CA-2+-ACTIVATED K+ CURRENT INVOLVEMENT IN NEURONAL FUNCTION REVEALED BY INSITU SINGLE-CHANNEL ANALYSIS IN HELIX NEURONS

1. The properties of single calcium‐activated potassium channels (or C‐channels) were studied in cell‐attached patches using the patch‐clamp technique. Experiments were performed on identified Ca2(+)‐dependent U cells in juvenile specimens (1‐2 months old) of Helix aspersa. 2. The criteria used to identify C‐channels were based on comparison between macroscopic C‐currents and currents reconstructed from unitary recordings. Both currents had a slow activation rate at large positive potentials which turned into fast activation after large Ca2+ entries. Both currents were blocked by intracellularly injected EGTA. 3. The unitary conductance in normal (5 mM) or reduced (0.5 mM) [K+]o ranged from 24 to 65 pS (mean +/‐ S.D., 48 +/‐ 13; n = 64). With 85‐110 mM [K+]o, which is approximately equal to the internal [K+], the conductance was 64 pS and the reversal potential was approximately 0 mV. 4. C‐channels in U cells were distributed in clusters of three to ten channels (mean 5.05 channels in seventy‐five patches). Calcium channels were present in patches containing clustered C‐channels. C‐channels within clusters behaved independently. 5. With patch electrode containing 8 mM‐calcium, C‐channels opened transiently upon patch depolarization. Reopenings in quiescent depolarized patches were induced by whole‐cell spikes triggered by current pulses applied to an intracellular electrode. Apparent inactivation of C‐channels in depolarized patches was in fact due to a decrease in [Ca2+]i resulting from inactivation of Ca2+ channels. 6. Calcium‐free saline solutions in the patch electrodes prevented C‐channels from opening upon patch depolarization. Entry of calcium through the surrounding membrane induced delayed openings in the patch. Peak opening probability Po occurred 330 +/‐ 30 ms after a brief Ca2+ entry with a lag period of 50‐80 ms. With patch electrodes filled with Ca2(+)‐containing saline solutions and under conditions which maximized C‐channel opening, peak Po was reached in 20‐50 ms. The same value was observed for the whole‐cell C‐current. 7. The peak Po at a given patch potential and in response to a whole‐cell spike was not altered by a previous long‐lasting patch depolarization, or by producing several successive Ca2+ entries. Thus, C‐channels did not appear to be inactivated by depolarization or increase in [Ca2+]i. 8. C‐channels were found to be relatively highly voltage dependent, with an e‐fold increase in Po per 14.9 mV increase in potential.(ABSTRACT TRUNCATED AT 400 WORDS) © 1990 The Physiological Society