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Gentamicin blocks ACh‐evoked K + current in guinea‐pig outer hair cells by impairing Ca 2+ entry at the cholinergic receptor
Author(s) -
Blanchet Christophe,
Eróstegui Carlos,
Sugasawa Masashi,
Dulon Didier
Publication year - 2000
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 - 10.1111/j.1469-7793.2000.t01-1-00641.x
Subject(s) - chemistry , nicotinic agonist , acetylcholine , acetylcholine receptor , biophysics , cholinergic , aminoglycoside , receptor , pharmacology , endocrinology , biochemistry , biology , antibiotics
1 Aminoglycoside antibiotics such as gentamicin are known to block the medial olivocochlear efferent system. In order to determine whether this inhibition takes place at the postsynaptic cholinergic receptors in outer hair cells (OHCs), we studied the effects of these polycationic molecules on cholinergic currents evoked in isolated guinea‐pig OHCs. 2 The cholinergic response of OHCs involves nicotinic‐like receptors (nAChRs) permeable to Ca 2+ ions that activate nearby Ca 2+ ‐sensitive K + channels (K Ca(ACh) channels). The extracellular application of gentamicin and neomycin reversibly blocked ACh‐evoked K + current ( I K(ACh) ) with IC 50 values of 5.5 and 3.2 μ m , respectively. The results showed that the blocking mechanism of I K(ACh) was due to inhibition of Ca 2+ influx via nAChRs. 3 Our study also provides interesting insights into the functional coupling between nAChRs and K Ca(ACh) channels in OHCs. By directly recording the cation current flowing through nAChRs ( I n(ACh) ) using an intracellular solution containing 10 m m BAPTA, we measured an EC 50 near 110 μ m for ACh‐evoked I n(ACh) . This EC 50 for ACh is one order of magnitude higher than that measured indirectly on I K(ACh) . This reveals a rather low affinity of ACh for its receptor but a very efficient coupling between nAChRs and K Ca(ACh) channels. 4 We also show that a high external Ca 2+ concentration reverts the gentamicin inhibition of I K(ACh) and that gentamicin directly alters the cation current flowing through the nAChRs of OHCs. We propose that gentamicin acts as a non‐competitive cholinergic blocker by displacing Ca 2+ from specific binding sites at the nAChRs. This block of the nAChRs at the level of the postsynaptic membrane in OHCs could explain the inhibitory effect of gentamicin reported on the crossed medial olivocochlear efferent system in vivo.