MICROPERFUSION STUDY OF L-HISTIDINE TRANSPORT BY THE RAT NEPHRON

Renal handling of L-histidine studied by continuous microperfusion and free flow micropuncture in the rat

Renal tubular reabsorption of L-histidine (His) was measured in vivo et situ by continuous microperfusion and free flow micropuncture of single proximal convoluted tubules of the rat kidney. The reabsorption is shown to be saturable. A permeability coefficient (P) of less than 29 microns 2 . s-1, a maximum reabsorption rate (J max) of 2.75 +/- 1.05 greater than J max greater than 1.97 +/- 0.86 (SEM) nmol . m-1 . s-1 and an affinity constant (Km) of 13.8 +/- 4.2 greater than Km greater than 10.9 +/- 4.0 (SEM) mol . 1-1 (lower values for P = 29 microns 2 . s-1, higher values for P = 0) were calculated from the microperfusion data. Using these constants and taking backflux of His and water reabsorption into account a good fit with the concentration profile of His along the proximal tubule--measured by free flow micropuncture--was obtained. Varying the buffered pH-values of the perfusion fluids (5.0 or 7.4) influenced neither the active reabsorption nor passive permeability of His. This indicates that the charge of the imidazol group of His does not play a significant role in His reabsorption. Further experiments showed that the addition of 20 mmol . 1-1 L-arginine--a strong inhibitor of the reabsorption system for dibasic amino acids--did not have a significant effect on the reabsorption of L-histidine. It is concluded, therefore, that His is reabsorbed by a system for neutral amino acids. Non ionic diffusion does not play an important role for His reabsorption.

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

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.

Amino acid reabsorption in the proximal tubule of rat kidney: stereospecificity and passive diffusion studied by continuous microperfusion

Renal tubular reabsorption of glycine and of the L- and D-isomers of histidine, serine, phenyl-alanine, methionine, proline and cystine was investigated in vivo et situ by continuous microperfusion of single proximal convolutions of the rat kidney. In the case of glycine and the L-isomers, tubular reabsorption is saturable to a great extent. The D-amino acids are reabsorbed much more slowly than the respective L-forms. Furthermore in the case of methionine and perhaps also of proline, serine and phenylalanine, the fractional reabsorption decreases in the presence of high concentrations of the L-form. This indicates that the D-isomers also have a measurable affinity for the reabsorption mechanisms of the renal tubule. The very poor reabsorption of D-amino acids in the presence of their L-isomers indicates that simple passive diffusion plays only a relatively small role in tubular amino acid reabsorption. Permeability coefficients estimated from these findings are in the range from 1--5 X 10(-7) cm2 - s-1. These values are very similar to those found for other organic molecules of comparable molecular weights.

Amino acid reabsorption in the rat nephron. Free flow micropuncture study

The concentration of nine endogenous free L-alpha-amino acids (ALA, LEU, ILE, PHE, TYR, LYS, GLU, PRO, GLY) and of taurine were determined simultaneously along the nephron of the rat kidney using free-flow micropuncture techniques without altering plasma amino acid concentration or kidney function. The amount of each amino acid was determined after dansylation (14C-labelled dansyl-chloride) in the micropuncture sample followed by thinlayer chromatography. The main site of reabsorption is the proximal tubule. After 15-20% of the proximal tubule length the bulk of reabsorption has taken place (18.9 plus or minus 3.4% S.E. of the filtered load remaining). Net reabsorption continues to a small but significant extent along the distal nephron (disal tubule and collecting duct). Reabsorption of taurine is less rapid (% remaining of filtered load at the early proximal tubule 37.0 plus or minus 4.6%). The transtubular concentration ratio of all amino acids except taurine follows a homogeneous course. Under the experimental conditions of this study no distction with respect to different systems of reabsorption "neutral", "basic", "acidic", "imino-glycine") could be made.