Proton modulation of a Ca(2+)-activated K+ channel from rat skeletal muscle incorporated into planar bilayers.
The effect of pH on the activation of a Ca-activated K+ [K(Ca)] channel from rat skeletal muscle incorporated into planar lipid bilayers was studied. Experiments were done at different intracellular Ca2+ and proton concentrations. Changes in pH modified channel kinetics only from the Ca-sensitive face of the channel. At constant Ca2+ concentration, intracellular acidification induced a decrease in the open probability (Po) and a shift of the channel activation curves toward the right along the voltage axis. The displacement was 23.5 mV per pH unit. This displacement was due to a change in the half saturation voltage (Vo) and not to a change in channel voltage dependence. The shifts in Vo induced by protons appeared to be independent of Ca2+ concentration. The slope of the Hill plot of the open-closed equilibrium vs. pH was close to one, suggesting that a minimum of one proton is involved in the proton-driven channel closing reaction. The change in Po with variations in pH was due to both a decrease in the mean open time (To) and an increase in the mean closed time (Tc). At constant voltage, the mean open time of the channel was a linear function of [Ca2+] and the mean closed time was a linear function of 1/[Ca2+]2. Changes in the internal pH modified the slope, but not the intercept of the linear relations To vs. [Ca2+] and Tc vs. 1/[Ca2+]2. On the basis of these results an economical kinetic model of the effect of pH on this channel is proposed. It is concluded that protons do not affect the open-closed reaction, but rather weaken Ca2+ binding to all the conformational states of the channel. Moreover, competitive models in which Ca2+ and H+ cannot bind to the same open or closed state are inconsistent with the data.
Laurido
C
C
Departamento de Química, Facultad de Ciencia, Universidad de Santiago, Chile.
Candia
S
S
Wolff
D
D
Latorre
R
R
D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology / Q000648:ultrastructure; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D006863:Hydrogen-Ion Concentration; D015640:Ion Channel Gating / Q000187:drug effects; D008051:Lipid Bilayers / Q000378:metabolism*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009132:Muscles / Q000166:cytology / Q000502:physiology / Q000648:ultrastructure*; D015221:Potassium Channels / Q000187:drug effects* / Q000502:physiology / Q000648:ultrastructure; D011522:Protons; D051381:Rats; D013997:Time Factors
D008051:Lipid Bilayers; D015221:Potassium Channels; D011522:Protons; D002118:Calcium