1. The ability of isolated membrane fractions of cardiomyocytes, aortic smooth muscle and erythrocytes from spontaneously hypertensive rats to bind calcium was investigated isotopically.

2. Plasma membranes of aortic smooth muscle and erythrocytes of spontaneously hypertensive rats possess a lower calcium-binding ability than preparations from normotensive controls.

3. Sarcoplasmic reticulum from the myocardium of spontaneously hypertensive rats had a lower ability to accumulate calcium.

4. Changes in the calcium-binding ability of the erythrocyte membrane of spontaneously hypertensive rats was related to the diminution of the amount of exchangeable calcium on the inner part of the membrane. The latter may be the cause of the increase of erythrocyte membrane permeability for sodium in spontaneously hypertensive rats and in patients with essential hypertension.

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