Body fluid volumes in prehypertensive spontaneously hypertensive rats

To ascertain the effect of aldosterone on body fluid volumes in neonatal, prehypertensive spontaneously hypertensive rats (SHR), we studied these animals at 12 days using age-matched Wistar-Kyoto (WKY) as normotensive controls. Some pups of each strain were treated with spironolactone (1.5 micrograms/g body wt) on days 10-12. Total body water (TBW, by dessication) and extracellular fluid (ECF, Na2 35SO4 space) volumes were significantly larger in SHR than in WKY, whereas plasma volumes (125I-serum albumin space) were not different. Thus the enlarged ECF was due to preferential expansion of the interstitial fluid (ISF) space. Treatment of SHR with spironolactone reduced TBW and ISF to values not different from untreated WKY and also reduced plasma volume to some extent. These results indicate 1) significant ISF volume expansion occurs in SHR prior to elevation of blood pressure, and 2) the previously observed elevation in plasma aldosterone in SHR at this age probably mediates the volume expansion.