title: Electrophysiological properties of BK channels in Xenopus motor nerve terminals.
abstract: Single channel properties of Ca(2+)-activated K(+) (BK or Maxi-K) channels have been investigated in presynaptic membranes in Xenopus motoneurone-muscle cell cultures. The occurrence and density of BK channels increased with maturation/synaptogenesis and was not uniform: highest at the release face of bouton-like synaptic varicosities in contact with muscle cells, and lowest in varicosities that did not contact muscle cells. The Ca(2+) affinity of the channel (K(d)= 7.7 microM at a membrane potential of +20 mV) was lower than those of BK channels that have been characterized in other terminals. Hill coefficients varied between 1.5 and 2.8 at different potentials and open probability increased e-fold per 16 mV change in membrane potential over a range of [Ca(2+)](i) from 1 microM to 1 mM. The maximal activation rate of ensembled single BK channel currents was in the submillisecond range at > or =+20 mV. The activation rate increased approximately 10-fold in response to a [Ca(2+)](i) increase from 1 to 100 microM, but increased only approximately 2-fold with a voltage change from +20 to +130 mV. The fastest activation kinetics of BK channels in cell-attached patches resembled that in inside-out patches with [Ca(2+)](i) of 100 microM or more, suggesting that many BK channels are located very close to calcium channels. Given the low Ca(2+) affinity and rapid Ca(2+) binding/unbinding properties, we conclude that BK channels in this preparation are adapted to play an important role in regulation of neurotransmitter release, and they are ideal reporters of local [Ca(2+)] at the inner membrane surface.
authors.affiliation: Department of Physiology, David Geffen School of Medicine at UCLA, Los Angles, CA 90095, USA.
authors.forename: Xiao-Ping
authors.initials: XP
authors.lastname: Sun
authors.forename: Bruce
authors.initials: B
authors.lastname: Yazejian
authors.forename: Alan D
authors.initials: AD
authors.lastname: Grinnell
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000502:physiology; D020013:Calcium Signaling / Q000502:physiology; D002478:Cells, Cultured; D018920:Coculture Techniques; D004594:Electrophysiology; D019706:Excitatory Postsynaptic Potentials / Q000502:physiology; D007700:Kinetics; D051036:Large-Conductance Calcium-Activated Potassium Channels; D008564:Membrane Potentials / Q000502:physiology; D009046:Motor Neurons / Q000502:physiology* / Q000648:ultrastructure; D009132:Muscles / Q000166:cytology / Q000502:physiology; D018408:Patch-Clamp Techniques; D010455:Peptides / Q000494:pharmacology; D026902:Potassium Channel Blockers / Q000494:pharmacology; D024681:Potassium Channels, Calcium-Activated / Q000502:physiology*; D017729:Presynaptic Terminals / Q000502:physiology*; D012964:Sodium / Q000378:metabolism; D014982:Xenopus laevis
chemical_list: D002121:Calcium Channel Blockers; D015220:Calcium Channels; D051036:Large-Conductance Calcium-Activated Potassium Channels; D010455:Peptides; D026902:Potassium Channel Blockers; D024681:Potassium Channels, Calcium-Activated; C064719:iberiotoxin; D012964:Sodium