Ca2+-activated K+ conductance of the human red cell membrane: voltage-dependent Na+ block of outward-going currents.
Human red cells were prepared with various cellular Na+ and K+ concentrations at a constant sum of 156 mM. At maximal activation of the K+ conductance. gK(Ca). the net efflux of K+ was determined as a function of the cellular Na+ and K+ concentrations and the membrane potential. Vm, at a fixed [K+]ex of approximately 3.5 mM. Vm was only varied from (Vm-EK) approximately equal to 25 mV and upwards, that is, outside the range of potentials with a steep inward rectifying voltage dependence (Stampe & Vestergaard-Bogind, 1988). gK(Ca) as a function of cellular Na+ and K+ concentrations at Vm = -40.0 and 40 mV indicated a competitive, voltage-dependent block of the outward current conductance by cellular Na+. Since the present Ca2+-activated K+ channels have been shown to be of the multi-ion type, the experimental data from each set of Na+ and K+ concentrations were fitted separately to a Boltzmann-type equation, assuming that the outward current conductance in the absence of cellular Na+ is independent of voltage. The equivalent valence determined in this way was a function of the cellular Na+ concentration increasing from 0.5 to 1.5 as this concentration increased from 11 to 101 mM. Data from a previous study of voltage dependence as a function of the degree of Ca2+ activation of the channel could be accounted for in this way as well. It is therefore suggested that the voltage dependence of gK(Ca) for outward currents at (Vm-Ek) greater than 25 mV reflects a voltage-dependent Na+ block of the Ca2+-activated K+ channels.
Stampe
P
P
Zoophysiological Laboratory B, August Krogh Institute, University of Copenhagen, Denmark.
Vestergaard-Bogind
B
B
D002118:Calcium / Q000494:pharmacology*; D004553:Electric Conductivity / Q000187:drug effects; D004910:Erythrocyte Membrane / Q000187:drug effects*; D006801:Humans; D007473:Ion Channels / Q000187:drug effects*; D008564:Membrane Potentials / Q000187:drug effects; D008954:Models, Biological; D010795:Phthalic Acids; D011188:Potassium / Q000097:blood; D015221:Potassium Channels / Q000187:drug effects; D012964:Sodium / Q000097:blood / Q000494:pharmacology*
D007473:Ion Channels; D010795:Phthalic Acids; D015221:Potassium Channels; C032279:phthalic acid; D012964:Sodium; D011188:Potassium; D002118:Calcium