Characterization of the high-conductance Ca(2+)-activated K+ channel in adult human skeletal muscle.
Ca(2+)-activated K+ channels of a large conductance (BKCa) in human skeletal muscle were studied by patch clamping membrane blebs and by using the three microelectrode voltage-clamp recording technique on resealed fibre segments. Single-channel recordings in bleb-attached and inside-out modes revealed BKCa conductances of 230 pS for symmetrical and 130 pS for physiological K+ distributions. Open probability increased with membrane depolarization and increasing internal [Ca2+]. The Hill coefficient was 2.0, indicating that at least two Ca2+ ions are required for full activation. Kinetic analysis revealed at least two open and three closed states. An additional long-lived inactivated state, lasting about 0.5-20 s, was observed following large depolarizations, when extracellular K+ was lowered to physiological values. BKCa were blocked by three means: (1) externally by tetraethylammonium which reduced single-channel amplitude (IC50 approx. 0.3 mM); (2) internally by polymyxin B which decreased the open probability (IC50 approx. 5 micrograms/ml); and (3) externally by charybdotoxin which caused long-lasting periods of inactivation (IC50 < 10 nM). Measurements on resealed fibre segments at physiological [K+] were in accordance with the single-channel data: only when intracellular [Ca2+] was elevated did charybdotoxin (50 nM) reduce the macroscopic membrane K+ conductance with depolarizing voltage steps.
Lerche
H
H
Department of Applied Physiology, University of Ulm, Germany.
Fahlke
C
C
Iaizzo
P A
PA
Lehmann-Horn
F
F
D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000502:physiology*; D002122:Calcium Chloride / Q000494:pharmacology; D016257:Fura-2; D006801:Humans; D008564:Membrane Potentials / Q000187:drug effects; D008839:Microelectrodes; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism*; D012508:Sarcolemma / Q000378:metabolism
D015221:Potassium Channels; D002110:Caffeine; D002122:Calcium Chloride; D002118:Calcium; D016257:Fura-2