Muscarinic activation and calcium permeation of nonselective cation currents in airway myocytes.
We examined the activation and Ca2+ permeation of nonselective cation channels in voltage-clamped (nystatin), fura 2-loaded equine tracheal myocytes at 35 degrees C. Methacholine (50 microM) induced a biphasic increase in intracellular Ca2+ concentration ([Ca2+]i) and a biphasic inward current consisting of a large, rapidly inactivating Ca(2+)-activated Cl current [ICl(Ca)] and a smaller, sustained nonselective cation current (Icat) ICl(Ca) but not Icat was activated by caffeine. Neither Icat nor the sustained rise in [Ca2+]i was blocked by nisoldipine, whereas both were rapidly blocked by Ni2+; Icat was determined to be Ca2+ permeant, since 1) a sustained elevation of [Ca2+]i occurred when Icat was activated, and blockade of Icat produced a rapid decline in [Ca2+]i; 2) increasing extracellular Ca2+ during Icat increased [Ca2+]i; 3) 110 mM extracellular Ca2+ shifted the reversal potential of Icat to 12 mV (Ca(2+)-to-Cs+ permeability ratio = 3.6); and 4) instantaneous voltage-clamp steps to negative potentials during Icat increased the current and [Ca2+]i, whereas depolarizing steps decreased the current and [Ca2+]i. The fraction of Icat carried by Ca2+ under physiological conditions was estimated to be 14% at -60 mV.
Fleischmann
B K
BK
Department of Animal Biology, School of Veterinary Medicine, University of Pennsylvania, Philadelphia 19104-6046, USA.
Wang
Y X
YX
Kotlikoff
M I
MI
D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002412:Cations / Q000378:metabolism*; D002712:Chlorides / Q000502:physiology; D004553:Electric Conductivity; D006736:Horses; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D009116:Muscarine / Q000378:metabolism*; D009130:Muscle, Smooth / Q000166:cytology / Q000378:metabolism*; D010539:Permeability; D014132:Trachea / Q000166:cytology / Q000378:metabolism*
D002412:Cations; D002712:Chlorides; D007473:Ion Channels; D002110:Caffeine; D009116:Muscarine; D002118:Calcium