HIGH CA2-INDEPENDENT LONG SHUT INTERVALS IN BK CHANNELS FROM RAT MUSCLE( CONCENTRATIONS INDUCE A LOW ACTIVITY MODE AND REVEAL CA2+)

1. Large-conductance calcium-activated K+ channels (BK channels) often display long closed intervals at higher levels of Ca2+. To gain furth er insight into possible mechanisms for these intervals, currents were recorded from single BK channels, using the patch clamp technique, fr om patches of membrane excised from primary cultures of rat skeletal m uscle. 2. High intracellular calcium concentrations ([Ca2+](i); 10-100 0 mu M) induced a lour activity mode and revealed isolated long shut i ntervals. Neither of thee phenomena were dui to the Ba2+ that typicall y contaminates reagent grade salts. 3. The low activity mode was chara cterized by typically single brief open intervals with mean durations of 0.1 ms, separated by long shut intervals with mean durations of 100 ms. The very low open probability of about 0.001 during the low activ ity mode would make a sojourn to this mode functionally equivalent to a sojourn to an inactive state. The durations of sojourns in the low a ctivity mode were exponentially distributed, with the mean durations r anging from about 1 s in 10 mu M Ca-i(2+), to 4.5 s in 1000 mu M Ca-i( 2+). With increased filtering, the brief open intervals would escape d etection so that a sojourn to the low activity mode would appear as a single shut interval. A typical channel spent less than 5% of its time in the low activity mode for [Ca2+](i) < 10 mu M. This increased to a bout 30% for [Ca2+](i) > 100-1000 mu M. A kinetic model with three clo sed states and two open states could approximate the gating of the low activity mode. 4. The isolated long shut intervals were not from the low activity mode, suggesting a different underlying mechanism. Their frequency of occurrence of about 0.3 s(-1) did not increase with incre asing [Ca2+](i), indicating that they did not arise from a slow Ca2+ b lock. Their durations were exponentially distributed, with a mean of 1 27 ms, which was independent of [Ca2+](i), suggesting that a single Ca 2+-independent closed state or block underlies the isolated long shut intervals. At higher [Ca2+](i), up to 60% of the shut time could be sp ent in the isolated long shut intervals. 5. These observations suggest that activation of BK channels by high [Ca2+](i) can be limited by so journs to a low activity mode and also by isolated long shut intervals , two additional phenomena that will have to be accounted for in the g ating of BK channels.