Sapecin B, a novel fly toxin, blocks macroscopic K+ currents in the GH3 rat pituitary cell line.
Sapecin B is structurally homologous to charybdotoxin (CTX), which is found in scorpion venom. This study investigated the effects of sapecin B on the Ca(2+)-activated K+ currents [IK(Ca)] and the rapidly inactivating K+ currents in clonal rat GH3 pituitary cells with whole cell voltage-clamp methods. Sapecin B (20 nM) reversibly blocked the CTX-sensitive Ix(Ca) (the BK current) in a dose-dependent manner, with a half-maximal inhibitory concentration of approximately 0.9 nM, comparable to that of 0.08-0.4 nM for CTX. The Ca2+ currents in GH3 cells, however, were not affected by sapecin B (40 nM), indicating that the blockade of IK(Ca) by sapecin B is not a secondary effect of Ca2+ current inhibition. The effect of sapecin B on IK(Ca) resembled that of CTX, as expected from the structural similarities shared by CTX and sapecin B. We also found that sapecin B largely inhibited the 4-aminopyridine-sensitive, rapidly inactivating K+ currents in a dose-dependent manner, with a half-maximal inhibitory concentration of approximately 40 nM, whereas CTX had little effect on this current in GH3 cells. Sapecin B may thus provide a useful tool, complementary to CTX, for probing the functional role of molecular domains in the BK channels and the structural similarities common to the BK and the rapidly inactivating A-type K+ channels.
Suzuki
N
N
Department of Physiology, School of Medicine, Kitasato University, Sagamihara, Japan.
Hirono
M
M
Kawahara
K
K
Yoshioka
T
T
D015761:4-Aminopyridine / Q000494:pharmacology; D000818:Animals; D018999:Charybdotoxin / Q000494:pharmacology; D004175:Diptera; D019476:Insect Proteins / Q000494:pharmacology*; D007700:Kinetics; D051036:Large-Conductance Calcium-Activated Potassium Channels; D018408:Patch-Clamp Techniques; D010903:Pituitary Gland, Anterior; D010911:Pituitary Neoplasms; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000502:physiology*; D024681:Potassium Channels, Calcium-Activated; D051381:Rats; D012604:Scorpion Venoms / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology; D013997:Time Factors; D014407:Tumor Cells, Cultured
D019476:Insect Proteins; D051036:Large-Conductance Calcium-Activated Potassium Channels; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D024681:Potassium Channels, Calcium-Activated; D012604:Scorpion Venoms; C080667:sapecin B protein, Sarcophaga peregrina; D018999:Charybdotoxin; D013779:Tetrodotoxin; D015761:4-Aminopyridine