Tertiapin Potently and Selectively Blocks Muscarinic K+ Channels in Rabbit Cardiac Myocytes

• Published in: The Journal of pharmacology and experimental therapeutics 2000-04, Vol.293 (1), p.196-205
• Main Authors: Kitamura, H, Yokoyama, M, Akita, H, Matsushita, K, Kurachi, Y, Yamada, M
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Animals; ATP-Binding Cassette Transporters; Bee Venoms - pharmacology; Cell Membrane - drug effects; Cell Separation; Electrophysiology; G Protein-Coupled Inwardly-Rectifying Potassium Channels; Heart - drug effects; In Vitro Techniques; KATP Channels; Male; Membrane Potentials - drug effects; Molecular Sequence Data; Muscarinic Antagonists - pharmacology; Myocardium - cytology; Myocardium - metabolism; Patch-Clamp Techniques; Potassium Channel Blockers; Potassium Channels - drug effects; Potassium Channels, Inwardly Rectifying; Rabbits; Receptors, Muscarinic - drug effects
• ISSN: 0022-3565; 1521-0103

Tertiapin is a 21-residue peptide isolated from honey bee venoms. A recent study indicated that tertiapin is a potent blocker of certain types of inwardly rectifying K + (Kir) channels ( Jin and Lu, 1998 ). We examined the effect of tertiapin on ion channel currents in rabbit cardiac myocytes using the patch-clamp technique. In the whole-cell configuration, tertiapin fully inhibited acetylcholine (1 μM)-induced muscarinic K + (K ACh ) channel currents in atrial myocytes with the half-maximum inhibitory concentration of ∼8 nM through ∼1:1 stoichiometry. The potency of tertiapin in inhibiting K ACh channels was not significantly different at −40 and −100 mV. Tertiapin also inhibited the K ACh channel preactivated by intracellular guanosine 5′- O -(3-thiotriphosphate), a nonhydrolyzable GTP analog. A constitutively active Kir channel, the I K1 channel, was at least 100 times less sensitive to tertiapin. Another Kir channel in cardiac myocytes, the ATP-sensitive K + channel, was virtually insensitive to tertiapin (1 μM). The voltage-dependent K + and the L -type Ca 2+ channels were not affected by tertiapin (1 μM). At the single-channel level, tertiapin inhibited the K ACh channel from the outside of the membrane by reducing the NP o (N is the number of functional channels, and the P o is the open probability of each channel) without affecting the single-channel conductance or fast kinetics. Therefore, tertiapin potently and selectively blocks the K ACh channel in cardiac myocytes in a receptor- and voltage-independent manner. Tertiapin is a novel pharmacological tool to identify the functional role of the K ACh channel in the parasympathetic regulation of the heart beat.