Quantitative Changes in Cardiac Na+, K+ -Adenosine Triphosphatase of Spontaneously Hypertensive Rats

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Abstract
Na pumps of cardiac membranes were studied in young, spontaneously hypertensive rats (SHR) and in their normotensive controls (Wistar-Kyoto; WKY) using the 2 following methods. The enzymatic activity and its sensitivituy to ouabain were measured as the Na+,K+-dependent ATP hydrolysis, and the number of pumps was estimated by [3H]ouabain binding. The following observations were made: concentrations of ouabain as low as 10-10 M inhibited 10-15% of the enzyme activity in both strains; Na+,K+-ATPase activity in membranes from SHR was double that in membranes from WKY (16.5 .+-. 3.2 .mu.mol Pi/h per mg protein vs. 8.2 .+-. 1.2 .mu.mol Pi/h per mg protein for 10-7 M ouabain; P < 0.01); sensitivity to 3 different cardiac glycosides, ouabain, digoxin and digitoxigenin, was identical in SHR and WKY vesicles; and the binding capacity of [3H]ouabain was significantly higher in SHR than in WKY vesicles, but the Kd did not appear to differ between the 2 substrains. These studies, performed on 3-wk-old rats before the appearance of hypertension, showed the existence of a Na+,K+-ATPase of very high affinity in the rat heart, and that cardiac sarcolemmal membranes from SHR had a greater number of NA pumps than those from WKY and thus a greater ability to extrude Na.

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