Influence of hypernatraemia and urea excretion on the ability to excrete a maximally hypertonic urine in the rat.

Rats normally excrete 20-25 mmol of sodium (Na+) + potassium (K+) per kilogram per day. To minimize the need for a large water intake, they must excrete urine with a very high electrolyte concentration (tonicity). Our objective was to evaluate two potential factors that could influence the maximum urine tonicity, hypernatraemia and the rate of urea excretion. Balance studies were carried out in vasopressin-treated rats fed a low-electrolyte diet. In the first series, the drinking solution contained an equivalent sodium chloride (NaCl) load at 150 or 600 mmol l(-1). In the second series, the maximum urine tonicity was evaluated in rats consuming 600 mmol l(-1) NaCl with an 8-fold range of urea excretion. Hypernatraemia (148 +/- 1 mmol l(-1)) developed in all rats that drank 600 mmol l(-1) saline. Although the rate of Na+ + K+ excretion was similar in both saline groups, the maximum urine total cation concentration was significantly higher in the hypernatraemic group (731 +/- 31 vs. 412 +/- 37 mmol l(-1)). Only when the rate of excretion of urea was very low, was there a further increase in the maximum urine total cation concentration (1099 +/- 118 mmol l(-1)). Thus hypernatraemia was the most important factor associated with a higher urine tonicity.