Kidney Protein Dynamics and Ammoniagenesis in Humans with Chronic Metabolic Acidosis GIACOMO GARIBOTTO,* ANTONELLA SOFIA,* CRISTINA ROBAUDO,* STEFANO SAFFIOTI,* MARIA RITA SALA,* DANIELA VERZOLA,* MONICA VETTORE, † RODOLFO RUSSO,* VANESSA PROCOPIO,* GIACOMO DEFERRARI,* and PAOLO TESSARI † *Division of Nephrology, Department of Internal Medicine, University of Genoa, and † Department of Metabolic Diseases, University of Padova, Italy Abstract. 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 acido- sis. 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 produc- tion 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 ef- fect, which is likely induced by the increase in ammonia excretion and partition into the urine, is potentially responsible for kidney hypertrophy. Both in vitro and in vivo studies in animals have shown that metabolic acidosis causes several biochemical and morpho- logic changes in tubule cells, which are ultimately associated with kidney hypertrophy (1,2). Protein synthesis and degrada- tion, the determinants of protein metabolism, are key factors in the hypertrophy process. However, information regarding the signals and mechanisms by which metabolic acidosis might cause kidney hypertrophy is still incomplete, and whether these effects take place in the human kidney is unknown. A further complicating element is that the effects of acidosis on protein turnover rates vary in different tissue types, with catabolic events occurring in peripheral tissues and splanchnic organs (3,4). Chronic acidosis causes several metabolic alterations in the renal proximal tubule, including increased H + secretion (1,5), ammonia synthesis (6), and citrate reabsorption (6,7). These changes in tubule function are associated with changes in the activities of a number of proteins, including the apical mem- brane Na + /H + antiporter (7), glutaminase and glutamate de- hydrogenase (8), and phosphoenolpyruvate carboxykinase (9), which may affect intracellular substrate availability for protein turnover, amino acid metabolism and/or transport, and glu- coneogenesis. It has been shown that in tubule epithelial cells, the associated increase in ammonia production, rather than the acidosis per se, is responsible for favoring tubular hypertrophy (10 –12). This effect is related to the inhibition of protein degradation, owing to changes in lysosomal pH and cathepsin activity (12). In addition, other mechanisms may be responsi- ble for tubular hypertrophy. Ammonium chloride, used to induce acidosis, may decrease amino acid oxidation (11), an- other effect that could account for the increase in tubular protein content. Furthermore, chronic acidosis activates early genes, which are associated with growth and could therefore promote protein synthesis (6). In a previous in vivo study in the rat, kidney hypertrophy was shown to be associated with both a decrease in protein degradation and an increase in protein synthesis (13). We previously evaluated protein turnover across the human kidney in studies on the basis of the organ mass balance associated with leucine isotope kinetics (14,15). Kidney pro- tein turnover, as compared with muscle and splanchnic turn- over, is characterized by the highest rates of protein synthesis and amino acid oxidation. These effects are mainly the expres- sion of tubular epithelial cell metabolism, because glomeruli make up only 5% of the kidney weight (16). In terms of mass Received October 24, 2003. Accepted March 12, 2004. Correspondence to Dr. Giacomo Garibotto, Dipartimento di Medicina Interna, Divisione di Nefrologia, Viale Benedetto XV, 6, 16132 Genoa, Italy; Phone: +390103538989; Fax: +390103538959; E-mail: gari@unige.it 1046-6673/1506-1606 Journal of the American Society of Nephrology Copyright © 2004 by the American Society of Nephrology DOI: 10.1097/01.ASN.0000127865.26968.36 J Am Soc Nephrol 15: 1606–1615, 2004