Renal haemodynamics and total body sodium in immature spontaneously hypertensive and Wistar-Kyoto rats.

We have investigated the renal haemodynamics and sodium metabolism in spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats during the first 10 weeks of life. Using constant infusion clearance measurements of 51Cr-EDTA and 125I-hippuran in unrestrained, conscious, 4-week-old rats we found a significant reduction in glomerular filtration rate (GFR) and renal blood flow (RBF), respectively, in SHR compared with WKY. The arterial blood pressure (BP) and renal vascular resistance were also significantly elevated in SHR at this age. Acute oral treatment with hydralazine increased SHR GFR and RBF to equal that of WKY, but acute angiotensin converting enzyme inhibition with oral MK 421 caused the GFR and RBF of SHR to exceed that of WKY. In addition, the plasma renin activity of 4-week-old SHR was significantly elevated compared with WKY. Total body sodium was measured from 1 to 10 weeks of age by determining the whole-body radioactivity of rats equilibrated on 22Na-labelled NaCl of constant specific activity. Indirect blood pressure and sodium intake were measured from weeks 4 to 10 inclusive. Relative sodium retention was observed in SHR both before and during the early developmental phase of hypertension. The sodium intake and BP of SHR were consistently higher than WKY throughout weeks 4 to 10. We propose that increased renal vascular resistance in immature SHR alters the pressure natriuresis relationship to cause sodium retention, with the subsequent development of hypertension.