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

1. The influx of Zn(2+) through the channels of fetal and adult mouse muscle nicotinic acetylcholine receptors (gamma- and epsilon -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 ZnCl(2), with half-maximal inhibitory concentrations of 3 and 1 mM for gamma- and epsilon -AChRs, respectively. 3. Both gamma- and epsilon -AChR channels were permeable to Zn(2+), as shown by fluorescence measurements using Zn(2+)-sensitive dyes. The fractional current carried by Zn(2+) (P(f,Zn); 0.5 mM Zn(2+) in Ca(2+) and Mg(2+)-free medium) through gamma- and epsilon -AChR channels was 1.7 and 4 %, respectively. 4. P(f,Zn) increased with the concentration of ZnCl(2), but was little affected by physiological concentrations of Ca(2+) and Mg(2+) 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 ZnCl(2) (1 or 3 mM). Simultaneous application of ACh and Zn(2+) to the extra-patch membrane lengthened channel open duration (tau (op)) by 50%. No obvious increment of tau (op) was observed following exposure of inside-out patches to Zn(2+). 6. The possible physiological relevance of zinc-induced modulation of AChR channels is discussed.