High-affinity Ca2+,Mg(2+)-ATPase in plasma membrane-rich preparations from olfactory epithelium of Atlantic salmon

High-affinity Ca2+,Mg(2+)-ATPase was identified in a plasma membrane-rich fraction of olfactory epithelium from Atlantic salmon (Salmo salar). The enzyme required both Ca2+ and Mg2+ for activation. The apparent Km for Ca2+ was 9.5 nM and Vmax was 0.85 mumol Pi/mg of protein per min. Stimulation by Ca2+ was optimal at 5-100 microM MgCl2. Bovine brain calmodulin had no effect on Ca2+,Mg(2+)-ATPase, even after multiple washes of the membrane preparation with EDTA or EGTA. Endogenous calmodulin was somewhat resistant to removal and could be detected with immunoblotting after multiple washes of the membrane preparation with EDTA or EGTA. This endogenous calmodulin may regulate Ca2+,Mg(2+)-ATPase activity because the activity was inhibited by calmidazolium. Vanadate inhibited Ca2+,Mg(2)-ATPase activity and thapsigargin, a specific inhibitor for Ca2+,Mg(2+)-ATPase of endoplasmic reticulum, had no effect on the enzyme activity. High affinity Ca2+,Mg(2+)-ATPase exists in both ciliary and nonciliary membranes with a similar Km for Ca2+. Ca2+,Mg(2+)-ATPase activity is greater in cilia preparations than in membranes from the deciliated olfactory epithelium. As a putative plasma membrane Ca2+ pump, this high-affinity Ca2+,Mg(2+)-ATPase may play an important role in the regulation of intracellular Ca2+ in olfactory epithelia. In particular, the ciliary membrane may play a prominent role in the removal of Ca2+ from ciliated olfactory receptor cells after odorant stimulation.