抄録
In bovine ciliary muscle cells, carbachol (CCh) evokes inward currents resulting from opening of two types of muscarinic receptor-operated non-selective cation channels (NSCCL & NSCCS) with very different unitary conductances (35 pS & 100 fS)1. Here we compared the dependence of their openings on the agonist concentration. Whole-cell membrane currents were recorded at 30°C under voltage clamp at -50 mV in myocytes freshly isolated from the bovine ciliary body with collagenase. The cells were superfused with HEPES-buffered physiological saline (PSS; pH 7.4). Pipettes were filled with K+-free solution containing 100 mM-Cs aspartate, 5 mM-BAPTA and 100 μM-GTP (pH 7.0). To record NSCCL current selectively, the free Ca2+ concentration in the pipette [Ca2+]p was set slightly high (200 nM) so as to suppress opening of NSCCS1. For selective measurement of NSCCS currents, Na+, K+, Ca2+ and Mg2+ in PSS were isotonically replaced with Ba2+, [Ca2+]p was kept at 70 nM, and GTP was omitted from the pipette1. The total amounts of charge carried by the CCh-evoked currents (0.1-100 μM) for 30 s were estimated by numerical integration. The present experiments estimated the concentration of CCh required for half-maximal activation and the Hill coefficient to be 0.5 ± 0.1 μM and 0.97 ± 0.08 for NSCCS (n=37) and 8.5 ± 2.1 μM and 1.1 ± 0.1 for NSCCL (n=43). The difference of the agonist sensitivities suggests the link of the channels with M3-receptor through different signalling pathways. 1. Takai et al (2004) J. Physiol. 559, 899-922 [Jpn J Physiol 55 Suppl:S138 (2005)]
