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Two types of non‐selective cation channel opened by muscarinic stimulation with carbachol in bovine ciliary muscle cells
Author(s) -
Takai Yoshiko,
Sugawara Ryoichi,
Ohinata Hiroshi,
Takai Akira
Publication year - 2004
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.2004.065607
Subject(s) - carbachol , chemistry , biophysics , tonic (physiology) , patch clamp , stimulation , ciliary muscle , muscarinic acetylcholine receptor , bapta , intracellular , endocrinology , biochemistry , biology , receptor , accommodation , neuroscience
In the ciliary muscle, the tonic contraction requires a sustained influx of Ca 2+ through the cell membrane. However, little has hitherto been known about the route(s) of Ca 2+ influx in this tissue that lacks voltage‐gated Ca 2+ channels. To identify ion channels as the Ca 2+ entry pathway we studied the effects of carbachol (CCh) on freshly isolated bovine ciliary muscle cells by whole‐cell voltage clamp. Experiments were carried out using pipettes filled with K + ‐free solution containing 100 m m caesium aspartate, 5 m m BAPTA and 180 μ m GTP (pH 7.0; the intracellular free Ca 2+ concentration, [Ca 2+ ] i = 70 n m ). CCh evoked an inward current showing polarity reversal at a holding potential near 0 mV. Analysis of the current noise distinguished two types of non‐selective cation channel (NSCCL and NSCCS) with widely different unitary conductances (35 pS and 100 fS). The ratios of the permeabilities to Li + , Na + , Cs + , Mg 2+ , Ca 2+ , Sr 2+ and Ba 2+ , estimated by cation replacement procedures, were 0.9: 1.0: 1.5: 0.2: 0.3: 0.4: 0.5 for NSCCL, and 1.0: 1.0: 1.8: 2.5: 2.6: 3.2: 5.0 for NSCCS. NSCCS, but not NSCCL, was strongly inhibited by elevation of [Ca 2+ ] i . Both NSCCL and NSCCS were dose‐dependently inhibited by 1–100 μ m SKF96365, La 3+ and Gd 3+ , which also inhibited the tonic component of the contraction produced in muscle bundles by CCh without markedly affecting the initial phasic component. NSCCL and/or NSCCS may serve as a major Ca 2+ entry pathway required for sustained contraction of the bovine ciliary muscle. RT‐PCR experiments in the bovine ciliary muscle (whole tissue) detected mRNAs of several transient receptor potential (TRP) channel homologues (TRPC1, TRPC3, TRPC4 and TRPC6), which are now regarded as possible molecular candidates for receptor‐operated cation channels.