Submitted on July 7, 2004
Accepted on January 13, 2005
GLUCOCORTICOIDS INCREASE OSMOTIC WATER PERMEABILITY (Pf) OF NEONATAL RABBIT RENAL BRUSH BORDER MEMBRANE VESICLES
Jaap Mulder1, Sumana Chakravarty1, Maha N Haddad1, Michel Baum2, and Raymond Quigley1*
1 Pediatrics, University of Texas Southwestern Medical Center, Dallas, TX, USA
2 Pediatrics, University of Texas Southwestern Medical Center, Dallas, TX, USA; Internal Medicine, University of Texas Southwestern Medical Center, Dallas, TX, USA
* To whom correspondence should be addressed. E-mail: raymond.quigley{at}utsouthwestern.edu.
During postnatal maturation there is an increase in renal brush border membrane vesicle (BBMV) osmotic water permeability and a parallel increase in aquaporin-1 (AQP1) protein abundance. The mechanisms responsible for these changes remain unknown. Because serum glucocorticoid levels rise postnatally and have previously been linked to other maturational changes in renal function, we examined the effects of glucocorticoids on osmotic (Pf) and diffusional (PDW) water permeability and AQP1 protein abundance of renal BBMV. Neonatal rabbits were treated with dexamethasone (10 µg/100 g) for three days and compared to control neonates and adults. Pf and PDW were measured at 20°C with a stopped-flow apparatus using light scattering and aminonaphthalene trisulfonic acid (ANTS) fluorescence, respectively. Pf was significantly higher in BBMV from dexamethasone treated neonates compared to vehicle treated neonates, but remained lower than in BBMV from adults (p < 0.05). PDW in dexamethasone and vehicle treated neonatal BBMV was lower than in adult BBMV. Pf/PDW ratio increased from neonate (5.1 ± 0.3) to dexamethasone (7.0 ± 0.1) and adult BBMV (6.3 ± 0.1). AQP1 expression was increased by dexamethasone treatment to adult levels. Membrane fluidity, which is inversely related to generalized polarization (GP) of steady-state laurdan fluorescence, was significantly higher in neonatal BBMV than both dexamethasone and adult BBMV (GP: neonate 0.285 ± 0.002, dexamethasone 0.302 ± 0.006 and adult 0.300 ± 0.005; p < 0.05). These combined results show that dexamethasone-treatment during days 4-7 of life increases BBMV water permeability despite a decrease in membrane fluidity. This occurs by increasing channel-mediated water transport, as reflected in an increase in AQP1 protein abundance and a higher Pf/PDW ratio. This mimics the maturational changes and suggests a physiological role for glucocorticoids in maturation of proximal tubule water transport.
