Effect of Diabetes on Cytosolic Free Ca²⁺ and Na⁺-K⁺-ATPase in Rat Aorta

We examined Na⁺-K⁺-ATPase activity and the levels of αI-, αII-, and β-subunit mRNA and protein in aortic cells of diabetic rats. Diabetes was induced by streptozocin. Na⁺-K⁺-ATPase activity was significantly reduced on the 2nd day of diabetes (9.4 ± 1.3 vs. 17.5 ± 2.1 μmol NADH · mg⁻¹ protein · h⁻¹, P < 0.05) and remained depressed on days 7 and 14. The levels of 5.3-kilobase (kb) mRNA band of the catalytic αII-subunit of Na⁺-K⁺-ATPase were also decreased on the 2nd day of diabetes, whereas the second band, 3.4 kb, was not affected. Both bands were significantly decreased on days 7 and 14. This was followed by a reduction in the levels of αII-protein (day 14). The levels of αI- and β-subunit mRNA and alpha I- protein were not affected by diabetes. A decrease in Na⁺-K⁺-ATPase activity was accompanied by a significant (P < 0.001) increase in the cytosolic free Ca²⁺ concentrations ([Ca²⁺]_i) in diabetic aortic cells (221 α 18 nM on the 7th day and 242 ± 17 nM on the 14th day vs. 153 ± 7 nM in controls). These findings are consistent with the hypothesis that decreased Na⁺-K⁺-ATPase activity and gene expression in vascular smooth muscle cells with accompanied rises in [Ca²⁺]_i may be an important pathogenetic factor in the development of hypertension and atherosclerosis in diabetes.