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Calcium Sensitivity of a Sodium-Activated Nonselective Cation Channel in Lobster Olfactory Receptor Neurons

¹ Whitney Laboratory, University of Florida, Gainesville, Florida 32601 ² Deparments of Zoology and Neuroscience, Center for Smell and Taste, and McKnight Brain Institute, University of Florida, Gainesville, Florida 32601

Submitted 26 February 2003; accepted in final form 27 June 2003

We report that a Na⁺-activated nonselective cation channel described previously in lobster olfactory neurons, in which phosphoinositide signaling mediates olfactory transduction, can also be activated by Ca²⁺. Ca²⁺ activates the channel in the presence of Na⁺, increasing the open probability of the channel with a K_1/2 of 490 nM and a Hill coefficient of 1.3. Ca²⁺ also increases the sensitivity of the channel to Na⁺. In some cells, the same channel is Ca²⁺ insensitive in a cell-specific manner. The nonspecific activator of protein phosphatases, protamine, applied to the intracellular face of patches containing the channel irreversibly eliminates the sensitivity to Ca²⁺. This effect can be blocked by okadaic acid, a nonspecific blocker of protein phosphatases, and restored by the catalytic subunit of protein kinase A in the presence of MgATP. The Ca²⁺-sensitive form of the channel is predominantly expressed in the transduction zone of the cells in situ. These findings imply that the Ca²⁺ sensitivity of the channel, and possibly its regulation by phosphorylation, play a role in olfactory transduction and help tie activation of the channel to the canonical phosphoinositide turnover pathway.

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