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 new born rats. Macroscopic currents, carried by 10 mM Ba2+, were measured in whole-cell configuration. The toxin at the saturating dose of 2.5 m u M reversibly produced an approximate to 67% block of the voltage-dep endent Ca2+ current by a fast mechanism of action. The current inhibit ion and recovery were reached in less than 1 min. Inhibition was conce ntration-dependent, with a half-effective dose of 0.25 mu M. The curre nt block was practically voltage-independent, whereas the steady-state inactivation h(infinity) was significantly affected by HLTx (approxim ate to 10 mV). The toxin did not affect the activation and inactivatio n kinetics of the Ca2+ current. Experiments with other Ca2+ channel bl ockers showed that HLTx abolished omega-conotoxin GVIA-sensitive Ca2currents, as well as omega-Aga-IVA- and dihydropyridine-sensitive Ca2 currents. These drugs had virtually no effect when HLTx was applied f irst. The present results indicate that HLTx produce a high-potency bl ockage of the three pharmacologically distinct Ca2+ current components .