PROPERTIES OF SINGLE CALCIUM-ACTIVATED POTASSIUM CHANNELS OF LARGE-CONDUCTANCE IN RAT HIPPOCAMPAL-NEURONS IN CULTURE

Patch-clamp recordings were made on rat hippocampal neurons maintained in culture. In cell-attached and excised inside-out and outside-out p atches a large single-channel current was observed. This channel had a conductance of 220 and 100 pS in 140 mM [K+](i)/140 mM [K+](0) and 14 0 mM [K+](i)/3 mM [K+](0) respectively. From the reversal potential th e channel was highly selective for K+, the P-K+/P-Na+ ratio being 50/1 . Channel activity was voltage-dependent, the open probability at 100 nM [Ca2+](i) increasing by e-fold for a 22 mV depolarization. It was a lso dependent on [Ca2+](i) at both resting and depolarized membrane po tentials. Channel open states were best described by the sum of two ex ponentials with time constants that increased as the membrane potentia l became more positive. Channel activity was sensitive to both externa l (500 mu M) and internal (5 mM) tetraethylammonium chloride. These da ta are consistent with the properties of maxi-K+ channels described in other preparations, and further suggest a role for maxi-channel activ ity in regulating neuronal excitability at the resting membrane potent ial. Channel activity was not altered by 8-chlorophenyl thio cAMP, con canavalin A, pH reduction or neuraminidase. In two of five patches lem akalim (BRL 38227) increased channel activity. Internal ruthenium red (10 mu M) blocked the channel by shortening the duration of both open states. This change in channel gating was distinct from the 'mode swit ching' seen in two patches, where a channel switched spontaneously fro m normal activity typified by two open states to a mode where only sho rt openings were represented.