Proximal tubular reabsorption of some organic acids in the rat kidney in vivo

Molecular cloning and functional reconstitution of a urate transporter/channel

Maintenance of urate homeostasis requires urate efflux from urate-producing cells with subsequent renal and gastrointestinal excretion. The molecular basis for urate transport, however, has not been identified. A novel full-length cDNA encoding a 322-amino acid protein, designated UAT (urate transporter), has been cloned from a rat renal cDNA library by antibody screening. UAT mRNA transcripts that approximate 1.55 kilobases are present, but differentially expressed in various rat tissues. Recombinant UAT protein that was expressed from the cloned cDNA in Escherichia coli and purified via immobilized metal affinity chromatography has been functionally reconstituted as a highly selective urate transporter/channel in planar lipid bilayers. The IgG fraction of the polyclonal antibody that was used to select the UAT clone from the cDNA library, but not nonimmune IgG, blocked urate channel activity. Based on the wide tissue distribution of the mRNA for UAT we propose that UAT provides the molecular basis for urate flux across cell membranes, allowing urate that is formed during purine metabolism to efflux from cells and serving as an electrogenic transporter that plays an important role in renal and gastrointestinal urate excretion.

Excretion of para-aminohippurate in the isolated perfused rat kidney: net secretion and net reabsorption

1. The excretion of para-aminohippurate (PAH) in the isolated perfused rat kidney was examined over a wide range of perfusate PAH concentrations (15 microM to 6 mM). PAH excretion increased steadily over the range of perfusate concentrations, reaching a maximal excretion rate of 3.28 mumol/min at a free-PAH concentration of 6 mM. 2. Tubular transport of PAH was evaluated from the difference between ultrafiltered PAH and excreted PAH. Net PAH secretion was observed at low perfusate free PAH concentrations. Net PAH transport was zero at a perfusate free PAH concentration of 2.1 mM. Above this level there was progressive net reabsorption. 3. Probenecid (2.5 mM) decreased PAH secretion to 18% of the initial value at 129 microM-free PAH (P less than 0.05). Probenecid had no effect on net reabsorption of PAH at high perfusate levels of the anion. 4. Alanine (5 mM) decreased net PAH secretion by 50% at low free PAH concentrations (P less than 0.05) and decreased net PAH reabsorption by 50% at at a free PAH concentration of 6 mM (P less than 0.05). These effects could not be related to effects of PAH, probenecid or alanine on glomerular filtration rate (GFR), vascular resistance or electrolyte excretion. 5. The results confirm the existence and integrity of the proximal tubular organic anion secretory system in the isolated kidney. In addition, net PAH reabsorption occurs at high perfusate levels.

Inhibition of renal accumulation of lysozyme (basic low molecular weight protein) by basic proteins and other basic substances

Together the two rat kidneys accumulated a total of 31.7 +/- 1.6% of the intravenously injected amount of 7 nmoles egg-white-lysozyme (measured as iodine 125 lysozyme) within 10 min. The low molecular weight protein lysozyme and other basic substances were injected simultaneously in order to evaluate whether these basic substances can inhibit the renal lysozyme accumulation. The inhibitory effect of various basic compounds was dose-dependent with a maximal reduction of lysozyme accumulation to 11.7 +/- 0.08%. The basic substances could be divided into three groups depending upon the micromolar amount injected at which a 50% inhibition was achieved (0.3-1.2 micromoles: cytochrome C, ribonuclease; 10.9 micromoles; spermine; 501-688 micromoles: L-arginine, L-lysine). The neutral myoglobin had no effect on renal lysozyme accumulation. The inhibitory potency appeared to increase with increasing molecular weight and pI value of the substance tested. Microperfusion experiments of proximal convoluted tubules of rat kidney revealed that luminal reabsorption of the basic lysozyme can be inhibited by the basic protein cytochrome C in a dose-dependent fashion. In these experiments the perfusion solution contained 57 micromol .l-1 lysozyme, an intratubular lysozyme concentration at which the tubular lysozyme reabsorption was found to be about 80% saturated. A 50% inhibition of the tubular endocytic lysozyme reabsorption was achieved a cytochrome C concentration of 102 micromol.l-1.

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.

Factors affecting urate reabsorption in the rat kidney

1. Urate transport in the rat appears to be saturable. However, affinity of the transport system for urate is very low and transport far from saturated at physiological plasma concentrations. 2. Since increase of the nonionized fraction of uric acid by a factor of five failed to increase urate reabsorption, transport cannot be due to nonionic diffusion but rather involves ionized urate. 3. Increases in luminal flow rate markedly depress urate reabsorption in the loop of Henle, which results in wash out of medullary urate.

Influence of glomerular filtration rate on the rate of para-aminohippurate secretion by the rat kidney: micropuncture and clearance studies

Net secretion rate of para-aminohippurate (PAH) in the proximal convolution of the rat kidney changes concomitantly with single nephron glomerular filtration rate (GFR) and intratubular flow rate. Reabsorption of PAH in the proximal convolution is negligible. The PAH concentration profile along the length of the proximal convolution does not change markedly with variations in GFR. Net PAH secretion by single nephrons, measured at the end of proximal convolutions, is about one-half that measured at the beginning of distal convolutions and in final urine. As in the entire kidney, at constant renal plasma flow and concentration of PAH, renal secretion rate of PAH also changes concomitantly with GFR. It is concluded that PAH secretion along the loop of Henle (i.e., probably along the pars recta) is also related to single nephron GFR, as is PAH secretion in the proximal convolution.

Urate-2-14C transport in the rat nephron

Intrarenal transport of urate-2-(14)C was studied in anesthetized rats using the microinjection technic. During saline diuresis, small volumes of urate-2-(14)C (0.24-0.48 mM) and inulin-(3)H were injected into surface proximal and distal convoluted tubules, and ureteral urine was collected serially. Total (74-96%) and direct (57-84%) urate recovery increased significantly the more distal the puncture site. Delayed recovery (+/-20%) remained approximately the same regardless of localization of the microinjection. After proximal injections, total and direct recoveries of urate-2-(14)C were significantly higher in rats treated with probenecid, pyrazinoate, or PAH than during saline diuresis alone, while the excretion rates were comparable after distal injection. Delayed recovery was not altered by drug administration. The decreased proximal reabsorption of urate is presumably due to an effect of the drugs on the luminal membrane of the nephron. For perfusion at high urate concentrations, nonradioactive urate was added to the injectate (0.89-1.78 mM). Urate-2-(14)C recovery was almost complete and there was no delayed excretion, demonstrating saturation kinetics. These findings are compatible with a carrier-mediated mechanism for urate transport probably located at the luminal border of the proximal tubular epithelium. No definitive evidence for urate secretion was found in these studies.