The ability of renal cortical slices of newborn and young rats to accumulate a nonmetabolizable sugar, a-methylglucoside, is slight and does not reach adult capacity until 25 days of age. However, a rudimentary sugar transport system is present, as indicated by a further decrease in accumulation in the presence of phlorizin or absence of sodium ion.Amino acid uptake in immature kidney tissue is not deficient;on the contrary,the tissue tookup and concentrated more glycine and lysine than adult tissue. Decreased amino acid efflux from the immature cells appears to be the explanation. Concentration dependence of amino acid uptake was the same in 5-day-old and adult tissue.These differences between the transport characteristics of a model sugar and representative amino acids during development indicate separate transport systems for the two types of substrate.

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Abstract.-The impaired ability of neonatal rat kidney cortex slices to take up L-cystine at a time when the ability to accumulate cysteine is similar to that of adult tissues indicates the separate nature of the transport processes for these amino acids. Dissimilarities in dependence on oxygen and temperature are also indicative of different transport systems. The intracellular form of the amino acid was largely cysteine when either cystine or cysteine was the transported substrate although significant amounts of both were incorporated into reduced glutathione. No difference in intracellular forms was found between neonatal and adult tissue.