Na⁺-dependent Ca²⁺ Extrusion Governs Response Recovery in Frog Olfactory Receptor Cells

To study the mechanism by which Ca²⁺, which enters during the odor response, is extruded during response recovery, recordings were made from isolated frog olfactory receptor cells using the suction pipette technique, while superfusing the olfactory cilia with solutions of modified ionic composition. When external Na⁺ was substituted with another cation, the response to odor was greatly prolonged. This prolongation of the response was similar irrespective of whether Na⁺ was replaced with Li⁺, which permeates the cyclic nucleotide-gated conductance, or choline, which does not. The prolonged current was greatly reduced by exposure to 300 μM niflumic acid, a blocker of the calcium-activated chloride channel, indicating that it is carried by this conductance, and abolished if Ca²⁺ was omitted from the external solution, demonstrating that Ca²⁺ influx is required for its generation. When the cilia were exposed to Na⁺-free solution after odor stimulation, the recovery of the response to a second stimulus from the adaptation induced by the first was greatly reduced. We conclude that a Na⁺-dependent Ca²⁺ extrusion mechanism is present in frog olfactory cilia and that it serves as the main mechanism that returns cytoplasmic Ca²⁺ concentration to basal levels after stimulation and mediates the normally rapid recovery of the odor response and the restoration of sensitivity after adaptation.