The effect of ω-conotoxin (ω-CgTX) on calcium currents of rat pituitary GH3 cells was studied by the voltage clamp method on a whole cell, under tight junction conditions. Two different components of the inward calcium current were observed in a solution containing 15 mM Ca2+. The first was activated with a holding potential of -80 mV and by testing pulses more positive than -55 mV. A shift of holding potential to -40 mV led to steady-state inactivation of this low-threshold component of the current. ω-CgTX at the initial moment after its application had an activating action on both components of the calcium current: low-threshold and high-threshold, but the increase in the first was much greater. In the present experiments the currents increased as early as 30 sec after replacement of the external solution; later the drop of current took place with temporal parameters characteristic of the spontaneous current drop in the control solution during cell dialysis. Incubation of the cells in growth medium containing 5 μM ω-CgTX for 2 h led to an increase in the density of both types of calcium currents in the GH3 cells, which was reduced after incubation for 2 h in the same medium. Thus ω-CgTX was found to have an activating action on calcium currents of GH3 cells at the initial moment after application of the toxin. The absence of a marked blocking action of ω-CgTX on the calcium currents of the test cells confirms the high tissue specificity of action of the toxin as a blocker of high-threshold calcium channels in the nerve cell membrane.
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