ATP receptor: A putative receptor-operated channel in PC-12 cells

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 ATP^4-. (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.