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 alpha 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/l caused biphasic inotropic effects. The maximal negative inotropic effect at external K(+) of 2.5 mmol/l 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/l 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/l, n=4) or by K(+) decrease following inhibition of Na(+)-H(+) exchanger by dimethyl-amiloride (50 micromol/l, 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 alpha(1) 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 alpha(1) 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.