Tetraethylammonium ion sensitivity of a 35-pS CA2(+)-activated K+ channel in GH3 cells that is activated by thyrotropin-releasing hormone.

Single Ca2(+)-activated K+ channels were studied in membrane patches from the GH3 anterior pituitary cell line. We have previously demonstrated the coexistence of large-conductance and small-conductance (280 pS and 11 pS in symmetrical 150 mM K+, respectively) Ca2(+)-activated K+ channels in this cell line (Lang and Ritchie 1987). Here we report the existence of a third type of Ca2(+)-activated K+ channel that has a conductance of about 35 pS under similar conditions. In excised inside-out patches, this channel can be activated by elevations of the internal free Ca2+ concentration, and the open probability increases as the membrane potential is made more positive. In excised patches, the sensitivity of this 35-pS channel to internal Ca2+ is low; at positive membrane potentials, this channel requires Ca2+ concentrations greater than 10 microM for activation. However, 35-pS channels have a much higher sensitivity to Ca2+ in the first minute after excision (activated by 1 microM Ca2+ at -50 mV). Therefore, it is possible that the Ca2+ sensitivity of this channel is stabilized by intracellular factors. In cell-attached patches, this intermediate conductance channel can be activated (at negative membrane potentials) by thyrotropin-releasing hormone-induced elevations of the intracellular Ca2+ concentration and by Ca2+ influx during action potentials. The intermediate conductance channel is inhibited by high concentrations of external tetraethylammonium ions (Kd = 17 mM) and is relatively resistant to inhibition by apamin.