Ontogeny of Iminoglycine Transport in Mammalian Kidney

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Abstract
Renal tubular absorption of proline, hydroxyproline, and glycine by the newborn of most mammals is inefficient compared to that of the adult. Cortex slices from seven-day-old rat kidney also transport proline and glycine at reduced initial rates compared to mature kidney. Nonetheless, newborn slices achieve higher intracellular concentrations during prolonged incubation; the latter reflects a reduced rate of efflux, a characteristic peculiar to the membrane of postnatal kidney. The postnatal reduction of initial uptake rates is observed clearly only at substrate concentrations in or below the physiological range; it correlates with the absence of two high-affinity systems which normally serve proline and glycine transport independently at these concentrations in mature kidney, in conjunction with a "common" low-affinity system. The low-affinity system alone performs the observed uptake in the newborn kidney. Specific activity of the high-affinity systems for proline and glycine increases asynchronously after birth, suggesting independent genetic control of the three systems for iminoglycine transport in mammalian kidney.