Relationship between sodium clearance and proximal tubular fluid output in conscious unoperated rats adapted to different dietary sodium contents.

The relationship between the proximal tubular fluid output (Vprox) and the sodium clearance (CNa) was examined in conscious unoperated diabetes insipidus rats adapted to different dietary sodium contents ranging from 5 to 300 mmol/kg. The lithium clearance (CLi) and the urine flow (V) were used as measures of Vprox, CLi being equal to and V being about one-third of Vprox. Variations in the dietary sodium content had almost no influence on CLi and V. However, repeated 2-hour clearance measurements on different days in the same rat showed considerable intraindividual variations of CLi and V, and at each dietary sodium level CNa was linearly correlated to CLi (r = 0.83-0.90) and to V (r = 0.57-0.89). Errors in bladder emptying could not explain these correlations. The slopes of the regression lines increased with increasing dietary sodium content, and the regression lines all intercepted the abscissa at approximately the same value of CLi, i.e. on the average about 40 microliter/min./100 g body weight. The results suggest that Vprox is almost independent of the dietary sodium content. Spontaneous intraindividual variation of Vprox is considerable and is associated with linearly related variation of CNa. The change of CNa for a given change of Vprox increases with the dietary sodium content.