)-influx-mediating transporter upregulation occurred within 5 days following constriction. In normal and hypertrophied hearts, mRNA of all Na(+)-influx-mediating transporters was expressed in order of abundance as: apex > septum approximately free wall of left ventricles. A transmural gradient in expression was also evident in normal hearts (midcardium > endo- and epicardium), which was attenuated under hypertrophic development. Myocardial hypertrophy is associated with significant changes in the spatial distribution and expression levels of Na(+) transporters. The upregulation of Na influx transporters during hypertrophy may contribute to the remodeling process, modulate contractility and promote arrhythmias.