LANTHANIDE PROLINE COTRANSPORT ACROSS RABBIT RENAL BRUSH-BORDERS

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: the Ki and Jmax were 0.05 mM and 17 1 sec-1 in Na+; and 0.28 mM and 73 Pmol mg-1 sec-1 in TB3+ Tb1+. The higher Jmax 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 a K0.5 of 21 mM, while Tb3+ activated with a Hill coefficient of 0.88 and a K0.5 of 28 mM. 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.