EXTRACELLULAR PROTONS MODULATE THE CA2-BLOCKABLE MONOVALENT CATION CHANNEL IN CHICK-EMBRYO( BLOCK OF A CA2+)

Some epithelial nonselective cation channels although in an open confo rmation are impermeable because they are blocked by extracellular Ca2 ions. Such a channel is present in the apical membrane of the ectoder mal cells of the chick gastrula. This single-channel study shows that protonation of a site located inside the pore of this embryonic channe l reduces the Ca2+ block thus allowing Na+ to pass. The apparent pK fo r Ca2+ at -50 mV shifts, from 5.7 at pH 7.4 to 3.5 at pH 4.5. Unexpect edly, the change in Ca2+ affinity does not originate from a destabiliz ation of the binding but comes from a large reduction of the associati on rate.