Effects of a Lasiodora spider venom on Ca2+ and Na+ channels

The venom of a Brazilian spider, Lasiodora sp (Mygalomorphae, Theraphosidae ), was screened for activity against ion channels using Ca2+ imaging and wh ole-cell patch clamp in GH3 cells. When tetrodotoxin (TTX) was present to b lock Na+ channels, the venom abolished the Ca2+ oscillations that are norma lly present in these cells and reduced the basal level of intracellular Ca2 +. Under patch clamp, the venom reduced the L-type Ca2+ channel conductance and caused a positive shift in its voltage dependence of activation. In ad dition to these effects, when applied without TTX, the venom also caused a slow and noisy increase in intracellular Ca2+. The sensitivity of this seco nd effect to TTX suggested an effect on Na+ channels, which was tested usin g patch clamp. Control Na+ currents inactivated completely as a single expo nential. Treatment with the venom did not affect the amplitude of I-Na, but caused it to divide in two slower exponential components plus a sustained component, all of which were suppressed by TTX. The venom also caused a neg ative shift in the voltage dependence of activation and steady-state inacti vation of I-Na. The observed effects of this venom on whole-cell currents e xplain the changes it causes in intracellular C2+ in GH3 cells and demonstr ate that the venom of this spider is a source of toxins active against ion channels. (C) 2001 Elsevier Science Ltd. All rights reserved.