Funnel-web Spider Venom and a Toxin Fraction Block Calcium Current Expressed from Rat Brain MRNA in Xenopus Oocytes

Injection of rat brain mRNA into Xenopus oocytes has been shown to induce a calcium current (ICa) that is insensitive to dihydropyridine and omega-conotoxin. We examined the effect of funnel-web spider venom on two aspects of this expressed ICa: (i) the calcium-activated chloride current [ICl(Ca)] and (ii) the currents carried by barium ions through calcium channels (IBa). In the presence of 1.8 mM extracellular calcium, ICl(Ca) tail current became detectable between -30 and -40 mV from a holding potential of -80 mV and reached a maximal amplitude between 0 and +10 mV. Total spider venom partially (83%) and reversibly blocked the calcium-activated chloride current without changing its voltage sensitivity. A chromatographic toxin fraction from the venom also blocked this current (64%). The venom had a minimal effect on INa and IK. Direct investigation of inward current mediated by calcium channels was carried out in high-barium solution. IBa had a higher threshold of activation (-30 to -20 mV) and reached its maximal amplitude at about +20 mV. Total venom or a partly purified chromatographic toxic fraction blocked IBa partially and reversibly without changing its current-voltage characteristics. Furthermore, the extent of the total venom block depended on the concentration of extracellular barium. Only 35% of the IBa was blocked in 60 mM Ba2+, whereas the block increased to 65% and 71%, respectively, for 40 and 20 mM Ba2+. On the basis of these results, we propose that the calcium channels expressed from rat brain mRNA in Xenopus oocytes is similar to the recently discovered P-type channels.