The interaction between solute and water in epithelial transport is represe
nted by the solute reflection coefficient. Because the osmotic water transp
ort process changes in the rabbit proximal tubule during maturation, there
is a potential for the solute reflection coefficients to also undergo matur
ational changes. In the present study, we directly examined solute reflecti
on coefficients in neonatal and adult brush border membrane vesicles (BBMV)
using the stop-flow lightscattering technique. Reflection coefficients for
NaCl, KCl, NaHCO3 and urea were found to be identical in the neonatal and
adult BBMV and were not different from 1. Thus, although the water transpor
t pathway undergoes changes in the proximal tubule during maturation, there
is no evidence for changes in solute and water interaction. Because the re
flection coefficients are not different from 1, there is no evidence for so
lvent drag in the proximal tubule apical membrane in either the neonatal or
adult tubule.