INACTIVATION OF A HIGH-CONDUCTANCE CALCIUM-DEPENDENT POTASSIUM CURRENT IN RAT HIPPOCAMPAL-NEURONS

Inactivating, high conductance BK-type currents have been recorded fro m inside-out patches (internal and external K+ of 140 mM and 5 mM, res pectively), obtained from cultured rat hippocampal neurons. The presen ce of prominent inactivation, not normally associated with BK channel activity, was dependent on two factors: a depolarizing step to 0 mV fr om a holding level of -80 mV and internal calcium at a concentration o f 0.7 mu M. Without the prior conditioning step to a negative potentia l, unitary currents were not evident at 0 mV; in addition, such curren ts were not elicited with the stimulus protocol if the internal Ca2+ w as reduced to a level of 0.3 mu M. Concomitant with current inactivati on was the finding of a delayed activation of BK currents following th e depolarizing step. Higher internal calcium, at 100 mu M, led to pers istent and sustained channel activity at 0 mV which was not dependent on a prior step to -80 mV. These results may be relevant to the comple x nature of the repolarizing neuronal current Ic which is the macrosco pic analogue of the unitary BK current.