Two distinct calcium‐activated potassium currents in a rat anterior pituitary cell line.

1. The single ‘giga‐seal’ patch‐electrode technique (Hamill, Marty, Neher, Sakmann & Sigworth, 1981) was used to record whole‐cell currents in the GH3 rat anterior pituitary cell line. 2. GH3 cells have a rapidly inactivating, voltage‐dependent K+ current that is selectively inhibited by 4‐aminopyridine (4‐AP) but not by tetraethylammonium chloride (TEA). 3. The majority of the Ca2+‐activated K+ current in these cells is blocked by TEA with an inhibitory concentration that is half‐maximal at 1 mM. An additional Ca2+‐activated K+ current is also present that is relatively resistant to TEA and is blocked by the polypeptide apamin. The apamin‐sensitive component represents less than 18% of the total Ca2+‐activated K+ current at 0 mV. 4. The time course of the slowly declining components of the Ca2+‐activated K+ tail currents measured at the ‐50 mV holding potential was usually biexponential with time constants of 0.21 +/‐ 0.02 and 1.75 +/‐ 0.23 s (mean +/‐ S.E. of mean, n = 14). Both of the two slowly decaying components contribute to the TEA‐ and apamin‐sensitive currents. 5. It is concluded that GH3 cells have at least two pharmacologically distinct Ca2+‐activated K+ currents and a 4‐AP‐sensitive voltage‐dependent K+ current.