Tetraethylammonium blockade of apamin-sensitive and insensitive Ca2(+)-activated K+ channels in a pituitary cell line.

1. The pharmacological sensitivities and physiological contributions of two types of Ca2(+)-activated K+ channels (BK and SK) in GH3 cells were examined by the outside-out, whole-cell and cell-attached modes of the patch-clamp technique. 2. BK channels (250-300 pS in symmetrical 150 mM-K+) in outside-out patches were blocked by external tetraethylammonium (TEA) and by 50 nM-charybdotoxin (CTX), but were not blocked by apamin. 3. SK channels (9-14 pS in symmetrical 150 mM-K+) in outside-out patches were blocked by external TEA and by apamin, but were not blocked by 50 nM-CTX. 4. The dissociation constant (Kd) for TEA block of SK channels (3.1 +/- 0.37 mM) was 12-fold greater than the Kd for the BK channels (260 +/- 21 microM). The TEA blockade of both channels was not strongly voltage dependent: for both channels the TEA binding site sensed less than 20% of the membrane electric field. 5. Application of blockers of the BK channels (1 mM-TEA and 50 nM-CTX) to whole cells under current clamp prolonged action potential duration; whereas application of apamin, a selective blocker of the SK channel, inhibited a slowly decaying after-hyperpolarization and had little effect on action potential duration. Apamin also increased the firing rate in 30% of the spontaneously pacing cells. 6. It is suggested that BK channels contribute to action potential repolarization: whereas SK channels contribute to the regulation of action potential firing rate.