Changes in the expression of Na+/K+-ATPase isoenzymes in the left ventricle of diabetic rat hearts: effect of insulin treatment

Abstract

Na+/K+-ATPase related strophanthidin sensitive 3-O-methylfluorescein-phosphatase activity, [3H]ouabain binding and expression of Na+/K+-ATPase subunit isoforms were measured in the left ventricle of the heart of normal and streptozotocin-diabetic rats with and without insulin treatment. Compared to control animals, the enzyme activity was 0.75 +/- 0.09 and 0.62 +/- 0.06 times lower in rats diabetic for 2 and for 4 weeks, respectively. This was associated with a proportional decrease of the [3H]ouabain binding sites. Immunoblots indicated a 0.76 +/- 0.08 and 0.61 +/- 0.08-fold decrease of alpha1, a 0.68 +/- 0.09 and 0.41 +/- 0.04-fold decrease of alpha2 subunit in 2- and 4-week diabetic rats, respectively relative to controls. Beta1 subunit decreased proportionally 0.71 +/- 0.07 and 0.38 +/- 0.06-fold, and beta2 decreased 0.75 +/- 0.08 and 0.31 +/- 0.06-fold, respectively. Northern blot analysis revealed a significant reduction in mRNA level of Na+/K+-ATPase subunit isoforms after 2 and 4 weeks of diabetes (for alpha1 66.2 +/- 8.2 and 55.9 +/- 7.8% of controls for alpha2 91.7 +/- 12.1 and 41.1 +/- 7.1% of controls and for beta subunit 93.4 +/- 11.1 and 49.8 +/- 6.8% of controls, respectively). Although, mRNA levels of isoform reverted to even higher levels than the control values after insulin treatment, insulin caused only a partial recovery of enzyme activity, [3H]ouabain binding capacity and protein expression. We have obtained evidence that in cardiac left ventricle there are more than one type of Na+/K+-ATPase alpha and beta subunit isoforms which are affected in diabetes and by insulin treatment. The time course of diabetes induced changes and the degree of involvement suggest that the Na+/K+-ATPase isoforms are altered individually.