Enhanced reabsorption of bicarbonate and phosphorus by the addition of amino acids in the isolated perfused rat kidney

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In the isolated perfused kidney, a defect in urinary acidification has been noted [1]. Nevertheless, this preparation which used rat kidney with albumin saline solution as perfusate including glucose as substrate has also been used for the study of distal acidification, because of urinary acidification reached in the extreme metabolic acidosis [2, 3]. Recently, in a model of the isolated perfused rat kidney, it was demonstrated clearly that adding amino acids results in the improvement and stability of renal function which are associated with the amelioration of histological damage of the thick ascending limb of Henle [4]. This observation prompted us to re-evaluate acid-base parameters of this preparation with respect to the effect of amino acids. Our results show that the addition of amino acids mixture leads to significant increase and stability of the reabsorption of HCO3 and phosphorus as well as the constant urine pH. Methods. Kidneys from male Sprague-Dawley rats weighing 340 to 400 g were perfused in vitro as previously described [2, 5] except for using 18-gauge metal needles instead of glass cannulas. Calculated mean artery pressure was maintained at 90 mm Hg during the experiments. Perfusion was carried out for 85 mm with the last 60 mm divided into four clearance periods with 6.7% bovine albumin Krebs-Henseleit saline solution, equilibrated with 97% 02/5% CO2. The perfusion medium contained 5 mM glucose with (N = 5), or without amino acids (N = 5), the composition of which was similar to that of a previous study [4]. Prior to each experiment 2 ml of concentrated essential amino acid solution (M. A. Bioproducts, Walkersville, Maryland, USA) were added to 100 ml of a prepared perfusate. The amino acid solution contained 12 amino acids including cystine (2.4 mg) and nine other amino acids individually. This procedure provides the similar composition of amino acids as previously described [4] except for the presence of cystine. Perfusate pH was adjusted to about 7.40. Urine and perfusate samples were obtained at 15-mm intervals; urine samples were collected under mineral oil. Analysis. Pco2 and pH of perfusate and urine were measured by a blood gas system (BMS3-MK2 , Radiometer, Copenhagen, Denmark). Sodium and potassium were measured with a flame photometer, and '4C-inulin was used for the determination of