Roles of Ca²⁺ influx through ATP-activated channels in catecholamine release from pheochromocytoma PC12 cells

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.