Pharmacology of the high-affinity apamin receptor in rabbit heart

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 (K_D = 8.07 ± 2.14 pM and B_max = 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.