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 (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.
Schetz
J A
JA
Department of Neuroscience, College of Medicine, University of Florida, Gainesville 32610 USA.
Anderson
P A
PA
D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001030:Apamin / Q000037:antagonists & inhibitors / Q000378:metabolism* / Q000494:pharmacology; D015220:Calcium Channels / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D002712:Chlorides / Q000494:pharmacology; D004095:Dihydropyridines / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D005374:Filtration; D006863:Hydrogen-Ion Concentration; D007457:Iodine Radioisotopes; D008722:Methods; D009206:Myocardium / Q000378:metabolism*; D015221:Potassium Channels / Q000378:metabolism*; D011485:Protein Binding / Q000187:drug effects; D011803:Quinine / Q000494:pharmacology; D011817:Rabbits; D012604:Scorpion Venoms / Q000494:pharmacology; D013696:Temperature
D015220:Calcium Channels; D002712:Chlorides; D004095:Dihydropyridines; D007457:Iodine Radioisotopes; D015221:Potassium Channels; D012604:Scorpion Venoms; C033799:apamin receptor; C056574:leiurotoxin I; D001030:Apamin; D000584:Amiloride; D011803:Quinine