[CA2-MEDIATED RELEASE OF CA2+ FROM INTRACELLULAR STORES IN RAT CHROMAFFIN CELLS(](I) ELEVATIONS DETECTED BY BK CHANNELS DURING CA2+ INFLUX AND MUSCARINE)

Submembrane [Ca2+](i) changes were examined in rat chromaffin cells by monitoring the activity of an endogenous Ca2+-dependent protein: the large conductance Ca2+- and voltage-activated K+ channel (also known a s the BK channel). The Ca2+ and voltage dependence of BK current inact ivation and conductance were calibrated first by using defined [Ca2+]( i) salines. This information was used to examine submembrane [Ca2+](i) elevations arising out of Ca2+ influx and muscarine-mediated release of Ca2+ from intracellular stores. During Ca2+ influx, some BK channel s are exposed to [Ca2+](i) of at least 60 mu M. However, the distribut ion of this [Ca2+](i) elevation is highly nonuniform so that the avera ge [Ca2+](i) detected when all BK channels are activated is only simil ar to 10 mu M. Intracellular dialysis with 1 mM or higher EGTA spares only the BK channels activated by the highest [Ca2+](i) during influx, whereas dialysis with 1 mM or higher BAPTA blocks activation of all B K channels. Submembrane [Ca2+](i) elevations fall rapidly after termin ation of short (5 msec) Ca2+ influx steps but persist above 1 mu M for several hundred milliseconds after termination of long (200 msec) inf lux steps. In contrast to influx, the submembrane [Ca2+](i) elevations produced by release of intracellular Ca2+ by muscarinic actetylcholin e receptor (mAChR) activation are much more uniform and reach peak lev els of 3-5 mu M. Our results suggest that during normal action potenti al activity only 10-20% of BK channels in each chromaffin cell see suf ficient [Ca2+](i) to be activated.