Extracellular ATP evokes catecholamine release concomitant with depolarization in pheochromocytoma PC12 cells. Roles of Ca2+ influx through ATP-activated channels during the catecholamine release were investigated. Norepinephrine or dopamine release induced by ≥100-μM concentrations of ATP was insensitive to 300 μM Cd2+, whereas the release induced by increasing extracellular KCl (50-150 mM) was completely blocked by this concentration of Cd2+. ATP (100 μM) increased the intracellular free Ca2+ concentration measured with fura-2. The increase was not affected by 300 μM Cd2+ or 100 μM nicardipine, suggesting that Ca2+ influx through ATP-activated channels but not through voltage-gated Ca2+ channels contributes to the ATP-evoked catecholamine release. Inward currents permeating through voltage-gated Ca2+ channels were measured using the whole-cell voltage clamp. In the presence of 10 μM ATP, a concentration that induces an ATP-activated channel-mediated current equivalent to that induced by 100 μM ATP during the depolarization in 'non-voltage clamped' cells, the Ca2+ current activated by a voltage step to +10 mV was reduced. The reduction in the Ca2+ channel- mediated current was not observed when the extracellular Ca2+ was replaced with Ba2+. The ATP (100 μM)-evoked dopamine release was inhibited by 300 μM Cd2+ when measured with extracellular Ba2+ instead of Ca2+. This effect of Ba2+ may not be related to K+ channel-blocking activity, because the ATP-evoked dopamine release obtained with 5 mM tetraethylammonium (TEA) was not inhibited by Cd2+. The results suggest that Ca2+ influx through ATP-activated channels plays dual roles by serving as a Ca2+ source for the catecholamine release and, at the same time, inactivating the Ca2+ channels.
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