External ATP induces [3H]dopamine ([3H]DA) release in rat pheochromocytoma cells (PC-12 cells). The ATP-induced release is a saturable process with half-effective concentration of EC50 = 80 μM. ADP is a poor secretagogue of [3H]DA (one-sixth of ATP) and AMP is devoid of secretory capabilities. Adenosine and the non-hydrolyzable analogues of ATP, AppNHp and AppCp are ineffective as inducers of [3H]DA, release, or as inhibitors of the ATP-induced [3H]DA release. The most potent antagonist of ATP-induced release is Coomassie Blue (IC50 = 25 MM), compared to ADPβS (IC50 = 500 μM). The overall rank order of potency is ATP > ADP ≫ AMP > adenosine, which is characteristic of the P2-purinergic receptor. ATP-induced secretion is absolutely Ca2+ dependent, indicating an exocytotic process and is independent of Mg2+ (up to 2 mM) suggesting that the active species is not ATP4-. (a) The ATP-induced 45Ca2+ influx into the cells is in good correlation to ATP induction of release (IC50 = 80 and 90 μM, respectively) and is carried over to ADP which has a diminished ability to induce both release and 45Ca2+ influx. (b) Divalent cations (Ba2+ > Sr2+ > Ln3+ > Mn2+) replace Ca2+ and support ATP-induced release similar to their effectiveness in supporting bradykinin- and K+ (50 mM)-induced release in PC-12 cells (Weiss, C., Sela, D., and Atlas, D. (1990) Neurosci. Lett. 119, 241-245). Combined together the absolute requirement of [Ca2+]ex for release, inhibition of release by Gd3+ (IC50 = 100 μM), Ni2+, and Co2+ (IC50 = 1 mM), and support of release by Ba2+, Sr2+, and Mn2+, we suggest that ATP induces Ca2+ entry via ligand-operated Ca2+ channels as previously suggested for ATP in smooth muscle cells (Benham, C. D., Bolton, T. B., Byren, N. G., and Large, W. A. (1987) J. Physiol. (Lond.) 387, 473-488). No significant inhibition by 1 μM verapamil, 10 μM nifedipine, or 2 mM Cd2+ argues against ATP activation of voltage-dependent Ca2+ channels as similarly shown for ATP-induced [3H]noradrenaline release (Inoue, K., Nakazawa, K., Fujimoro, K., and Takanaka, A. (1989) Neurosci. Lett. 106, 294-299). Thus, the widely distributed ATP receptor might play an essential role in Ca2+ homeostasis of the cell by introducing Ca2+ into the cell via specific ligand-gated Ca2+ channels.
|Original language||American English|
|Number of pages||5|
|Journal||Journal of Biological Chemistry|
|State||Published - 25 Sep 1991|