Muscarinic receptors modulate intracellular Ca²⁺ concentration in hyaline cells of the chicken basilar papilla

Abstract.

Nonsensory hyaline cells border the sensory epithelium of the auditory end-organ (basilar papilla) in birds and reptiles. Their innervation by cochlear cholinergic efferent fibers and the presence of contractile proteins suggest that hyaline cells may actively regulate basilar membrane mechanics. The cholinergic pharmacology of hyaline cells was studied by measuring the intracellular calcium concentration ([Ca²⁺]_i) of fura-2-loaded cells in the chicken cochlea in vitro. Superfusion of the cholinergic agonist carbachol produced a dose-dependent increase in hyaline cell [Ca²⁺]_i (EC₅₀=1.05 µmol l^–1) and small responses in short hair cells. Calcium increases in hyaline cells were evoked by the muscarinic agonists oxotremorine (10 µmol l^–1) and muscarine (100 µmol l^–1) whereas nicotine (100 µmol l^–1, 200 µmol l^–1) was without effect. Carbachol-evoked responses were blocked by the muscarinic antagonist atropine (≥10^–13 mol l^–1) and were unaffected by the nicotinic antagonists d-tubocurare (100 µmol l^–1, 1 mmol l^–1) and hexamethonium (100 µmol l^–1). Responses persisted in the absence of extracellular Ca²⁺ and were abolished by thapsigargin (1 µmol l^–1). These results indicate that the cholinergic-stimulated increase in hyaline cell [Ca²⁺]_i is due to a muscarinic-mediated release of Ca²⁺ from intracellular stores. This is the first evidence that hyaline cells possess a muscarinic receptor whose activation causes mobilization of intracellular Ca²⁺.