Proline uptake by rat renal brush border membrane vesicles from animals 7 days of age and older has been examined to delineate developmental changes in membrane function that may underlie the physiological hyperprolinuria of young animals. Although the two proline transport systems normally present in adult membranes were found in membranes from young animals, the proline "overshoot" resulting from a sodium ion gradient is minimal and increases with age of the animal from which the membranes were isolated. This is associated with a severalfold faster entry of 22Na into vesicles of the 7-day-old animal compared to entry into membranes prepared from adult kidneys. The very rapid dissipation of the sodium gradient thus diminishing the driving force for transmembrane proline movement may explain the changes in proline overshoot observed in membranes from young animals. The altered sodium permeability is consistent with the fact that young animals have a generalized inability to reabsorb other amino acids whose transport is known to be sodium gradient stimulated.Urinary hyperexcretion of proline and indeed of most amino acids is characteristic ofneonatal rat (1), dog (2), and man (3). Subsequent postnatal development of the nephron, however, reduces urinary amounts of these acids to much lower levels (4, 5). Although the subject of numerous investigations (6-9), the nature of the developmental changes occurring in the kidney that result in decreased amino acid excretion is as yet undefined. An understanding of these changes should not only increase our knowledge of normal transport processes but also provide insight into human disorders of renal amino acid transport, such as familial iminoglycinuria, in which the affected individual continues to excrete excessive quantities of certain amino acids in the urine.Previous studies of the development of renal amino acid transport in rat have relied upon the use of the cortical slice (10, 11) and isolated renal tubule fragment (1,8). Data obtained from these models, however, are influenced by factors such as bidirectional solute movement, substrate penetration, and substrate metabolism. The use of isolated membrane vesicles obviates these problems and age-related differences in solute transport by these vesicles may be examined directly.In our recent studies, proline uptake by renal brush border membrane vesicles prepared from newborn rats by free-flow electrophoresis, unlike vesicles from adult animals, did not show a Na+-gradient-stimulated overshoot but did exhibit a Na+-gradient-enhanced rate of early proline entry (12). To better understand the events accompanying the maturation of renal solute transport we have therefore examined characteristics of proline and Na+ uptake by renal brush border membrane vesicles prepared from rats of various ages ranging from 1 week oflife to adulthood. The results indicate that, although the two proline transport systems normally present in adult membranes are also found in membranes from young animals, the proline overshoot re...