Abnormal relationship between sodium excretion and hypertension in spontaneously hypertensive rats

Inulin clearance, single nephron glomerular filtration rate (SNGFR), Na and K excretion were studied following an acute saline infusion in spontaneously hypertensive (Okamoto strain) rats (SH). 1. Hypertonic saline load: experiments were performed in adult and young SH rats. As compared to control normotensive Wistar rats (NI), the sodium excretion rose much less following the load in SH. During the 75 min following the beginning of the load, adult SH excreted 15 degrees (NT: 58%) and young SH 9%(NT = 38%) of the sodium load (P less than 0.01 in both cases). Cin were similar in SH and NT during control period, and a similar increase was observed following the load. The superficial to juxtamedullary SNGFR ratio was 0.80 plus or minus 0.05 in 5 non diuretic adult SH, a value not different from that found in normal rats, and 0.87 plus or minus 0.07 in 3 salt loaded SH, indicating that no significant intrarenal GFR redistribution occurs in SH following an acute hypertonic saline load. 2. Following an isotonic saline load, the results were not different: the percentage of the sodium load excreted was 44 plus or minus 5% and 10 plus or minus 2% in NT and SH adult rats respectively. The possibility that the observed defect in sodium excretion of SH rats involves distal tubular function is suggested by the pattern of evolution in Na and K excretion.