Isoform-specific regulation of myocardial Na,K-ATPase alpha-subunit in congestive heart failure. Role of norepinephrine.

BACKGROUND

Myocardial ouabain-binding sites and Na,K-ATPase activity are reduced in congestive heart failure (CHF), but the mechanisms by which CHF reduces the Na,K-ATPase remain unknown. We proposed to investigate whether the changes are accompanied by isoform-specific reductions of the Na,K-ATPase alpha-subunit proteins in CHF and whether similar changes could be produced by exogenous norepinephrine administration.

METHODS AND RESULTS

CHF was induced in dogs by rapid ventricular pacing at a rate of 225 beats per minute for 8 weeks (protocol 1). A second group of dogs were paced at 100 beats per minute and served as controls. In protocol 2, norepinephrine was infused in normal dogs using a subcutaneous osmotic minipump for 8 weeks. The control dogs received normal saline through the pump. Animals were studied after 8 weeks of pacing or norepinephrine infusion. After the baseline hemodynamics and interstitial norepinephrine concentration had been obtained, the hearts were removed for measuring [3H]ouabain-binding sites and Na,K-ATPase alpha-subunit proteins using isoform-specific monoclonal antibodies.

RESULTS

Myocardial [3H]ouabain-binding sites were reduced in dogs with CHF and chronic norepinephrine infusion. The Western blot analysis showed that adult canine hearts possess both alpha 1 and alpha 3 isoforms of the Na,K-ATPase alpha-subunit but not the alpha 2 isoform protein. CHF and NE infusion had no effect on the Na,K-ATPase alpha 1-subunit protein but did reduce the alpha 3 isoform protein significantly. In addition, there was a significant inverse correlation between the amount of myocardial alpha 3 isoform protein and interstitial norepinephrine content in the dogs. In contrast, the specific activity of the sarcolemmal marker 5'-nucleotidase did not differ among the groups of animals.

CONCLUSIONS

The reduction of myocardial Na,K-ATPase in CHF is limited to the alpha 3 isoform. Furthermore, because similar changes in myocardial ouabain-binding sites and Na,K-ATPase alpha 3 isoform were produced by chronic norepinephrine infusion, the decrease in the Na,K-ATPase in CHF is most likely mediated via excess sympathetic stimulation.