AJP - Gastrointestinal and Liver Physiology, Vol 251, Issue 2 223-G229, Copyright © 1986 by American Physiological Society
Regulation of duodenal Ca2+ pump by calmodulin and vitamin D-dependent Ca2+-binding protein
W. E. Ghijsen, C. H. Van Os, C. W. Heizmann and H. Murer
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.
