Properties and the cytoskeletal control of Ca(++)-independent large conductance K+ channels in neonatal rat hippocampal neurons.

A member of the family of Ca(++)-independent large conductance K+ channels (termed BK channels) was identified in patch clamp experiments with cultured neonatal rat hippocampal neurons. Permeation was characterized (at 5 mmol/l external, 140 mmol/l internal K+; 135 mmol/l external Na+) by a conductance of 107 pS, a ratio PNa/Pk approximately 0.01, and outward rectification near the reversal potential. Channel activity was not voltage-dependent, could not be reduced by internal TEA or by a shift of internal pH from 7.4 to 6.8, i.e., discriminating features within the Ca(++)-independent BK channel family. Cytosolic proteolysis abolished the functional state of hippocampal Ca(++)-independent BK channels, in contrast to the pronase resistance of hippocampal Ca(++)-activated BK channels which suggests structural dissimilarities between these related channels. Cytoskeletal alterations had an activating influence on Ca(++)-independent BK channels and caused a 3-4-fold rise in Po, but patch excision and channel isolation from the natural environment provoked the strongest increase in Po, from 0.07 +/- 0.03 to 0.73 +/- 0.04. This activation process operated slowly, on a minute time scale and can be most easily explained with the loss of a membrane-associated inhibitory particle. Once activated, Ca(++)-independent BK channels reacted sensitively to a Mg-ATP supplemented brain tissue extract with a Po decline, from 0.60 +/- 0.06 to 0.10 +/- 0.05. Heated extracts failed to induce significant channel inhibition, providing evidence for a heat-unstable molecule with reassociates with the internal channel surface to reestablish channel inhibition. A dualistic channel control, by this membrane-associated molecule and by the cytoskeleton seems possible.