RENAL, INTESTINAL, AND ADRENAL RESPONSES TO SODIUM LOADING IN DAHL-IWAI SALT-SENSITIVE AND SALT-RESISTANT RATS

This study compared renal and intestinal handling of sodium in Dahl-Iw ai salt-sensitive (S) and salt-resistant (R) rats given a normal-salt diet (0.3% NaCl) and a high-salt diet (4.0% NaCl). Six-week-old female S and R rats (n = 7 each) were given a normal-salt diet for 14 days f ollowed by a high-salt diet for 3 weeks. Systolic blood pressure was s ignificantly higher in the S rats than in the R rats only at the end o f the high-salt diet period (170 +/- 5, mean +/- SEM, vs 152 +/- 1 mmH g, p < 0.01). Daily sodium intake, water intake, urine volume, and uri nary and fecal excretions did not significantly differ between the R a nd the S rats during the normal- and high-salt diets, except for a sli ght, although significant, decrease in fecal sodium excretion in the S rats as compared with the R rats in the 2nd week of the high-salt die t period. After switching from the normal-salt diet to the high-salt d iet, urinary sodium excretion increased by 17- to 18-fold and fecal so dium excretion increased by about 5-fold in the 1st week of salt loadi ng. The changes in urinary and fecal sodium excretions did not differ significantly between the groups. Cumulative sodium retention was simi lar in the two groups. The aldosterone/creatinine ratio in 24-hr urine , which was significantly lower in the S than in the R rats during the normal-salt diet, decreased to similar levels in both groups after sa lt loading, indicating a blunted response of aldosterone in the S rats . Thus, there were no discernible differences in renal and intestinal handling of sodium between the S and the R rats, except for a slight, but significant, difference in fecal sodium excretion in the 2nd week of the high-salt period. The results indicate that inappropriate suppr ession of aldosterone or some other mechanism induced by salt loading may be involved in blood pressure elevation in Dahl-Iwai S rats.