Tubular reabsorption of alpha-aminoisobutyric acid in the pre-steady-state. Evidence for a cell-to-lumen flux

We investigated time-dependent and concentration-dependent renal handling of alpha-aminoisobutyric acid (AIB) in the rat during seven consecutive 30-min clearance periods after onset of AIB infusion. Following low-level rapid venous infusion of AIB (9.75 mumol/kg in 30 s), plasma [AIB] fell exponentially in the initial three periods and in linear fashion (-0.00158 microM/min) thereafter. Although filtered AIB fell in proportion to plasma [AIB] during the early clearance periods (5--125 min), fractional excretion of AIB (FEAIB) rose (delta FEAIB = + 0.077 +/- 0.010, mean +/- SD) and stabilized only in later clearance periods (125--215 min). Saturable intrarenal binding does not explain rising FEAIB with falling plasma [AIB] because sequential infusions of AIB, 120 min apart, also elicited the phenomenon after each infusion. During continuous infusions of AIB, time dependence of FEAIB was also observed, in the early interval (5--95 min), after onset of AIB infusion at various concentrations of plasma [AIB] below 3 mM approximately. Measurement of concentration-dependent net tubular reabsorption of AIB in the fourth through seventh clearance periods (near the steady state) revealed progressive saturation over the whole range of plasma [AIB] (0--12 mM), yet with a collapse of net reabsorption in the plasma [AIB] range 2--4 mM. Measurements of renal tissue AIB in these experiments revealed that cellular AIB is of sufficient magnitude and bears such a relationship to plasma AIB and FEAIB that it could contribute to an observable cell-to-lumen movement of amino acid in the "pre-steady-state" situation and could account for the rise in FEAIB following AIB infusion and the collapse in tubular reabsorption of AIB at 2--4 mM plasma [AIB].

Effect of calciotropic hormones and cyclic nucleotides on aminoaciduria and phosphaturia

Parathyroid extract (PTE), dibutyrylcyclic AMP (dbcAMP), adenosine cyclic 3':5'-monophosphate (cAMP). calcitonin (CT), and calcium chloride were infused separately into anesthetized, sham-operated, or TPTX vitamin D-fed adult rats to examine the effect of these calciotropic agents on fractional excretion (FE) of alpha-aminoisobutyric acid (AIB), and phosphate anion (Pi). AIB is a nonmetabolizable amino acid. Inulin clearance, FEAIB, and FEPi were stable in the intact (n = 10) and TPTX rat (n = 10). TPTX decreased FEAIB, and FEPi significantly (P less than 0.001 for both). PTE and dbcAMP both increased FEAIB in the intact rat (P less than 0.001); failure to obtain this response in the TPTX animal was a key finding. PTE and dbcAMP increased FEPi (P less than 0.001) in both the intact and TPTX animal. CT was the only agent (versus PTE, dbcAMP, adenosine cyclic 3':5'-monophosphate, and CaCl2) to increase FEAIB (P less than 0.001) in the TPTX rat; furthermore, it was the only agent that did not increase FEPi in the TPTX rat although it had hypocalcemic and hypophosphatemic effects. Changes in inulin clearance or plasma concentration of AIB, following infusion of calciotropic agents, do not explain the unique responses in FEAIB in the TPTX rat. Our findings suggest that hyperaminoaciduria induced by parathyroid hormone and cyclic nucleotide in the intact animal may be mediated by CT. Hyperphosphaturia is not a necessary response to small-dose (25 milliunits/kg.hr) infusions of CT.

Abnormalities of carbohydrate metabolism in idiopathic Fanconi syndrome

Various metabolic studies were performed in a patient with the idiopathic Fanconi syndrome in whom constant ketonuria suggested that organic acidemia might contribute to the metabolic acidosis. Glucose intolerance with a diminished insulin release was found after PO or IV glucose loads and after glucagon administratio. An insulinopenic "diabetes-like" state has not previously been described in such patients. The patient had impaired galactose-glucose interconversion, elevated blood lactate levels, reduced pyruvate levels, and an increased lactate:pyruvate ratio. Hepatomegaly and hypoglycemia were not present, and liver and muscle biopsies revealed no enzymatic evidence of glycogenosis. The erythrocyte UDP galactose transferase activity was normal. The patient failed to convert fructose to glucose and had a rise in blood lactate after ethanol administration. Further studies revealed no production of glucose after alanine or glycerol administraion, each test being associated with elevated blood lactate levels and, after alanine, an increased lactate:pyruvate ratio. The lactate:pyruvate ratio was elevated after glucagon administration with increased lactate and reduced pyruvate concentrations.

Treatment of gyrate atrophy of the choroid and retina with low arginine diet

In gyrate atrophy the blood ornithine is grossly elevated, due to deficiency of ornithine ketoacid transaminase, which converts ornithine towards glutamic acid. Two patients with gyrate atrophy have been treated with a low arginine diet and their blood ornithine levels have been reduced to near normal. At this level hyperammonemia may result from overtreatment, but this can be quickly cleared by a small dose of arginine. There has also been some improvement in vision, but no clearing of the gyrate areas. Future care with this regimen seems possible and improvements in handling of these patients are likely.

Role of epithelial architecture and intracellular metabolism in proline uptake and transtubular reclamation in PRO/re mouse kidney

The homozygous PRO/Re mouse has less than 1 percent of the very high proline oxidase activity that characterizes normal kidney cortex. In PRO/Re mouse the endogenous proline concentration is eight times normal in plasma and four times normal in kidney cortex cell, but 50 times normal in urine. The integrity of the membrane transport systems for proline uptake at the antiluminal surface of absorbing epithelium is retained in PRO/Re kidney, as determined by the slice method. Clearance studies in vivo under steady-state conditions indicate that the integrity of the luminal uptake system shared by glycine and proline, and serving proline absorption, is also intact. The exaggerated renal clearance of proline in PRO/Re mice (50 times normal) is explained when its raised intracellular concentration, caused by impaired proline oxidation, is considered. Backflux into urine flowing down the nephron will occur under these conditions, thus impairing net reclamation of proline in PRO/Re kidney. The findings reveal that membrane transport and intracellular metabolism of a substrate are, indeed, independent functions, but that metabolism of a substance can influence its transcellular transport.