A suspension of cortical tissue fragments prepared by collagenase digestion of renal cortex obtained from fed and chronically acidotic (NH4Cl) rats was separated into four bands on a Percoll density gradient. By microscopic examination, vital staining with trypan blue, and histologic staining technique (periodic acid-Schiff) the F4 band was shown to contain only (greater than 98%) proximal tubules, whereas the F1 band was significantly enriched (70%) with distal tubules contaminated by glomeruli and short segments of proximal tubules. Intra/extracellular ratios for PAH of 15 were measured in the F4 band and of 2 in F1 band. ATP was 1.4 and 2.8 mumol/g in the F4 and F1 bands, respectively, and was stable for at least 60 min. The proximal F4 band was shown to be gluconeogenic (L-glutamine or L-lactate 2.5 mM as substrate) and to adapt to metabolic acidosis. The distal F1 band was shown to be glycolytic (glucose 2.5 mM) with no changes with acid-base status. All fractions were shown to metabolize glutamine, but the metabolic fate of this amino acid was different in proximal and distal structures. A F4/F1 activity ratio for the proximal cytoplasmic phosphoenolpyruvate carboxykinase enzyme of 2.6 and 4.3 was observed in normal and acidotic rats, respectively. In contrast, a F4/F1 ratio of 0.13 and 0.22 was observed for the distal cytoplasmic hexokinase enzyme. This preparation, therefore, allows the metabolism of a homogeneous population of proximal tubular fragments to be studied and can be used to obtain information on enzyme location within the nephron.
Acute metabolic acidosis was induced by an i.v. administration of hydrochloric acid to dogs and rats to decrease the plasma bicarbonate concentration from 22 to 12 mM in dogs and from 26 to 10 mM in rats. Chronic metabolic acidosis was also induced in dogs by ammonium chloride feeding for 5 days. Rats also were given ammonium chloride for 24 hours. The renal metabolite profile was determined on the freeze-clamped renal tissue before and after 100 min (dogs) or 30 to 240 min (rats) of acsute acidosis. Measurements on chronically acidotic dogs and rats with 24-hour acidosis were obtained also for comparison with acute acidosis. In both species, kidney glutamine, glutamate, and alpha-ketokglutarate concentrations decreased drastically following induction of acute or chronic acidosis, In the dog, or in the rat during the first 2 hours of acidosis, malate concentration was unchanged. Malate concentration fell significantly in the rat kidney only after 2 hours of acidosis without change in phosphoenolpyruvate (PEP) concentration. In chronically acidotic dogs, malate and oxaloacetate rose fivefold with no change in PEP concentration. Phosphoenolpyruvate carboxykinase (PEPCK) activity was not stimulated by chronic metabolic acidosis in the dog in contrast to the rat. Acute acidosis by hydrochloric acid increased net renal glutamine extraction in the rat but not in the dog. These data suggest that an increased metabolic flux occurs between alpha-ketoglutarate and malate in both rat and dog kidney during acute metabolic acidosis. In the rat, however, after 2 hours, PEPCK activation modifies the kidney metabolite profile. Intrarenal glutamine transport seems to be a rate-limiting factor for adaptation to acute acidosis in the dog but not in the rat kidney.
Preparations of distinct nephron segments were obtained from dog kidneys by collagenase treatment. Four morphologically different tissues were isolated: glomeruli, proximal tubules, thick ascending limbs, and papillary collecting ducts. Each segment possessed a characteristic assay of membrane-bound and cytoplasmic enzymes. Specific metabolic characteristics also were found: gluconeogenesis and ammoniagenesis in proximal tubules, glycolytic aerobic metabolism in thick ascending limbs, and glycolytic anaerobic metabolism in papillary collecting ducts. The assay of Na+–K+ ATPase, H+-ATPase, and Ca2+-ATPase activities in these nephron segments demonstrated a specific enrichment of Na+–K+ ATPase in thick ascending limbs, and of H+-ATPase in proximal tubules and papillary collecting ducts. Tubular respiration in the absence or presence of ouabain, 1,3-dicyclohexylcarbodiimide, or furosemide demonstrated that the respiration of each segment could be correlated to the activity of specific ion motive ATPases. Furthermore, a tight coupling between ion transport, ATP turnover, and substrate oxidation was demonstrated. These isolated tubular structures are thus viable and capable of transepithelial transport. Our preparation provides large amounts of defined population of tubules and are thus useful for the study of biochemical and functional heterogeneity along the nephron.
The capacity of chronically hemodialyzed patients to metabolize acetate during conventional hemodialysis was evaluated using a retrospective study in 219 patients dialyzed for up to ten years under similar dialysis conditions. For each patient, and using all available data, a regression line relating the changes of plasma total CO2 during dialysis as a function of the pre-dialysis value was calculated. The intercept of this function indicates the plasma concentration where the losses of bicarbonate in the dialysate is matched by the generation of bicarbonate arising from the metabolism of acetate. This value therefore represents an individual index of the capacity of each patient to metabolize acetate. A value for this index smaller than 18.0 mM was considered abnormal. It was shown that around 10% of chronically hemodialyzed patients are clearly unable to metabolize acetate optimally. This defect is not related to the duration of dialysis, body weight or quality of hemodialysis treatments but is strongly related to sex, 19 of the 22 "acetate intolerant" patients being women. In a prospective study, all the 60 patients of the same population undergoing active dialysis were studied, and this index identified 12 abnormal (11 women, 1 man) patients and 48 normal patients. Plasma acetate measured at the end their dialysis treatments were significantly higher in abnormal than in normal patients. It is concluded: that this index is useful to identify the patients unable to metabolize acetate optimally; that only around 10% of hemodialyzed patients present a severe problem when dialyzed against acetate and should be dialyzed against bicarbonate; that dialysis against acetate does not fully correct the metabolic acidosis even in "normal" patients.
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