Natriuresis and renal Na-K-ATPase activity in kidneys of normotensive and spontaneously hypertensive rats.

The isolated perfused kidneys of spontaneously hypertensive (SHR) rats aged 10-12 weeks (early hypertensive phase) exhibited pressure natriuresis curves already shifted to the right, as compared with kidneys of the normotensive Wistar rats and Wistar-Kyoto rats. Sodium excretion in relation to tubular sodium load was higher in kidneys of SHR rats as compared with those of the normotensive controls. Renal Na-K-ATPase activity was significantly lowered only in the cortex during chronic hypertension but not in the outer medulla. In both parts of the kidney no differences could be detected in the apparent Km value for sodium stimulation or the apparent activation energies. Mg-ATPase activity was lowered in SHR rats during chronic hypertension. Protein content was significantly decreased in the outer medulla of SHR rats in the early hypertensive phase as well as during chronic hypertension. The results suggest that the functional changes observed in kidneys of young SHR rats are not due to a genetically determined defect in renal Na-K-ATPase.