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Unformatted text preview: Heteropodatoxins: Peptides Isolated from Spider Venom that Block Kv4.2 Potassium Channels MICHAEL C. SANGUINETTI, 1 JANICE H. JOHNSON, LANCE G. HAMMERLAND, PAUL R. KELBAUGH, ROBERT A. VOLKMANN, NICHOLAS A. SACCOMANO, and ALAN L. MUELLER NPS Pharmaceuticals Inc., Salt Lake City, Utah 84108 (M.C.S., J.H.J., L.G.H., A.L.M.), and Pfizer Inc., Groton, Connecticut 06340 (P.R.K., R.A.V., N.A.S.) Received September 26, 1996; Accepted November 14, 1996 SUMMARY Toxins isolated from scorpion, snake, and spider venoms are valuable tools to probe the physiologic function and structure of ion channels. In this study, we have isolated three new toxins (heteropodatoxins) from the venom of a spider, Heteropoda venatoria . These toxins are structurally similar peptides of 29 to 32 amino acids and share sequence homology with hanatoxins isolated from the venom of a Chilean tarantula. The heteropo- datoxins prolonged the action-potential duration of isolated rat ventricular myocytes, suggesting that the peptides block K 1 currents. The effect of toxins on cardiac K 1 currents were studied using voltage clamp techniques. The toxins blocked the transient outward K 1 current but not other K 1 currents in isolated rat cardiac myocytes. The mechanism of block was studied further using Kv4.2, a cloned channel believed to un- derlie transient outward K 1 current in rat myocytes. The toxins blocked Kv4.2 current expressed in Xenopus laevis oocytes in a voltage-dependent manner, with less block at more positive potentials. In addition, the toxins slowed the time course of current activation and inactivation and shifted the voltage de- pendence of current inactivation to more positive potentials. The heteropodatoxins represent new pharmacologic probes to study the role of Kv4.2 channels in cardiac and neural tissue. Voltage-sensitive K 1 channels modulate excitability and synaptic transmission of neurons and repolarization of car- diac myocytes. A large number of K 1 channels have been cloned recently, indicating the great diversity of these chan- nels compared with other types of voltage-sensitive channels. The study of their physiologic roles has been aided greatly by the discovery of specific blockers. For example, charybdotoxin isolated from venom of the scorpion, Leiurus quinquestriatus , was used to define the function of large conductance Ca 2 1- activated K 1 channels (1), and the structural determinants of function in Shaker-like K 1 channels (2–4). Potassium channel toxins also have been used to distinguish the role of a particular channel type to net current resulting from the overlap of many different types of channels (5, 6) and be- tween different subtypes of a single class of K 1 channels (7)....
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- Spring '11
- Ecology, MV, Venom, rat ventricular myocytes, voltage dependence