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

Abstract

High-affinity Ca 2+,Mg 2+-ATPase was identified in a plasma membrane-rich fraction of olfactory epithelium from Atlantic salmon ( Salmo salar). The enzyme required both Ca 2+ and Mg 2+ for activation. The apparent K m for Ca 2+ was 9.5 nM and V max was 0.85 μmol P i/mg of protein per min. Stimulation by Ca 2+ was optimal at 5–100 μM MgCl 2. Bovine brain calmodulin had no effect on Ca 2+,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 immunobloting after multiple washes of the membrane preparation with EDTA or EGTA. This endogenous calmodulin may regulate Ca 2+,Mg 2+-ATPase activity because the activity was inhibited by calmidazolium. Vanadate inhibited Ca 2+,Mg 2-ATPase activity and thapsigargin, a specific inhibitor for Ca 2+,Mg 2+-ATPase of endoplasmic reticulum, had no effect on the enzyme activity. High affinity Ca 2+,Mg 2+-ATPase exists in both ciliary and nonciliary membranes with a similar K m for Ca 2+. Ca 2+,Mg 2+-ATPase activity is greater in cilia preparations than in membranes from the deciliated olfactory epithelium. As a putative plasma membrane Ca 2+ pump, this high-affinity Ca 2+,Mg 2+-ATPase may play an important role in the regulation of intracellular Ca 2+ in olfactory epithelia. In particular, the ciliary membrane may play a prominent role in the removal of Ca 2+ from ciliated olfactory receptor cells after odorant stimulation.