Zinc permeates mouse muscle ACh receptor channels expressed in BOSC 23 cells and affects channel function

1. The influx of Zn²⁺ through the channels of fetal and adult mouse muscle nicotinic acetylcholine receptors (γ- and ε-AChRs) and its effects on receptor function were studied in transiently transfected human BOSC 23 cells, by combining patch-clamp recordings with digital fluorescence microscopy. 2. ACh-induced whole-cell currents were reversibly reduced by external ZnCl₂, with half-maximal inhibitory concentrations of 3 and 1 mM for γ- and ε-AChRs, respectively. 3. Both γ- and ε-AChR channels were permeable to Zn²⁺, as shown by fluorescence measurements using Zn²⁺-sensitive dyes. The fractional current carried by Zn²⁺ (P(f,Zn); 0.5 mM Zn²⁺ in Ca²⁺-and Mg²⁺-free medium) through γ- and ε-AChR channels was 1.7 and 4%, respectively. 4. P(f,Zn) increased with the concentration of ZnCl₂, but was little affected by physiological concentrations of Ca²⁺ and Mg²⁺ in the external medium. 5. The conductance of ACh-evoked unitary events, measured by cell-attached or outside-out recordings, decreased when the patched membrane was exposed to ZnCl₂ (1 or 3 mM). Simultaneous application of ACh and Zn²⁺ to the extra-patch membrane lengthened channel open duration (τ(op)) by 50%. No obvious increment of τ(op) was observed following exposure of inside-out patches to Zn²⁺. 6. The possible physiological relevance of zinc-induced modulation of AChR channels is discussed.