Stefano Saffioti

Kidney, splanchnic, and leg protein turnover in humans. Insight from leucine and phenylalanine kinetics.

The rate of kidney protein turnover in humans is not known. To this aim, we have measured kidney protein synthesis and degradation in postabsorptive humans using the arterio-venous catheterization technique combined with 14C-leucine, 15N-leucine, and 3H-phenylalanine tracer infusions. These measurements were compared with those obtained across the splanchnic bed, the legs (approximately muscle) and in the whole body. In the kidneys, protein balance was negative, as the rate of leucine release from protein degradation (16.8 +/- 5.1 mumol/min.1.73 m2) was greater (P < 0.02) than its uptake into protein synthesis (11.6 +/- 5.1 mumol/min. 1.73 m2). Splanchnic net protein balance was approximately 0 since leucine from protein degradation (32.1 +/- 9.9 mumol/min. 1.73 m2) and leucine into protein synthesis (30.8 +/- 11.5 mumol/min. 1.73 m2) were not different. In the legs, degradation exceeded synthesis (27.4 +/- 6.6 vs. 20.3 +/- 6.5 mumol/min. 1.73 m2, P < 0.02). The kidneys extracted alpha-ketoisocaproic acid, accounting for approximately 70% of net splanchnic alpha-ketoisocaproic acid release. The contributions by the kidneys to whole-body leucine rate of appearance, utilization for protein synthesis, and oxidation were approximately 11%, approximately 10%, and approximately 26%, respectively; those by the splanchnic area approximately 22%, approximately 27%, and approximately 18%; those from estimated total skeletal muscle approximately 37%, approximately 34%, and approximately 48%. Estimated fractional protein synthetic rates were approximately 42%/d in the kidneys, approximately 12% in the splanchnic area, and approximately 1.5% in muscle. This study reports the first estimates of kidney protein synthesis and degradation in humans, also in comparison with those measured in the splanchnic area, the legs, and the whole-body.

Protein-energy wasting and mortality in chronic kidney disease.

Protein-energy wasting (PEW) is common in patients with chronic kidney disease (CKD) and is associated with an increased death risk from cardiovascular diseases. However, while even minor renal dysfunction is an independent predictor of adverse cardiovascular prognosis, PEW becomes clinically manifest at an advanced stage, early before or during the dialytic stage. Mechanisms causing loss of muscle protein and fat are complex and not always associated with anorexia, but are linked to several abnormalities that stimulate protein degradation and/or decrease protein synthesis. In addition, data from experimental CKD indicate that uremia specifically blunts the regenerative potential in skeletal muscle, by acting on muscle stem cells. In this discussion recent findings regarding the mechanisms responsible for malnutrition and the increase in cardiovascular risk in CKD patients are discussed. During the course of CKD, the loss of kidney excretory and metabolic functions proceed together with the activation of pathways of endothelial damage, inflammation, acidosis, alterations in insulin signaling and anorexia which are likely to orchestrate net protein catabolism and the PEW syndrome.

Amino acid and protein metabolism in the human kidney and in patients with chronic kidney disease.

The progressive loss of kidney function in patients with chronic kidney disease (CKD) is associated with a number of complications, including cardiovascular diseases, anemia, hyperparathyroidism, inflammation, metabolic acidosis, malnutrition and protein-energy wasting. The excess cardiovascular risk related to CKD is due in part to a higher prevalence of traditional atherosclerotic risk factors, in part to non-traditional, emerging risk factors peculiar to CKD. While even minor renal dysfunction is an independent predictor of adverse cardiovascular prognosis, nutritional changes are more often observed in an advanced setting. In addition, factors related to renal-replacement treatment may be implicated in the pathogenesis of heart disease and protein-energy wasting in dialysis-treated patients. Progressive alterations in kidney metabolism may cause progressive effects on cardiovascular status and nutrition. Altered kidney amino acid/protein metabolism and or excretion may be a key factor in the homeostasis of several vasoactive compounds and hormones in patients with more advanced disease. In this discussion recent research regarding the kidney handling of amino acids and protein turnover and their potential link with cardiovascular disease, progressive kidney dysfunction and nutritional status are reviewed.

Apoptosis and myostatin mRNA are upregulated in the skeletal muscle of patients with chronic kidney disease

Apoptosis and myostatin are major mediators of muscle atrophy and might therefore be involved in the wasting of uremia. To examine whether they are expressed in the skeletal muscle of patients with chronic kidney disease (CKD), we measured muscle apoptosis and myostatin mRNA and their related intracellular signal pathways in rectus abdominis biopsies obtained from 22 consecutive patients with stage 5 CKD scheduled for peritoneal dialysis. Apoptotic loss of myonuclei, determined by anti-single-stranded DNA antibody and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling assays, was significantly increased three to fivefold, respectively. Additionally, myostatin and interleukin (IL)-6 gene expressions were significantly upregulated, whereas insulin-like growth factor-I mRNA was significantly lower than in controls. Phosphorylated JNK (c-Jun amino-terminal kinase) and its downstream effector, phospho-c-Jun, were significantly upregulated, whereas phospho-Akt was markedly downregulated. Multivariate analysis models showed that phospho-Akt and IL-6 contributed individually and significantly to the prediction of apoptosis and myostatin gene expression, respectively. Thus, our study found activation of multiple pathways that promote muscle atrophy in the skeletal muscle of patients with CKD. These pathways appear to be associated with different intracellular signals, and are likely differently regulated in patients with CKD.

Effects of a Protein Meal on Blood Amino Acid Profile in Patients with Chronic Renal Failure

Arterial whole blood levels of amino acids were determined in patients with chronic renal failure and in healthy subjects before and after 270 min after the ingestion of a grilled beefsteak (4 g/kg). In patients, total nonessential amino acids increased significantly more (+46%) than in controls owing to an exaggerated rise of serine, glutamine, proline, glycine, cyst(e)ine and alanine. Total essential amino acids increased as much as in controls; however, threonine, histidine and phenylalanine showed greater increases, while tryptophan had a smaller increment. Abnormalities in amino acid levels were even more evident in the postprandial period than in the postabsorptive state owing to reduced levels of valine, leucine, tryptophan, tyrosine, aspartate and glutamate and higher levels of glutamine, proline, glycine, cyst(e)ine, threonine, histidine and phenylalanine. Moreover, after the meal, the ratios total essential amino acids/total nonessential amino acids, valine/glycine, and branched-chain amino acids/total amino acids rose but persisted to be reduced whereas tryptophan/total amino acids and tyrosine/phenylalanine ratios increased in controls, but did not change in patients. In conclusion, in chronic renal failure, protein ingestion enhances the imbalance in amino acid levels already present in the postabsorptive state. The all data indicate that in patients with chronic renal failure, the metabolism of exogenous protein is impaired and the flow of amino acids to the organs is altered during the phase of body nitrogen replenishment.

Sonographic detection of renal changes in pseudoxanthoma elasticum

Pseudoxanthoma elasticum (PXE) is a rare hereditary disease whose basic abnormality is degeneration of elastic tissue, with calcification of the tonaca media of the arteries, and typical yellowish papules of the skin. We report the sonographic findings observed in a young patient with PXE at the level of the kidneys: fine hyperechogenic spots suggesting small calcifications were diffuse at the corticomedullary junction; however, normal waveforms were obtained at the level of intraparenchymal renal vessels. The presence of this structural pattern in a young patient with dermatologic abnormalities should lead to the consideration of PXE in the differential diagnosis list.

Kidney Protein Dynamics and Ammoniagenesis in Humans with Chronic Metabolic Acidosis

To evaluate the effects of chronic metabolic acidosis on protein dynamics and amino acid oxidation in the human kidney, a combination of organ isotopic ((14)C-leucine) and mass-balance techniques in 11 subjects with normal renal function undergoing venous catheterizations was used. Five of 11 studies were performed in the presence of metabolic acidosis. In subjects with normal acid-base balance, kidney protein degradation was 35% to 130% higher than protein synthesis, so net protein leucine balance was markedly negative. In acidemic subjects, kidney protein degradation was no different from protein synthesis and was significantly lower (P < 0.05) than in controls. Kidney leucine oxidation was similar in both groups. Urinary ammonia excretion and total ammonia production were 186% and 110% higher, respectively, and more of the ammonia that was produced was shifted into urine (82% versus 65% in acidemic subjects versus controls). In all studies, protein degradation and net protein balance across the kidney were inversely related to urinary ammonia excretion and to the partition of ammonia into urine, but not to total ammonia production, arterial pH, [HCO(-)(3)], urinary flow, the uptake of glutamine by the kidney, or the ammonia released into the renal veins. The data show that response of the human kidney to metabolic acidosis includes both changes in amino acid uptake and suppression of protein degradation. The latter effect, which is likely induced by the increase in ammonia excretion and partition into the urine, is potentially responsible for kidney hypertrophy.

Ultrasonographic imaging and doppler analysis of renal changes in non-insulin-dependent diabetes mellitus

RATIONALE AND OBJECTIVES We used ultrasonographic imaging and Doppler analysis to assess renal changes in patients with non-insulin-dependent diabetes mellitus (NIDDM) and normal renal function, as established by normal serum creatinine levels and the absence of macroalbuminuria. METHODS Renal parenchymal echogenicity, renal volume, and resistive index (RI) were blindly evaluated for 85 NIDDM patients and 42 age-matched control subjects (C). Results were analyzed and correlated with the following clinical parameters: patient age, duration of diabetes, blood pressure, blood glucose and cholesterol levels, and the presence of microalbuminuria. RESULTS Normal renal parenchymal echogenicity was seen in all but one NIDDM patient; however, in comparison with C, diabetic patients had significantly higher renal volume (mean +/- standard deviation, 314.01 +/- 72.74 vs 227.64 +/- 58.76) and RI (mean +/- standard deviation, 0.71 +/- 0.05 vs 0.64 +/- 0.02). An RI higher than 0.70 was found in 55 of 85 (65%) NIDDM patients; an increased RI was directly correlated with patient age, whereas an inverse correlation existed between an increased RI and renal volume. No statistically significant differences were observed for the duration of diabetes, arterial hypertension, blood levels of glucose and cholesterol, and the presence of microalbuminuria. CONCLUSIONS NIDDM patients with normal renal function show a significant increase in renal volume and RI in comparison with C. Demonstration of these findings may aid in the detection of early renal involvement in NIDDM patients. However, further investigations are needed to understand fully the correlation of such changes with the pathology of diabetic nephropathy and to provide an interpretation of the pathophysiologic mechanisms underlying changes in intrarenal vascular impedance in NIDDM patients.

Successful Treatment of Mitomycin C-Associated Hemolytic Uremic Syndrome by Plasmapheresis

Therapeutic attempts have generally failed to reverse the rapid progression of renal failure after mitomycin C (MMC)-induced hemolytic uremic syndrome (HUS). A patient who developed HUS after MMC and who showed pathologic changes in the kidney consistent with thrombotic microangiopathy is reported. Treatment with plasmapheresis was followed by a favorable outcome and significant recovery of renal function. Accordingly, this therapeutic modality should be considered for patients with MMC-induced HUS.

Mechanisms of renal ammonia production and protein turnover

Renal synthesis and excretion of ammonia are critical for efficient removal of acids from the body. Besides the rate of ammonia production, the intrarenal distribution of produced ammonia is a crucial step in the renal regulation of acid-base balance. Various acid-base disorders are associated not only with changes in ammonia production but also with its distribution between the urine and the renal veins. The final effect of ammonia production on acid-base balance largely depends on the events that determine the distribution of ammonia produced between urine and blood. Several factors, among which urine pH, urine flow, total ammonia production “per se” and renal blood flow may affect the percent of ammonia excreted into urines in humans with different acid-base disturbances. Among these factors, urine pH is the most important. An additional effect of stimulated ammoniagenesis is kidney hypertrophy. In tubule epithelial cells, the associated increase in ammonia production, rather than the acidosis per se, is responsible for favoring tubular hypertrophy. This effect is related to the inhibition of protein degradation, owing to changes in lysosomal pH and cathepsin activity, without effects on cell cycle. Both changes of PI-3 kinase pathway and the suppression of chaperone-mediated autophagy are candidate mechanism for ammonia-mediated inhibition of protein degradation in tubule cells. Available data in humans indicate that the response of kidney to metabolic acidosis includes both changes in amino acid uptake and suppression of protein degradation. The latter effect is associated woth the increase in ammonia excretion and partition into the urine.