Basolateral uptake and tubular metabolism of L-citrulline in the isolated-perfused non-filtering kidney of the African clawed toad (Xenopus laevis)

Expression and function of arginine-producing and consuming-enzymes in the kidney

The kidney plays a key role in arginine metabolism. Arginine production is controlled by argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL) which metabolize citrulline and aspartate to arginine and fumarate whereas arginine consumption is dependent on arginine:glycine amidinotransferase (GAT), which mediates creatine and ornithine synthesis. Histological and biochemical techniques have been used to study the distribution and activity of these enzymes in anatomically dissected segments, in isolated fragments of tubules and in whole tissues. ASS and ASL mRNAs and proteins are expressed in the proximal tubule. Within this nephron segment, the proximal convoluted tubule has a higher arginine synthesis capacity than the proximal straight tubules. Furthermore, this arginine-synthesizing portion of the nephron matches perfectly with the site of citrulline reabsorption from the glomerular filtrate. The kidney itself can produce citrulline from methylated arginine, but this capacity is limited. Therefore, intestinal citrulline synthesis is required for renal arginine production. Although the proximal convoluted tubule also expresses a significant amount of GAT, only 10% of renal arginine synthesis is metabolized to guanidinoacetic acid, possibly because GAT has a mitochondrial localization. Kidney arginase (AII) is expressed in the cortical and outer medullary proximal straight tubules and does not degrade significant amounts of newly synthesized arginine. The data presented in this review identify the proximal convoluted tubule as the main site of endogenous arginine biosynthesis.

Production of arginine by the kidney is impaired in a model of sepsis: early events following LPS

Lipopolysaccharide (LPS) is used experimentally to elicit the innate physiological responses observed in human sepsis. We have previously shown that LPS causes depletion of plasma arginine before inducible nitric oxide synthase (iNOS) activity, indicating that changes in arginine uptake and/or production rather than enhanced consumption are responsible. Because the kidney is the primary source of circulating arginine and renal failure is a hallmark of septicemia, we determined the time course of changes in arginine metabolism and kidney function relative to iNOS expression. LPS given intravenously to anesthetized rats caused a decrease in mean arterial blood pressure after 120 min that coincided with increased plasma nitric oxide end products (NOx) and iNOS expression in lung and liver. Interestingly, impairment of renal function preceded iNOS activity by 30–60 min and occurred in tandem with decreased renal arginine production. The baseline rate of renal arginine production was ∼60 μmol·h−1·kg−1, corresponding to an apparent plasma half-life of ∼20 min, and decreased by one-half within 60 min of LPS. Calculations based on the systemic production and clearance show that normally only 5% of kidney arginine output is destined to become nitric oxide and that <25% of LPS-impaired renal production was converted to NOx in the first 4 h. In addition, we provide novel observations indicating that the kidney appears refractory to iNOS induction by LPS because no discernible enhancement of renal NOx production occurred within 4 h, and iNOS expression in the kidney was muted compared with that in liver or lung. These studies demonstrate that the major factor responsible for the rapid decrease in extracellular arginine content following LPS is impaired production by the kidney, a phenomenon that appears linked to reduced renal perfusion.

Renal transepithelial secretion of ochratoxin A in the non-filtering toad kidney

Renal excretion is an important way of elimination for the nephrotoxin ochratoxin A (OTA). Because binding to proteins hinders filtration, excretion is mainly due to proximal tubular secretion. The goal of this study was to investigate the kinetics of secretion as well as the extent of urine and tissue accumulation in situ using the non-filtering amphibian kidney model. Transepithelial secretion is a saturable process (K(m) = 0.63.10(-6) mol/l) and leads to a concentration-dependent accumulation of OTA in the tubular lumen and in renal tissue. Maximum accumulation achieved is approximately 14-fold as compared to the perfusate concentration. There was no accumulation in the tubular lumen as compared to renal tissue (lumen-to-tissue concentration ratio approximately 1). Tissue and tubular lumen accumulation were reduced to approximately 40% of control in the presence of 10(-3) mol/l p-aminohippurate (PAH). Addition of 10(-3) mol/l alpha-ketoglutarate (KG) to PAH-containing perfusate did not lead to a further reduction of secretion. By contrast, addition of 10(-2) mol/l L-phenylalanine (L-Phe) reduced secretion further to approximately 25% of control. In the presence of 10(-3) mol/l probenecid tissue accumulation was reduced to 7% and tubular lumen accumulation to 1% of control. Lumen-to-tissue concentration ratio decreased to 0.15 in the presence of probenecid, indicating an inhibitory action at the luminal membrane. Addition of albumin to the perfusate, reduced secretion to only 50% of control, whereas the concentration of free OTA was reduced below 1% as compared to control. The results of this study show that transepithelial secretion is an effective way for accumulation of OTA in the tubular lumen and thus its urinary excretion. Transport via the basolateral organic anion and a basolateral amino acid carrier are the active steps in transepithelial secretion. Luminal exit of OTA is a passive process. Furthermore, tissue accumulation by the active transport across the basolateral membrane supports the toxic action of OTA on proximal tubular cells. Due to the qualitative similarity of organic anion transport our findings should also apply for the mammalian kidney.

Cationic amino acid fluxes beyond the proximal convoluted tubule of rat kidney

To investigate the fluxes of cationic amino acids beyond the proximal convolution, we micropunctured and microperfused superficial tubules of male Wistar rats in vivo et situ. In free-flow micropuncture experiments, the concentrations of endogenous L-arginine+, [Arg], and of intravenously infused L-homoarginine+, [HoArg], were determined by HPLC. Fluorescein isothiocyanate-labeled inulin was detected on-line in the same tubular fluid samples. To determine undirectional fluxes, radiolabeled Arg and inulin were (1) microperfused through short loops of Henle and (2) microinfused into different tubule segments to measure urinary recovery of the radiolabel. At a mean [Arg]plasma of 116 mumol/l, [Arg] was 9.3 mumol/l in the late proximal tubule (LPT), and 35.6 mumol/l in the early distal tubule (EDT) corresponding to fractional deliveries (FD) of 0.055 in LPT and 0.078 in EDT. Fractional urinary excretion (FE) of Arg was 0.00033 (P < 0.05 vs FDEDT). Infusion of HoArg (2.5 or 7.5 mumol/min) led to respective mean [HoArg]plasma values of 1.44 and 3.73 mmol/l, and resulted in respective FDLPT values for HoArg of 0.23 and 0.53, respective FDEDT values of 0.29 and 0.41, and finally, respective FE values for HoArg of 0.25 and 0.58. When short loops of Henle were microperfused with 1 or 50 mmol/l [14C]Arg (+[3H]inulin), fractional recovery (FR) of 14C (relative to inulin) in the EDT was 0.13 and 0.36, respectively. During microinfusion of radiolabeled Arg (1 or 50 mmol/l) and inulin into LPT, the urinary FR of the radiolabel was 0.14, or 0.59, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal synthesis of arginine in chronic renal failure: in vivo and in vitro studies in rats with 5/6 nephrectomy

Synthesis of arginine (Arg) from citrulline (Cit) by the kidney is a major source of Arg for the body. The high level of plasma Cit in chronic renal failure is often thought to result from the impairment of the renal conversion of Cit to Arg. To verify this assumption, we performed two studies in Sprague-Dawley rats with 5/6 nephrectomy (CRF rats) and in sham-operated rats (CONT rats). In study I synthesis of Arg by isolated proximal convoluted tubules (PCT; the nephron segment exhibiting the highest Arg synthesis) was measured in vitro with two concentrations of Cit (200 or 50 microM) corresponding to those observed in plasma of rats with or without renal failure. In study II the net renal uptake of Cit and release of Arg were determined in vivo by measuring PAH clearance and arterial and renal venous Arg, and Cit concentrations in anesthetized rats. The in vitro results showed that Arg synthesis increased only in proportion to the hypertrophy of remnant PCT (+50%), and was highly and similarly dependent on Cit concentration in PCT of remnant and normal kidneys (Arg production with 200 microM Cit was 3 times higher than with 50 microM Cit for both CONT and CRF). The in vivo results showed that renal Cit uptake and Arg release were not altered in CRF: -286 +/- 28 versus -326 +/- 16 nmol Cit.min-1 (NS), and + 390 +/- 47 versus + 399 +/- 22 nmol Arg.min-1 (NS) in CONT and CRF rats, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

On leaking into the lumen, amino acids cross the tubule cells. Secretion of L-citrulline in the isolated-perfused non-filtering kidney of the African clawed toad (Xenopus laevis)

In our previous studies we were able to show that L-citrulline is taken up across the contraluminal membrane into the tubule cells of the isolated perfused non-filtering kidney of the African clawed toad (Xenopus laevis). We have also described the tubular metabolism of the L-citrulline taken up. The remaining question was whether peritubular L-citrulline gains access to the tubular lumen and, if so, by what kind of mechanism. The method was the same as that described in our previous study: the perfusion of the kidneys was performed through both portal veins (2 ml/h into each) with a solution containing inulin. In order to stimulate the urine formation in the non-filtering kidney, we added p-aminohippurate to the perfusion solution. In the urine collected from the ureters no inulin could be detected. Thus, the glomeruli were not reached by the perfusate, and no glomerular filtration took place. Upon adding 65 mumol/l L-citrulline to the perfusion solution (corresponding to the physiological plasma concentration), a urinary excretion rate of this amino acid of 4.5 +/- 0.4 nmol.h-1.g-1 (n = 6) was determined. During perfusion under the same conditions with D-citrulline (65 mumol/l) the excretion rate was only 0.36 +/- 0.1 nmol.h-1.g-1 (n = 5). A similarly low excretion rate was also found with L-citrulline when L-phenylalanine (20 mmol/l) was added to the perfusate or when NaCl was substituted by mannitol. These data show that L-citrulline taken up from the contraluminal side of the renal tubular epithelium enters the tubular lumen to a much greater extent than its D-isomer.(ABSTRACT TRUNCATED AT 250 WORDS)