To explore the nature of differences in uptake by renal brush border vesicles from animals of different ages, vesicles were isolated from 7-day old and adult rats by a Mg-aggregation method. A number of criteria were compared in vesicles from the young and mature animals. The vesicles isolated from animals of both ages appear similar on electron microscopy, in response to osmotic changes, and in uptake kinetics for L-glucose. Despite these parameters which indicate no basic differences between the membranes of young and mature kidney, differences in proline and sodium handling are seen. When compared to the uptake pattern seen in vesicles from adult animals, the height of the sodium gradient-stimulated proline overshoot is diminished and sodium entry is faster in vesicles of the 7-day old rats. These are the same differences which were found in vesicles prepared by differential centrifugation from 7-day old animals. In addition, although sodium efflux was faster from vesicles of immature kidney and mirrored the faster sodium entry, proline efflux was slower. The data indicate a dissociation of proline and sodium fluxes in brush borders of the young rat kidney.

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