Distribution and role of Na+/K+ ATPase in endocardial endothelium

Objective: In mammalian cardiomyocytes, alpha isoforms of Na+/K+ ATPase have specific localisation and function, but their role in endocardial endothelium is unknown. Methods: Different a isoforms in endocardial endothelium and cardiomyocytes of rabbit Were investigated by measuring contractile parameters of papillary muscles, by RT-PCR, by Western blots and by immunocytochemistry. Results: Inhibition of Na+/ K+ ATPase by decreasing external K+ from 5.0 to 0.5 mmol/1 caused biphasic inotropic effects. The maximal negative inotropic effect at external K+ of 2.5 mmol/1 was significantly larger in +EE muscles (with intact endocardial endothelium) than in -EE muscles (with endocardial endothelium removed) (-22.5 +/- 2.4% versus -5.9 +/- 4.0%, n=7, P<0.05). Further decrease of K+ to 0.5 mmol/1 caused endothelium-independent positive inotropy (27.8 +/- 11.8% for +EE versus 18.6 +/- 11.3% for -EE, n=7, P>0.05). Inhibition of Na+/K+ ATPase either by dihydro-ouabain (10(-9) to 10(-4) mol/1, n=4) or by K+ decrease following inhibition of Na+-H+ exchanger by dimethyl-amiloride (50 mumol/1, n=6) caused endothelium-independent positive inotropic effects only. RT-PCR and Western Blot demonstrated alpha(1) and alpha(2) Na-K-ATPase isoforms in cardiomyocytes, but only a, in cultured endocardial endothelial cells. Immunohistochemistry showed that alpha(1), in endocardial endothelium was predominantly present at the luminal side of the cell (n=7) and that alpha(1) and alpha(2) displayed different localisation in cardiomyocytes. Conclusions: These results suggested that negative and positive inotropic effects of Na+/K+ ATPase inhibition in +EE muscles could be attributed to inhibition of endocardial endothelial a, and muscle alpha(2) isoform, respectively. Accordingly, the endocardial endothelial alpha(1) isoform of Na+/K+ ATPase may contribute to blood-heart barrier properties of this endothelium and may control cardiac performance via endothelial Na+/H+ exchange. (C) 2001 Elsevier Science B.V. All rights reserved.