Limits of Calcium Clearance by Plasma Membrane Calcium ATPase in Olfactory Cilia

Background In any fine sensory organelle, a small influx of Ca 2+ can quickly elevate cytoplasmic Ca 2+ . Mechanisms must exist to clear the ciliary Ca 2+ before it reaches toxic levels. One such organelle has been well studied: the vertebrate olfactory cilium. Recent studies have suggested that clearance from the olfactory cilium is mediated in part by plasma membrane Ca 2+ -ATPase (PMCA). Principal Findings In the present study, electrophysiological assays were devised to monitor cytoplasmic free Ca 2+ in single frog olfactory cilia. Ca 2+ was allowed to enter isolated cilia, either through the detached end or through membrane channels. Intraciliary Ca 2+ was monitored via the activity of ciliary Ca 2+ -gated Cl − channels, which are sensitive to free Ca 2+ from about 2 to 10 µM. No significant effect of MgATP on intraciliary free Ca 2+ could be found. Carboxyeosin, which has been used to inhibit PMCA, was found to substantially increase a ciliary transduction current activated by cyclic AMP. This increase was ATP-independent. Conclusions Alternative explanations are suggested for two previous experiments taken to support a role for PMCA in ciliary Ca 2+ clearance. It is concluded that PMCA in the cilium plays a very limited role in clearing the micromolar levels of intraciliary Ca 2+ produced during the odor response.