ROLES OF CA-2+ INFLUX THROUGH ATP-ACTIVATED CHANNELS IN CATECHOLAMINE RELEASE FROM PHEOCHROMOCYTOMA PC12 CELLS

1. 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. 2. Norepinephrine or dopamine release induced by greater-than-or-equal-to 100-muM concentrations of ATP was insensitive to 300 muM Cd2+, whereas the release induced by increasing extracellular KCl (50-150 mM) was completely blocked by this concentration of Cd2+. 3. ATP (100 muM) increased the intracellular free Ca2+ concentration measured with fura-2. The increase was not affected by 300 muM Cd2+ or 100 muM nicardipine, suggesting that Ca2+ influx through ATP-activated channels but not through voltage-gated Ca2+ channels contributes to the ATP-evoked catecholamine release. 4. Inward currents permeating through voltage-gated Ca2+ channels were measured using the whole-cell voltage clamp. In the presence of 10 muM ATP, a concentration that induces an ATP-activated channel-mediated current equivalent to that induced by 100 muM 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+. 5. The ATP (100 muM)-evoked dopamine release was inhibited by 300 muM 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+. 6. 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.