Lanthanide/proline cotransport across rabbit renal brush borders

Abstract

It has been suggested previously that La3+ can replace Na+ on various cotransport systems in renal brush border membranes. In the present study, we used rabbit renal brush border membrane vesicles to examine the specificity and kinetics of Ln3+/proline cotransport. Experiments were carried out under zero-trans, voltage clamped conditions using a rapid-mix/filtration technique. Initial experiments confirmed that La3+ produced the classical overshoot phenomenon. The initial rates of proline uptake relative to Na+ were Eu3+, Tb3+, Nd3+, Pr3+, Ho3+ (3.3)>Na+ (1.0)>La3+ (0.86) > choline+ (0.1). At a saturating salt concentration, uptake saturated with increasing proline concentration: theK t andJ max were 0.05mm and 17 pmol mg−1 sec−1 in Na+; and 0.28mm and 73 pmol mg−1 sec−1 in Tb3+. The higherJ max in Tb3+ indicates that the Tb3+-proline loaded carrier is more effective than the Na+-proline loaded carrier in overcoming some rate-limiting barriers in the transport process. Na+ activated proline uptake with a Hill coefficient of 1.6 and aK 0.5 of 21mm, while Tb3+ activated with a Hill coefficient of 0.88 and aK 0.5 of 28mm. The Hill coefficient for Na+ suggests two binding sites, whereas the Hill coefficient for Tb3+ may indicate negative cooperativity between the trivalent ligands at the binding sites. We conclude that lanthanides are able to substitute for Na+ on the brush border proline carrier and that the lanthanides may serve as useful probes for the ligand binding sites.