Olfactory neurons exhibit heterogeneity in depolarization-induced calcium changes

Olfactory neurons from the channel catfish, Ictalurus punctatus, were isolated by a brief (15 min) treatment with papain. After incubation with fura-2 acetoxymethyl ester (fura-2/AM) for 1 h, the isolated olfactory receptor cells are found to hydrolyze fura-2/AM to fura-2 free acid without detectable traces of intermediate products of hydrolysis. Intracellular calcium measured with fura-2 in single cells covers a wide range (from < 2 to 100 nM with a median of 17.6 nM, n = 140 cells). Twenty-one percent of the cells respond to potassium-induced depolarization with an increase in intracellular calcium mediated by influx of extracellular calcium. The L-type calcium channel antagonist nimodipine inhibits the increase in intracellular calcium triggered by membrane depolarization and blocks small unitary barium currents displaying the characteristics of L-type calcium currents (unitary conductance of 29 ± 5 pS in 55 mM BaCl₂ and high selectivity for Ba²⁺ over Na⁺ and K⁺) recorded from azolectin bilayers at the tip of patch pipettes into which isolated olfactory cilia membrane vesicles had been incorporated. Olfactory neurons are found to be functionally heterogeneous in their response to membrane depolarization and can be separated into three groups: one in which the increase in intracellular calcium is rapid and transient, another in which calcium increases slowly, and a third group of cells in which depolarization causes no change in intracellular calcium.