A comparison of the efflux rates of AIB from kidney cortex slices of mature and newborn rats

Earlier observations of the pattern of uptake of amino acids by rat kidney cortex slices taken from mature and from newborn animals indicated that although the initial rate of entry was invariably more rapid in the mature tissue, the concentration gradient eventually achieved was consistently higher in the newborn tissues. Assuming that the rate of entry was relatively constant, the final concentration differences could only be explained if there was a significantly more rapid efflux of amino acid from the mature tissues. Studies were therefore carried out to measure the rate of efflux of 14C-labelled α-aminoisobutyric acid (AIB) from newborn and mature tissues previously loaded with this nonmetabolized amino acid. It was observed that the rate of efflux was markedly greater in the case of the mature tissues. At 1 h the proportion of AIB which had effluxed was about twice as great in the mature as compared with the newborn tissue. A comparatively small fraction of the efflux from both types of tissue was shown to be dependent on the concentration of external AIB and may have resulted from an exchange diffusion process. This fraction did not account for any significant part of the difference in efflux rates, which probably was the result of different rates of passive diffusion.