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Muscarinic agonists and ATP increase the intracellular Ca2+ concentration in chick cochlear hair cells.
Author(s) -
Shigemoto T,
Ohmori H
Publication year - 1990
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1990.sp017904
Subject(s) - muscarine , acetylcholine , chemistry , muscarinic acetylcholine receptor , endocrinology , medicine , desensitization (medicine) , stimulation , cholinergic , intracellular , carbachol , muscarinic agonist , extracellular , biophysics , biology , biochemistry , receptor
1. Cholinergic muscarinic agonists applied by the pressure puff method increased intracellular Ca2+ concentration in Fura‐2‐loaded hair cells. The Ca2+ response outlasted the agonist application. 2. The Ca2+ response induced by acetylcholine (ACh) was ACh dose dependent with a KD of 200 microM. Desensitization was negligible, and almost identical Ca2+ responses were observed when two ACh puffs were separated by 150 s. The response was blocked by d‐tubocurarine (dTC). The KD of dTC blocking was 500 microM when 100 microM‐ACh induced the Ca2+ response. 3. The amplitude of the ACh‐induced Ca2+ responses were potentiated to 3 times the control by incubation with calcitonin gene‐related peptide (CGRP; 0.1‐1 microM). CGRP did not affect the resting Ca2+ concentration. Glycine (100 microM) potentiated the ACh response to 1.4 times the control, and also increased the resting Ca2+ concentration slightly. 4. The ACh‐induced Ca2+ response was suppressed by atropine. It was induced in Ca2(+)‐free extracellular medium, and in Ca2(+)‐free medium desensitization to a second ACh stimulation was significant. The amplitude of the second Ca2+ response was 44% of the first when two ACh puffs were separated by 117 s in Ca2+ free medium. 5. Muscarine and carbamylcholine induced similar Ca2+ responses, with KD values of 130 microM for muscarine and 340 microM for carbamylcholine. Desensitization of Ca2+ responses was negligible in both agonists. 6. ATP co‐exists with ACh in some presynaptic nerve terminals (Burnstock, 1981). Puff‐applied ATP (100 microM) generated a Ca2+ response with a rapid rising phase and a following slow phase. In Ca2(+)‐free medium the rapid phase disappeared and only the slow phase was observed. The rapid phase is due to the influx of Ca2+ ions and the slow phase is due to a release of Ca2+ ions from an intracellular reservoir. Under voltage clamp ATP induced a fast inward current and a following slow outward current. 7. Nicotine, adenosine, glycine, GABA, glutamate and bradykinin did not induce Ca2+ responses in the hair cell. 8. ACh induced hyperpolarization of the hair cell membrane under current clamp, most probably by the activation of Ca2+ activated K+ conductance. Therefore, a cholinergic muscarinic receptor may mediate the inhibitory effects of efferent innervation observed in hair cells.