ELECTROLYTE AND WATER-BALANCE IN YOUNG SPONTANEOUSLY HYPERTENSIVE RATS

In metabolic balance studies the intake and excretion of Na, K and water were measured in spontaneously hypertensive rats (SHR) of the Okamoto-Aoki strain and age-matched Wistar Kyoto rats (WKY) that were 3-13 wk of age. While fed their usual chow, young SHR exhibited differences in excretion as compared to WKY consuming essentially equivalent amounts of food and water. Fractional Na and water excretion (percent of amount ingested) by SHR wre significantly less during wk 4-6 and 6-7, respectively, due to lower rates of urinary excretion. Potassium excretion was less in SHR at 4-5 wk. Apparently, SHR retain more urinary Na, K and water during an early phase of hypertension than normotensive, age-matched WKY. After 8 wk of age, fractional excretion of electrolytes and water did not differ appreciably between strains. In another group of rats, Na intake was restricted and observations were made from 3-13 wk of age. Although SHR excreted slightly less Na, cumulative Na balance was similar in SHR and WKY. Na restriction slowed the increase in arterial pressure in SHR younger than 9 wk of age and reduced the magnitude of the hypertension in 10-13 wk old SHR. At the latter age, arterial pressure was not as high in Na-restricted SHR as in SHR on the standard Na diet, but it was elevated above that in either WKY groups. Thus, dietary Na restriction retards the development, but does not prevent the hypertension in SHR.