Abstract
We previously reported that in the rabbit, the vitamin D-dependent calcium binding protein 28K (CaBP 28K) increases calcium (Ca
2+) transport in the distal tubule by opening a high affinity Ca
2+ channel in the luminal membrane. Since Na
+ and Ca
2+ transports are interdependent in this membrane, we questioned whether the calbindin has any influence on Na
+ transport. Luminal membranes from rabbit proximal and distal tubules were purified and
22Na uptake by the membrane vesicles was measured using the rapid filtration technique. The vesicles were loaded with 280 mM mannitol and 20 mM Tris–Hepes pH 7.4, with either 3 μM CaBP or the carrier. Incubation medium contained 1 mM
22NaCl, 278 mM mannitol, and 20 mM Tris–Hepes pH 7.4. The presence of 3 μM CaBP 28K in the distal luminal membrane vesicles increased the 0.5 mM Ca
2+ uptake from 0.91±0.21 to 1.84±0.33 pmol/μg/10 s (
P<0.01) and decreased 1 mM Na
+ uptake from 0.62±0.15 to 0.27±0.08 pmol/μg/10 s (
P<0.05). A similar decrease of Na
+ uptake was observed in proximal luminal membrane experiments. The effect on Na
+ uptake by the distal membrane was dose-dependent with a IC
50 of 4.5 μM. Addition of 2 mM Ca
2+ to the incubation medium decreased 1 mM Na
+ uptake from 0.62±0.15 to 0.49±0.12 pmol/μg/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 involves the Na
+/H
+ exchanger activity.