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.
