Indian red scorpion (Mesobuthus tamulus concanesis, Pocock) venom prolongs repolarization time and refractoriness of the compound action potential of frog sciatic nerve in vitro through calcium dependent mechanism.

Effects of Indian red scorpion (M. tamulus concanesis; MBT) venom on the compound action potential (CAP) of sciatic nerve in vitro were examined. MBT venom (0.1-6.0 micrograms/ml) prolonged the repolarization time and refractory period of the CAP in a concentration-dependent manner with maximal potentiation occurring at 6 micrograms/ml (about 100-200 times of the initial). At 1 microgram/ml of venom the prolongations were 40-50 times the initial durations and this concentration was used for subsequent experiments. Rise time, threshold, and conduction velocity of CAP were not altered by MBT venom (1 microgram/ml). In Ca(2+)-free medium, the venom-induced prolongations were only 2-6 times the initial response but addition of Ca2+ in the same medium then prolonged than by 50-70 times. The Ca2+ channel antagonists (nifedipine, 10 microM or Mg2+ ions, 5 mM) attenuated the venom (1 microgram/ml)-induced prolongation of repolarization time and refractory period. However, venom-induced prolongation of CAP responses were still significantly greater than the control in presence of these antagonists. The results indicate that MBT venom-induced increases in repolarization time and refractory period of the action potential greatly depend upon the presence of Ca2+ ions in the medium. The Ca2+ influx was through the L-type of Ca2+ channels.