Age-related changes in Na+ excretion in saline-loaded spontaneously hypertensive rats

The ability of 6- to 7-, 12- to 13-, and 16- to 17-wk-old spontaneously hypertensive rats (SHR) to excrete an acutely administered, isotonic saline load (equivalent to 4.5% body wt) was evaluated. Female SHR from two different colonies were studied. Age-matched female Wistar (W) and Wistar-Kyoto (WKY) rats served as controls. At age 6 wk, the fraction of administered Na+ load excreted by either SHR colony after 60 min was significantly lower than in the Wistar rats but not significank, one colony of SHR excreted significantly more sodium after 60 min than the other rats. Glomerular filtration rate during control periods was positively correlated with age in one SHR colony; no age-related changes were observed in glomerular function in the other groups. These data 1) suggest that WKY rats are more appropriate than the Wistar rats as controls for renal function in the SHR, 2) demonstrate that the ability of the SHR to excrete an acutely administered sodium load is equal to or greater than normotensive WKY controls, and 3) demonstrate that with respect to renal function parameters, the SHR is not a homogenous strain.