Exaggerated response to isotonic saline loading in genetically hypertension-prone rats
Ben-Ishay, D.; Knudsen, K.D.; Dahl, L.K.
Journal of Laboratory and Clinical Medicine 82(4): 597-604
1973
ISSN/ISBN: 0022-2143 PMID: 4755435 Document Number: 59291
Female rats from 2 strains, Brookhaven R (resistant to hypertension) and Brookhaven S (susceptible to hypertension), were used. Two types of diet were used, low-salt with 0.4% NaCl and high-salt with 8% NaCl. Water was allowed freely until 2 h before loading tests. In the first experiment 10 adult rats of each strain, maintained since weaning on the low-salt diet, were given 3 consecutive 6-ml intragastric loads of NaCl 0.6, 0.3 and 0.15 molar in that order with an interval of several days between tests. At the beginning of the experiment the blood pressure of the 2 groups of adults was similar and within normal limits. There was no significant difference between groups in urine volume, Na excretion, Na concentration in urine and urine osmolality in response to the first hypertonic load. When the load was 0.3 M urine volume and Na excretion in both groups was lower than when the load was 0.6 M, but not significantly different between groups. Urine osmolality and Na concentration were significantly lower in S than in R rats. In response to the isotonic load (0.15 M) the S rats had significantly higher urine volume and Na excretion and significantly lower urine osmolality and Na concentration than R rats. The effect of isotonic saline (0.15 M) loading was further tested on 10 rats of each strain aged 8 weeks which were normotensive after a low-salt diet from weaning. Urine volume and Na excretion in response to the load were significantly higher in the S than in R rats. The effect on urine osmolality was not significantly different between the groups. The experiment was repeated after 6 weeks on high-salt diet when S rats were hypertensive and after a further week on low-salt diet. While the rats were on high-salt diet urine volume was significantly greater and urine osmolality was significantly lower in S than in R rats in response to the load. Na excretion in response to loading was not significantly different between the groups while on high-salt diet but became significantly higher in the S than in the R group when the test was repeated after a week of low-salt diet. Possible explanations of the increased response to isotonic saline of normotensive S rats are considered.