Novel action of BAPTA series chelators on intrinsic K+ currents in rat hippocampal neurones

1. Whole-cell recordings were made from rat CA1 neurones in brain slices. When electrodes contained diazo-2 (2 mM) or dibromo BAPTA (1 mM) a large steady-state outward current (hundreds of picoamps) developed within 5 min of breakthrough at a V(H) of -60 mV. BAPTA itself (1 mM) caused qualitatively similar but smaller effects. 2. The outward current was accompanied by increased conductance with a null potential close to the calculated K(+) equilibrium potential (E(K)) of -110 mV. Development of outward current occurred concurrently with progressive loss of slow AHP tail current (I(SAHP)) evoked by brief depolarizations. The peak latency of I(SAHP) increased during the onset of chelator action. 3. The persistent outward current was reversibly inhibited by noradrenaline (10 mu M) or isoprenaline (2-5 mu M), and completely prevented by 8-bromoadenosine 3',5' cyclic monophosphate (8-Br cAMP; 100 mu M) or QX-314 (10 mM) in recording electrodes. After development of outward current, diazo-2 photolysis caused inward current and decreased conductance. Both. flash- and noradrenergic-sensitive responses were inwardly rectifying outward currents with null potentials close to E(K). 4. The outward current induced by dibromo BAPTA was not blocked by internal EGTA (10 mM). However, experiments incorporating Ca(2+) influx or Ca(2+) loading of the buffer indicate that Ca(2+) facilitated the outward current. 5. The outward currents induced by dibromo BAPTA or diazo-2 were not associated with significant changes in resting [Ca(2+)](i). Regions of the cell contributing to the outward current were deduced from measurements of fura-2 diffusion. These were compared with regions of [Ca(2+)](i) elevation during I(SAHP). 6. These results are consistent with the hypothesis that the BAPTA series Ca(2+) buffers can activate those Ca(2+)-activated K(+) channels that underlie the slow AHP, without the predicted elevation of bulk [Ca(2+)](i). Therefore these results cannot be interpreted solely in terms of Ca(2+) concentration changes, although the observations illustrate a novel, investigative role for these compounds in the study of Ca(2+)-dependent processes.