Regulation of duodenal Ca2+ pump by calmodulin and vitamin D-dependent Ca2+-binding protein

The Ca2+ pump in rat duodenal epithelium is studied as ATP-dependent Ca2+ uptake in a vesicle preparation with a 9-fold purification in Na+-K+-ATPase activity and a 20-fold purification of Na+-K+-ATPase with respect to an endoplasmic reticulum marker. ATP-dependent Ca2+ uptake is reduced by 60% by digitonin treatment of the vesicles, whereas high-affinity Ca2+-ATPase is stimulated by the same treatment. Different methods to deplete membrane preparations of calmodulin have been used. In EDTA osmotically shocked vesicles, calmodulin stimulated ATP-dependent Ca2+ transport up to 100% in a Ca2+ concentration-dependent manner. The duodenal Ca2+ pump is inhibited by calmodulin antagonists only at low Ca2+ concentrations and in membranes not depleted from calmodulin. Vitamin D-dependent Ca2+-binding protein (Mr = 10,000) in concentrations up to 5 microM did not affect the rate of ATP-dependent Ca2+ transport, either in Ca2+-EGTA-buffered solutions or in EGTA-free solutions. In membrane preparations from vitamin D-deficient rats, the effects of calmodulin and of Ca2+-binding protein were identical to the vitamin D-repleted control preparations. This excludes a specific effect of Ca2+-binding protein and calmodulin in the vitamin D dependency of duodenal Ca2+-ATPase.