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.
Lin
J W
JW
Department of Physiology and Biophysics, New York University Medical Center, NY 10016.
Rudy
B
B
Llinás
R
R
D000818:Animals; D001180:Arthropod Venoms / Q000494:pharmacology*; D001921:Brain / Q000502:physiology*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D005260:Female; D007700:Kinetics; D008565:Membrane Proteins / Q000235:genetics* / Q000502:physiology; D008845:Microinjections; D009865:Oocytes / Q000187:drug effects / Q000502:physiology*; D010456:Peptides, Cyclic / Q000494:pharmacology*; D012333:RNA, Messenger / Q000008:administration & dosage / Q000235:genetics*; D051381:Rats; D013111:Spider Venoms / Q000494:pharmacology*; D014982:Xenopus laevis; D020960:omega-Conotoxins
D001180:Arthropod Venoms; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D008565:Membrane Proteins; D010456:Peptides, Cyclic; D012333:RNA, Messenger; D013111:Spider Venoms; D020960:omega-Conotoxins; C053423:Conus magus toxin