1. Calmodulin and calcium effects on cardiac ouabain-sensitive adenosine triphosphatase (ATPase) activity were studied in young spontaneously hypertensive rats (SHR) and in their normotensive control Wistar—Kyoto rats (WKY).

2. Cardiac sarcolemmal membranes from SHR showed significantly higher ouabain-sensitive ATPase activity than membranes from WKY rats. This activity was unaffected by calmodulin or calcium alone.

3. However, when both calmodulin and calcium were added, ouabain-sensitive activity was significantly reduced without changes in the total ATPase activity.

4. The calcium-dependent calmodulin effect was dose-dependent and greater in SHR than in WKY membranes.

5. An altered interaction between the calcium-calmodulin system and sodium handling by the plasma membrane in SHR may play a role in the pathogenesis of hypertension.

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