Extract: Twenty puppies, 18 hr-40 days of age, were used to investigate the relation between maximal tubular reabsorption of glucose (TmG) and glomerular filtration rate (GFR) during maturation. The renal threshold for glucose was examined in an attempt to obtain information concerning functional heterogeneity. There was a slow increase in dry kidney weight (DKW) over the first 2 weeks of life, relative to the more rapid rate noted thereafter. Examination of the clearance of inulin (CIN) and TmG revealed a pattern of increase with age similar to that observed for the relation of DKW to age. The ratio of TmG to CIN initially was high (4.22), fell markedly over the first few days, and thereafter continued to decrease exponentially until levels were achieved that were similar to values found in adult animals (2.75–3.31). These data suggest that the documented anatomical glomerular preponderance in the infant kidney may not have a consistent functional correlate. Analysis of the glucose threshold as a function of age revealed a statistically significant positive regression; however, the increase with age was small (range 190–254 mg/100 ml), and all values fell within the 95% confidence limits for adult thresholds. A comparison of splay between the youngest and oldest puppies demonstrates an increase in splay and thus in functional heterogeneity of the younger animals, a finding which may reflect a population of nephrons with relative glomerular preponderance, despite the presence of glomerulotubular balance for the kidney as a whole.Speculation: Current evidence, including that of the present investigation, suggests that although functional tubular capacity at birth may be limited, glomerulotubular balance is maintained.
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