Net reabsorption of alpha-aminoisobutyric acid by rat kidney in vivo

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Abstract
Net renal reabsorption of .alpha.-aminoisobutyric acid (AIB) was studied by a clearance method in the anesthetized rat. Plasma [AIB] (0.24 .+-. 0.07 mM, mean .+-. SE, 9 rats) and fractional AIB excretion (FEAIB) (0.126 .+-. 0.033) were at steady state when measured 16 h after i.p. injection of 1.22 mmol/kg. Renal handling controlled plasma [AIB] in the rat under these conditions. Interanimal variability in FEAIB (range, 0.07-0.21, 9 rats) was 30-fold larger than intraindividual variation. Net reabsorption of AIB was altered by infusions of natural amino acids, and during its own reabsorption AIB interacted with the natural amino acids, in particular the short-chain neutral group. AIB uptake by renal tissue occurs preferentially in the medullary region of kidney. Natural amino acids decreased uptake in vivo while increasing FEAIB. Proline, a secondary amino acid that undergoes countertransport with AIB, abolished the difference between cortical and medullary AIB content. Following bolus i.v. infusion of AIB, FEAIB increased (.DELTA. = +0.078 .+-. 0.008, mean .+-. SE, six rats) while plasma [AIB] was decreasing (.DELTA. = -3.2 .+-. 0.3 .mu.M) during the initial 2 h period of the presteady state. This reduction in net efflux of AIB from the lumen appears to be the result of AIB backflux from the epithelial cellular pool during filling of the cells by uptake from extracellular pools.