Single apamin-blocked Ca-activated K+ channels of small conductance in cultured rat skeletal muscle

Nature ◽  
1986 ◽  
Vol 323 (6090) ◽  
pp. 718-720 ◽  
Author(s):  
Andrew L. Blatz ◽  
Karl L. Magleby
Keyword(s):  
Skeletal Muscle ◽  
Rat Skeletal Muscle ◽  
K Channels ◽  
Small Conductance

scholarly journals Potassium blocks barium permeation through a calcium-activated potassium channel.

1988 ◽  
Vol 92 (5) ◽  
pp. 549-567 ◽  
Author(s):  
J Neyton ◽  
C Miller
Keyword(s):  
Skeletal Muscle ◽  
Lipid Bilayers ◽  
Dissociation Rate ◽  
Planar Lipid Bilayers ◽  
Rat Skeletal Muscle ◽  
K Channels ◽  
Calcium Activated Potassium Channel ◽  
Micromolar Range ◽  
High Conductance ◽  
External K

Single high-conductance Ca2+-activated K+ channels from rat skeletal muscle were inserted into planar lipid bilayers, and discrete blocking by the Ba2+ ion was studied. Specifically, the ability of external K+ to reduce the Ba2+ dissociation rate was investigated. In the presence of 150 mM internal K+, 1-5 microM internal Ba2+, and 150 mM external Na+, Ba2+ dissociation is rapid (5 s-1) in external solutions that are kept rigorously K+ free. The addition of external K+ in the low millimolar range reduces the Ba2+ off-rate 20-fold. Other permeant ions, such as Tl+, Rb+, and NH4+ show a similar effect. The half-inhibition constants rise in the order: Tl+ (0.08 mM) less than Rb+ (0.1 mM) less than K+ (0.3 mM) less than Cs+ (0.5 mM) less than NH4+ (3 mM). When external Na+ is replaced by 150 mM N-methyl glucamine, the Ba2+ off-rate is even higher, 20 s-1. External K+ and other permeant ions reduce this rate by approximately 100-fold in the micromolar range of concentrations. Na+ also reduces the Ba2+ off-rate, but at much higher concentrations. The half-inhibition concentrations rise in the order: Rb+ (4 microM) less than K+ (19 microM) much less than Na+ (27 mM) less than Li+ (greater than 50 mM). The results require that the conduction pore of this channel contains at least three sites that may all be occupied simultaneously by conducting ions.

scholarly journals Time-irreversible Subconductance Gating Associated with Ba2+ Block of Large Conductance Ca2+-activated K+ Channels

1998 ◽  
Vol 111 (2) ◽  
pp. 343-362 ◽  
Author(s):  
Ricardo A. Bello ◽  
Karl L. Magleby
Keyword(s):  
Skeletal Muscle ◽  
Membrane Potential ◽  
External Solution ◽  
Single Step ◽  
Rat Skeletal Muscle ◽  
Patch Clamp Technique ◽  
K Channels ◽  
Open Time ◽  
Channel Open Time ◽  
A Site

Ba2+ block of large conductance Ca2+-activated K+ channels was studied in patches of membrane excised from cultures of rat skeletal muscle using the patch clamp technique. Under conditions in which a blocking Ba2+ ion would dissociate to the external solution (150 mM N-methyl-d-glucamine+o, 500 mM K+i, 10 μM Ba2+i, +30 mV, and 100 μM Ca2+i to fully activate the channel), Ba2+ blocks with a mean duration of ∼2 s occurred, on average, once every ∼100 ms of channel open time. Of these Ba2+ blocks, 78% terminated with a single step in the current to the fully open level and 22% terminated with a transition to a subconductance level at ∼0.26 of the fully open level (preopening) before stepping to the fully open level. Only one apparent preclosing was observed in ∼10,000 Ba2+ blocks. Thus, the preopenings represent Ba2+-induced time-irreversible subconductance gating. The fraction of Ba2+ blocks terminating with a preopening and the duration of preopenings (exponentially distributed, mean = 0.75 ms) appeared independent of changes in [Ba2+]i or membrane potential. The fractional conductance of the preopenings increased from 0.24 at +10 mV to 0.39 at +90 mV. In contrast, the average subconductance level during normal gating in the absence of Ba2+ was independent of membrane potential, suggesting different mechanisms for preopenings and normal subconductance levels. Preopenings were also observed with 10 mM Ba2+o and no added Ba2+i. Adding K+, Rb+, or Na+ to the external solution decreased the fraction of Ba2+ blocks with preopenings, with K+ and Rb+ being more effective than Na+. These results are consistent with models in which the blocking Ba2+ ion either induces a preopening gate, and then dissociates to the external solution, or moves to a site located on the external side of the Ba2+ blocking site and acts directly as the preopening gate.

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