Helothermine, a lizard venom toxin, inhibits calcium current in cerebellar granules

Helothermine (HLTx), a 25.5-kDa peptide toxin isolated from the venom of the Mexican beaded lizard (Heloderma horridum horridum), was found to be an inhibitor of Ca2+ channels in cerebellar granule cells of newborn rats. Macroscopic currents, carried by 10 mM Ba2+, were measured in whole-cell configuration. The toxin at the saturating dose of 2.5 microM reversibly produced an approximately 67% block of the voltage-dependent Ca2+ current by a fast mechanism of action. The current inhibition and recovery were reached in less than 1 min. Inhibition was concentration-dependent, with a half-effective dose of 0.25 microM. The current block was practically voltage-independent, whereas the steady-state inactivation h infinity was significantly affected by HLTx (approximately 10 mV). The toxin did not affect the activation and inactivation kinetics of the Ca2+ current. Experiments with other Ca2+ channel blockers showed that HLTx abolished omega-cono-toxin GVIA-sensitive Ca2+ currents, as well as omega-Aga-IVA- and dihydropyridine-sensitive Ca2+ currents. These drugs had virtually no effect when HLTx was applied first. The present results indicate that HLTx produce a high-potency blockage of the three pharmacologically distinct Ca2+ current components.