Development of renal potassium excretion capacity in the neonatal rat.

We investigated the capacity of newborn rats to excrete an acute potassium load to understand the development of a renal potassium excretion system. Three groups of the rats (7-14 d) were used to collect urine periodically over 6 h after oral infusion of potassium: control (no potassium loading) and low- and high-potassium-loaded rats. In the low-potassium-loaded group, infused with about 0.6 microEq of potassium chloride/g body wt., the rate of renal potassium excretion increased from 0.08 plus minus 0.02 (7 d) to 0.13 plus minus 0.02 (10 d) and 0.21 plus minus 0.03 (14 d) microEq/h/g body wt. The high-potassium-loaded rats (1.5-2.8 microEq/g body wt. potassium load) excreted potassium at a higher rate of 0.18 +/- 0.05 (7 d), 0.30 +/- 0.02 (10 d), and 0.45 +/- 0.10 (14 d) microEq/h/g body wt. They excreted 77% (7 d), 76% (10 d), and 95% (14 d) of the potassium load. These values were much larger than the rate of 0.026 microEq/h/g body wt. of the control rats and of 0.08 microEq/h/g body wt., a mean potassium excretion rate during development from 7 to 14 d calculated from the data in the previous study (Kanno T et al.: J. Pediatr. Gastr. Nutr. 24: 242-252, 1997). In the same period, serum potassium concentration in the newborn rats decreased significantly (p < 0.01) from 7.2 +/- 0.1 (7 d) to 6.7 +/- 0.1 mEq/l (14 d). All these results suggest that a renal potassium excretion system in the rat develops at least in the second week of life, and its capacity is high enough to excrete the daily potassium intake.