Effect of calbindin D 28K on sodium transport by the luminal membrane of the rabbit nephron

We previously reported that in the rabbit, the vitamin D-dependent calcium binding protein 28K (CaBP 28K) increases calcium (Ca2+) transport in the di stal tubule by opening a high affinity Ca2+ channel in the luminal membrane . Since Na+ and Ca2+ transports are interdependent in this membrane, we que stioned whether the calbindin has any influence on Na+ transport. Luminal m embranes from rabbit proximal and distal tubules were purified and Na-22 up take by the membrane vesicles was measured using the rapid filtration techn ique. The vesicles were loaded with 280 mM mannitol and 20 mM Tris-Hepes pH 7.4, with either 3 mu M CaBP or the carrier. Incubation medium contained 1 mM (NaCl)-Na-22, 278 mM mannitol, and 20 mM Tris-Hepes pH 7.4. The presenc e of 3 mu M CaBP 28K in the distal luminal membrane vesicles increased the 0.5 mM Ca2+ uptake from 0.91 +/- 0.21 to 1.84 +/- 0.33 pmol/mu g/10 s (P < 0.01) and decreased 1 mM Na+ uptake from 0.62 +/- 0.15 to 0.27 +/- 0.08 pmo l/mu g/10 s (P < 0.05). A similar decrease of Na+ uptake was observed in pr oximal luminal membrane experiments. The effect on Na+ uptake by the distal membrane was dose-dependent with a IC50 of 4.5 mu M. Addition of 2 mM Ca2 to the incubation medium decreased 1 mM Na+ uptake from 0.62 +/- 0.15 to 0 .49 +/- 0.12 pmol/mu g/10 s (P < 0.05), but did not influence the effect of CaBP 28K on Na+ uptake. Experiments performed in the presence and absence of ethyl isopropyl amiloride (EIPA) suggest that the effect of calbindin in volves the Na+/H+ exchanger activity. (C) 1999 Published by Elsevier Scienc e Ireland Ltd. All rights reserved.