SEPARATION OF GATING PROPERTIES FROM PERMEATION AND BLOCK IN MSLO LARGE-CONDUCTANCE CA-ACTIVATED K+ CHANNELS

In this and the following paper we have examined the kinetic and stead y-state properties of macroscopic mslo Ca-activated K+ currents in ord er to interpret these currents in terms of the gating behavior of the mslo channel. To do so, however, it was necessary to first find condit ions by which we could separate the effects that changes in Ca2+ conce ntration or membrane voltage have on channel permeation from the effec ts these stimuli have on channel gating. In this study we investigate three phenomena which are unrelated to gating but are manifest in macr oscopic current records: a saturation of single channel current at hig h voltage, a rapid voltage-dependent Ca2+ block, and a slow voltage-de pendent Ba2+ block. Where possible methods are described by which thes e phenomena can be separated from the effects that changes in Ca2+ con centration and membrane voltage have on channel gating. Where this is not possible, some assessment of the impact these effects have on gati ng parameters determined from macroscopic current measurements is prov ided. We have also found that without considering the effects of Ca2and voltage on channel permeation and block, macroscopic current measu rements suggest that mslo channels do not reach the same maximum open probability at all Ca2+ concentrations. Taking into account permeation and blocking effects, however, we find that this is not the case. The maximum open probability of the mslo channel is the same or very simi lar over a Ca2+ concentration range spanning three orders of magnitude indicating that over this range the internal Ca2+ concentration does not limit the ability of the channel to be activated by voltage.