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Cardiovascular Research 1995 30(5):755-762; doi:10.1016/S0008-6363(95)00114-X
© 1995 by European Society of Cardiology
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Copyright © 1995, European Society of Cardiology

Pharmacology of the high-affinity apamin receptor in rabbit heart

John A. Schetz* and Peter A.V. Anderson

Department of Neuroscience and the Whitney Laboratory, College of Medicine, University of Florida, Box 1-244, JHMHC, Gainesville, FL 32610 USA

* Corresponding author. Section of Neuropharmacology, National Institutes of Health, 9000 Rockville Pike, Bldg. 10, Room 5C108, Bethesda, MD 20892, USA. Tel. (+ 1-301) 496-6329; Fax (+1-301) 496-6609.

Apamin is a potent blocker of calcium-activated small conductance potassium (SK) channels in neurons, liver, skeletal muscle and ileum smooth muscle, but not in cardiac muscle. Cardiac muscle is devoid of SK channels; however, in isolated, single ventricular myocytes apamin is an extremely potent blocker of the L-type calcium current, and the anti-arrhythmic drug quinidine reverses apamin block. Objective: To characterize the receptor binding properties and pharmacology of the apamin receptor in heart. Methods: The binding properties of the apamin receptor were determined by rapid filtration of purified rabbit heart membranes. Results: Monoiodinated apamin binds to a labile, membrane-bound protein in heart membranes at a single, high-affinity site (KD = 8.07 ± 2.14 pM and Bmax = 686 ± 167 fmoles/mg protein, significant run test at P = 0.05 for a one site fit). 125I-apamin binding is dose-dependently inhibited by apamin, scyllatoxin, quinidine, amiloride, as well as a variety of di- and trivalent cations that are classical blockers of L-type calcium channels (e.g. Co2+, Cd2+, Mn2+, La3+, Gd3+). The cardiac apamin receptor is also critically dependent upon pH, temperature and KCl, and co-purifies in the same membrane fraction as L-type cardiac Ca2+ channels. Conclusions: The apamin receptor in rabbit heart P2 membranes has pharmacological and biochemical properties in common with both an SK channel and an L-type Ca2+ channel.

KEYWORDS Apamin receptor; Apamin; Calcium channels; Potassium channel, SK; Quinidine; Rabbit, ventricular myocytes


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