Scorpion toxins targeted against the sarcoplasmic reticulum Ca2+-release channel of skeletal and cardiac muscle

Abstract

We report the purification of two peptides, called "imperatoxin inhibitor" and "imperatoxin activator," from the venom of the scorpion Pandinus Imperator targeted against ryanodine receptor Ca2+-release channels. Imperatoxin inhibitor has a Mr of ≈10,500, inhibits [3H]ryanodine binding to skeletal and cardiac sarcoplasmic reticulum with an ED50 of ≈10 nM, and blocks openings of skeletal and cardiac Ca2+-release channels incorporated into planar bilayers. In whole-cell recordings of cardiac myocytes, imperatoxin inhibitor decreased twitch amplitude and intracellular Ca2+ transients, suggesting a selective blockade of Ca2+ release from the sarcoplasmic reticulum. Imperatoxin activator has a Mr of ≈8700, stimulates [3H]ryanodine binding in skeletal but not cardiac sarcoplasmic reticulum with an ED50 of ≈6 nM, and activates skeletal but not cardiac Ca2+-release channels. These ligands may serve to selectively "turn on" or "turn off" ryanodine receptors in fragmented systems and whole cells.