Micropuncture studies of the transport of individual amino acids by the Necturus proximal tubule

Abstract

Absorption of amino acids by the necturus proximal tubules was measured under free-flow conditions The coexistence of proximal tubular amino acid influx was determined by infusing saline into tubular lumens by the stopped-flow microperfusion technique. Under free-flow conditions, fractional absorption of individual amino acids ranged from 0.30 +/- 0.18 (glutamic acid) to 0.96 +/- 0.02 (proline), with 14 of 19 values greater than 0.75. The transport avidity for a given amino acid bore no relationship to its molecular weight, transport class, or plasma concentration. The values obtained for tubular fluid/plasma (TF/P) were very comparable to those reported for the rat. In stopped-flow microperfusion experiments, samples of isotonic saline residing in tubule lumens for 20 min were found to contain all the amino acids present in plasma (filtrate). The concentrations of all except the acidic anionic pair, glutamic acid and aspartic acid, were remarkably similar to those obtained by collection of end proximal samples in free-flow studies. The very high concentrations of the acidic amino acids may reflect their passive distribution across the luminal cell membrane, active absorption having been impaired by the absence of some substance normally present in glomerular filtrate.