title: Extracellular protons modulate the Ca2+ block of a Ca(2+)-blockable monovalent cation channel in chick embryo.
abstract: Some epithelial nonselective cation channels although in an open conformation are impermeable because they are blocked by extracellular Ca2+ ions. Such a channel is present in the apical membrane of the ectodermal cells of the chick gastrula. This single-channel study shows that protonation of a site located inside the pore of this embryonic channel reduces the Ca2+ block thus allowing Na+ to pass. The apparent pK for Ca2+ at -50 mV shifts from 5.7 at pH 7.4 to 3.5 at pH 4.5. Unexpectedly, the change in Ca2+ affinity does not originate from a destabilization of the binding but comes from a large reduction of the association rate.
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D002642:Chick Embryo; D003470:Culture Media; D005110:Extracellular Space / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007473:Ion Channels / Q000187:drug effects* / Q000378:metabolism*; D007700:Kinetics; D012964:Sodium / Q000378:metabolism
chemical_list: D002121:Calcium Channel Blockers; D003470:Culture Media; D007473:Ion Channels; D012964:Sodium; D002118:Calcium