Developmental Aspects of Renal β-Amino Acid Transport II. Ontogeny of Uptake and Efflux Processes and Effect of Anoxia

Abstract

SummaryRenal cortex slices from newborn, tweek, and 4-week-old Sprague-Dawley rats had reduced initial rates of taurine uptake compared io adult slices after short (< 30 min) incubation periods. From birth onward, steady-state accumulation occurred by at least two sodium-dependent uptake systems. The first system had an "apparent KmlW = 0.1 mM and a Vmax varying from 1.8 to 5.1 pmoles/ml ICF/120 min at four ages. The second uptake mode had an apparent Kmz = 12-16 mM and a Vmax of 45 pmoles/ml ICF/120 min. Emux of taurine was reduced in slices from younger animals possibly accounting for taurinuria. Only other p-amino acids inhibited accumulation.Anoxia inhibited uptake at high concentrations (> 1.0 mM) at each age, but taurine accumulation at low concentrations (< 0.4 mM) was relatively protected from anoxia in neonatal (< 36 hr of age) tissue. Preincubation in taurine-free medium for 120 min enhanced low concentration, but not high concentration uptake in neonatal and 2-week slices.After preincubation in dibutyryl cyclic AMP (dbcAMP) enhanced uptake of taurine was found in adult cortex, but not in neonatal cortex.The ontogeny of renal taurine transport in cortex slices a p peared to involve faster initial uptake rates and faster efflux as well as greater dependence on aerobic metabolism with maturation. Age-related differences in the response to preincubation and cyclic nucleotides were also indicative of maturational events in renal tubular amino acid transport. SpeculationTaurinuria found in the immature rat may involve decreased initial uptake rates and/or decreased efflux, but heterogeniety of transport processes across the antiluminal membrane was apparent at birth. Preincubation in taurine-free medium increased the rate of taurine accumulation by immature cortex, but changes in transport processes across the brush border membrane may account for the decrease in taurinuria found after the age of 2 weeks.

Developmental Aspects of Renal β-Amino Acid Transport. VI. The Role of Membrane Fluidity and Phospholipid Composition in the Renal Adaptive Response in Nursing Animals

“…(Pediatr Res 22: 163-167,1987) Immature animals of virtually all mammalian species have a "physiologic" generalized aminoaciduria which results in excretion of a higher fraction of the filtered amino acid load of all amino acid classes (1,2). Our laboratory has focused on physiologic factors that modify the development of amino acid transport systems in order to better understand the processes by which these transport sites achieve a mature transport profile (3)(4)(5)(6)(7). For an appropriate transport probe , we have used the l3-amino acid taurine, since it is relevant in the nutrition of young animals (8) and since one can manipulate the dietary level of sulfur amino acids to alter the renal handling of taurine (5,(9)(10)(11)(12)(13)(14).…”

Developmental Aspects of Renal β-Amino Acid Transport. IV. Brush Border Membrane Response to Altered Intake of Sulfur Amino Acids

“…Indeed, in some time course experiments, the initial (1-and 2-min) uptake was more brisk in tubules from the young animals. The differences are observed in our previous studies (12,18) would indicate that the efflux of taurine out of the slice or isolated tubule is impaired in immature animals. Since efflux is presumably occurring at the basolateral membrane surface, studies of transport differences in isolated basolateral membrane would appear to be important.…”

“…The mechanism of neonatal taurinuria in slices (1 2) and tubules (18) appears to involve a decreased initial rate of uptake with a reduction in the V,,, of taurine accumulation. A third mechanism, decreased efflux at the basolateral membrane in immature cortex, also appears to be relevant (2,3,10,12,18,33), but the kinetic characteristics of this diminished rate of exit are not clear. Finally, changes in "effective intravascular volume" may influence the transport of organic solutes (2 1).…”

Development Aspects of Renal β-Amino Acid Transport. V Brush Border Membrane Transport in Nursing Animals—Effect of Age and Diet

“…Thus, the renal adaptive response to changes in sulfur amino acid intake is found in the brush border membrane, but the urinary excretion pattern is only partially adapted. It is possible that delayed emux of taurine across the basolateral surface accounts for this excessive taurinuria, since our previous studies have indicated reduced basallateral efflux in immature rats (4,6,7).Although the adaptive response has been shown to partially exist in terms of urinary excretion and to fully exist at the apical surface in 28-day-old rats, little is known about the response in nursing animals at this membrane surface. Herein we describe the influence of the altered amino acid diets fed to the mothers of nursing rats on the renal handling and brush border accumu-…”

“…Thus, the renal adaptive response to changes in sulfur amino acid intake is found in the brush border membrane, but the urinary excretion pattern is only partially adapted. It is possible that delayed emux of taurine across the basolateral surface accounts for this excessive taurinuria, since our previous studies have indicated reduced basallateral efflux in immature rats (4,6,7).…”