Stimulation of Plasma Membrane Ca2+ Pump by Calbindin-D28k and Calmodulin is Additive in EGTA-Free Solutions

In enterocytes and erythrocytes a calmodulin-stimulated Ca2+-ATPase is the main Ca2+ efflux pathway. Previous studies have shown that in enterocytes this Ca2+-pumping ATPase could be stimulated by vitamin D-dependent Ca2+-binding protein, calbindin-D9k, in ethylene glycol-bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid (EGTA)-free solutions. In contrast, a similar stimulatory effect of calbindin-D9k was not observed in erythrocytes. We reinvestigated the effects of calbindin, parvalbumin and calmodulin on active Ca2+ uptake in membrane vesicles derived from porcine erythrocytes and from rat duodenum. In EGTA-containing solutions, neither calbindin-D28k nor parvalbumin influenced the rate of ATP-dependent Ca2+ uptake in red blood cell-derived vesicles. However, when EGTA-free solutions were used, calbindin D28k and parvalbumin significantly increased ATP-dependent Ca2+ uptake in erythrocyte as well as in enterocyte-derived membrane vesicles. In contrast, calmodulin significantly increased active Ca2+ uptake in erythrocyte vesicles in the absence as well as in the presence of EGTA. In addition, ATP-dependent Ca2+ uptake in the presence of 0.2 µM calmodulin was further increased by parvalbumin in the absence but not in the presence of EGTA. This observation precludes that parvalbumin and calbindin stimulate the plasma membrane Ca2+-ATPase by occupying the calmodulin binding site. Our results support the theoretical notion that calbindin and parvalbumin stimulate the Ca2+-starved pump by increasing the free Ca2+ in the immediate vicinity of the Ca2+ pump sites.