Character of function and size in kidney during normal growth of rats.

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Extract: During normal growth in male rats (3 weeks to 3 months of age) weighing from 50–400 g, kidney weight and glomerular filtration rate (GFR) increased at slower rates than did body weight; in contrast, the rate of increase in kidney weight and glomerular filtration rate were the same, and the ratio of GFR:g kidney weight was constant after 4–5 weeks of age. The ratio of maximal glucose reabsorption (TmG) to GFR increased only slightly with growth. Na-K-dependent ATPase activity/mg light microsomal protein from kidney cortex and QO2 did not change during growth. Kidney growth up to 200 g body weight at 16 weeks of age was due more to an increase in cell number; beyond then it was due more to an increase in cell size. The pattern of function-structure relation during growth differed from that observed in kidney hypertrophy secondary to uninephrectomy. It was not specifically determined from cell number or size but from some property proportional to total protein mass or to the product of cell number and cell mass. Speculation: Tubular functions of the nephron during growth increase in proportion to each other and in proportion to total renal mass. This pattern of increase differs in several respects from that which occurs following uninephrectomy. The inference is that growth response differs in fundamental biological character from the hypertrophy response.