Postnatal development of renal function: micropuncture and clearance studies in the dog

Postnatal renal development was studied in dogs between 2 and 77 days. Single, superficial nephrons were evaluated by micropuncture, concurrently with measurements of total renal function and morphometric analyses in the same animals. Glomerular filtration rate for the entire kidney increased linearly from 0.13 ml/min per g kidney weight at 2 days to 0.91 at 77 days. Extraction of p-aminohippurate increased from about 20 to 80%, and renal plasma flow per g kidney weight, measured as Cpah/Epah, increased threefold during the same period. Filtration fraction increased to the mature value during the first half of the postnatal period studied. The clearance of urea per unit of renal mass increased with age, whereas the fraction of filtered urea reabsorbed declined during the early part of the postnatal period. The pattern of fractional urea reabsorption may be due mainly to increased medullary recycling of urea and to a rise in the reabsorption of water from the medullary collecting duct. Urine osmolality was higher than plasma from birth onward and rose with age. Osmolal equality of collecting duct fluid and medullary interstitium reflected mature vasopressin (ADH)-induced water permeability. The rise in urinary concentration was predominantly due to increasing medullary sequestration of urea. Glomerular filtration rate of the superficial nephron increased from 3.2 nl/min at 21 days, when subcapsular nephrons were uniformly patent, to 23.1 at 77 days. Despite this rise in filtered load, fractional reabsorption of sodium and water in superficial proximal tubules was constant and at the mature level from the onset of intratubular perfusion. Changes in arterial plasma protein concentration, in filtration fraction, and in the hydrostatic pressure gradient between proximal tubule and peritubular capillary may interact to maintain glomerulotubular balance. The data, together with results of an accompanying morphological study, demonstrate a sequence of coordinated changes during postnatal renal maturation.