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 (7-14d) 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 μEq of potassium chloride/g body wt., the rate of renal potassium excretion increased from 0.08±0.02 (7d) to 0.13±0.02 (10d) and 0.21±0.03 (14d) μEq/h/g body wt. The high-potassium-loaded rats (1.52.81μEq/g body wt. potassium load) excreted potassium at a higher rate of 0.18±0.05 (7d), 0.30±0.02 (10d), and 0.45±0.10 (14d) μEq/h/g body wt. They excreted 77% (7d), 76% (10d), and 95% (14 d) of the potassium load. These values were much larger than the rate of 0.026 μEq/h/g body wt. of the control rats and of 0.08 μEq/h/g body wt., a mean potassium excretion rate during development from 7 to 14d 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 (7d) to 6.7±0.1 mEq// (14d). 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.
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