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

Background: In any fine sensory organelle, a small influx of Ca2+ can quickly elevate cytoplasmic Ca2+. Mechanisms must exist to clear the ciliary Ca2+ 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 Ca2+-ATPase (PMCA). Principal Findings: In the present study, electrophysiological assays were devised to monitor cytoplasmic free Ca2+ in single frog olfactory cilia. Ca2+ was allowed to enter isolated cilia, either through the detached end or through membrane channels. Intraciliary Ca2+ was monitored via the activity of ciliary Ca2+-gated Cl- channels, which are sensitive to free Ca2+ from about 2 to 10 mu M. No significant effect of MgATP on intraciliary free Ca2+ 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 Ca2+ clearance. It is concluded that PMCA in the cilium plays a very limited role in clearing the micromolar levels of intraciliary Ca2+ produced during the odor response.