Maturational Changes in Rabbit Renal Brush Border Membrane Vesicle Urea Permeability • 1830

Abstract

Urea transport in the proximal tubule is thought to occur by passive diffusion through the lipid bilayers of the cell membranes. The lipid composition of cell membranes changes during maturation and may directly affect urea permeability of proximal tubule membranes. The present study examined the maturation of urea transport in rabbit renal brush border membrane vesicles (BBMV). BBMV from adult and neonatal (9-to 11-d-old) New Zealand white rabbits were loaded with 500 mM urea and mixed with an iso-osmotic mannitol solution using a stop-flow instrument. Vesicle shrinkage, due to efflux of urea, was followed with light scattering and urea permeability was calculated from an exponential fit of the data. Urea permeability was significantly lower in the neonatal BBMV than the adult at 25°C (0.34 ± 0.04 × 10 −6 versus 0.56 ± 0.03 × 10 −6 cm/sec; p < 0.001, n = 7) and 37°C (0.45 ± 0.04 × 10 −6 versus 0.66 ± 0.03 × 10 −6 cm/sec; p = 0.001, n = 7). There was no effect of 250 µM phloretin on urea permeability in either adult or neonatal BBMV at either temperature. The activation energy for urea diffusion was higher in the neonatal than the adult BBMV. Because the maturational increase in urea permeability could potentially be due to a sodium-dependent urea transporter in the adult BBMV, the sodium dependence of urea uptake in adult BBMV was examined. There was no difference in urea permeability in the presence or absence of 20 mM NaCl. Permeability of the lipid-soluble molecule, glycerol, was also found to be the same in the neonatal and adult BBMV. Urea transport in the apical membrane of neonatal and adult proximal tubules is not phloretin sensitive, a finding consistent with diffusion of urea via the lipid bilayer. The rate of urea diffusion is lower in neonatal membranes and may be an important factor in overall urea excretion. This may also play a role in developing and maintaining a high medullary urea concentration and thus the ability to concentrate the urine during renal maturation.In most mammals, nitrogen waste from metabolism of amino acids is excreted in the urine as urea (1,2) . The proximal tubule reabsorbs approximately 50% of the filtered load of urea despite changes in hydration status and protein intake that alter the overall excretion of urea. Although initial studies suggested that the proximal straight tubule actively secreted urea ( and that membrane transport of urea in the proximal tubule may occur by a specific transporter (4) , most evidence indicates that urea transport in the proximal tubule occurs by passive diffusion through the lipid bilayer (1,2) . The phospholipid composition of membranes has a direct influence on solute permeabilities and thus may directly affect proximal tubule transport of urea (5) .During maturation, the proximal tubule of the rabbit is known to undergo many changes in active and passive transport characteristics (6,7) . The lipid composition of brush border membranes also changes during development and may play a role in the maturation of transport processes...