Multi-barrelled K channels in renal tubules.

Most ion-channels have several kinetically distinct open and closed states. However, a number of groups have reported the presence of sub-conductance states within the open states. It is not clear whether these sub-conductance states represent a partially-occluded open state (with restricted diffusion), or whether they indicate the presence of parallel conductive units. Here we present evidence for the presence of four parallel, equally-conductive subunits that make up a K-selective channel in the apical membrane of early distal tubule in the kidney of Amphiuma. The overall open probability of the channel is controlled by a main gate, the kinetics of which can be described by a simple two-state (open or closed) model. This main gate acts either to occlude or to expose the four subunits in unison. The subunits seem to gate independently both of the main gate and of one another, their open probability obeying the binomial distribution.