Electrolyte and water balance in young spontaneously hypertensive rats.

In metabolic balance studies the intake and excretion of sodium, potassium, and water were measured in spontaneously hypertensive rats (SHRs) of the Okamoto-Aoki strain and age-matched Wistar Kyoto rats (WKYs) that were 3 through 13 weeks of age. While fed their usual chow, young SHRs exhibited differences in excretion as compared to WKYs consuming essentially equivalent amounts of food and water. Fractional sodium and water excretion (percent of amount ingested) by SHRs were significantly less during Weeks 4-6 and 6-7, respectively, due to lower rates of urinary excretion. Potassium excretion was less in SHRs at 4-5 weeks. These observations indicate that SHRs retain more urinary sodium, potassium, and water during an early phase of hypertension than normotensive, age-matched WKYs. After 8 weeks of age, fractional excretion of electrolytes and water did not differ appreciably between strains. In another group of rats, sodium intake was restricted and observations were made from 3 through 13 weeks of age. Although SHRs excreted slightly less sodium, cumulative sodium balance was similar weeks of age and reduced the magnitude of the hypertension in 10- through 13-week-old SHRs. At the latter age, arterial pressure was not as high in sodium-restricted SHR as in SHR on the standard sodium diet, but it was elevated above that in either WKY groups. Thus dietary sodium restriction retards the development, but does not prevent the hypertension in SHR.