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.
Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston 77550-2779.
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Ritchie
D000200:Action Potentials; D001030:Apamin / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D002460:Cell Line; D018999:Charybdotoxin; D004553:Electric Conductivity; D010902:Pituitary Gland / Q000166:cytology / Q000378:metabolism*; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism*; D012604:Scorpion Venoms / Q000494:pharmacology; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology*
D015221:Potassium Channels; D012604:Scorpion Venoms; D013757:Tetraethylammonium Compounds; D018999:Charybdotoxin; D001030:Apamin; D019789:Tetraethylammonium; D002118:Calcium