Two vitamin D3-dependent calcium binding proteins increase calcium reabsorption by different mechanisms II. Effect of CaBP 9K

Abstract

The distal tubule is a major site of hormone regulated Ca2+ reabsorption. We have previously shown that in the rabbit, vitamin D depletion decreases both the Ca2+ uptake by the luminal membrane of nephron distal segments and the ATP-dependent Ca2+ transport by the basolateral membrane of these same segments. We also reported that in in vitro experiments, the vitamin D-dependent calcium binding protein (CaBP) 28K enhances Ca2+ uptake by the high-affinity Ca2+ transport system of the distal luminal membranes. The purpose of the present study was to investigate the role of CaBP 9K in Ca2+ transport by the two polar membranes of the proximal and distal nephron segments. Preloading the luminal membranes of the two series of tubules with 10 μM CaBP 9K did not modify 0.5 mM Ca2+ uptake by these membranes. In contrast, preincubation of distal tubule basolateral membranes with 10 μM CaBP 9K strongly enhanced ATP-dependent Ca2+ transport by these membranes. A similar although more modest effect was observed in proximal basolateral membranes. This effect was present only in calmodulin depleted membranes. Calmodulin also increased the ATP-dependent Ca2+ uptake by the depleted membranes. The two actions were not additive. A dose-response curve showed that the maximal CaBP 9K effect was obtained at 10-5 M. CaBP 9K increased the V(max) of ATP-dependent uptake from 0.43 ± 0.03 to 0.79 ± 0.06 nmol/mg/10 sec (P < 0.005), leaving the Km unchanged. It is concluded that CaBP 9K activates the Ca2+ pump in the basolateral membrane of both the proximal and distal tubule. In vivo, vitamin D enhances Ca2+ reabsorption in the distal segments exclusively because the two CaBPs (9K and 28K) are synthesized in these segments.