Relation between potassium excretion and proximal tubular fluid output in conscious unoperated rats adapted to different dietary potassium contents.

The effect of dietary potassium content on the proximal tubular fluid output (Vprox), and the relation between Vprox and potassium excretion was examined in conscious unoperated diabetes insipidus rats adapted to five dietary potassium contents in the range 25-400 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. Increase of the dietary potassium content from 25 to 400 mmol/kg led only to a slight, barely statistically significant increase of Vprox. However, repeated measurements in the same rats on different days showed considerable intraindividual variation of Vprox and the potassium excretion. At each dietary potassium level except the lowest one the potassium excretion was linearly correlated to CLi (r = 0.60-0.92) and to V (r = 0.56-0.85). Errors in bladder emptying could not explain these correlations. The slopes of the regression lines increased with increasing dietary potassium content, and the regression lines all intercepted the CLi axis at approximately the same CLi value, i.e., on the average about 70 microliter/min./100 g body weight. The results indicate that an increase of the dietary potassium content leads to a slight increase of Vprox. Spontaneous intraindividual variation of Vprox is considerable and is associated with linearly related variation of the potassium excretion. The change of the potassium excretion for a given change of Vprox increases with the dietary potassium content.