EFFECTS OF ATP AND UTP IN PHEOCHROMOCYTOMA PC12 CELLS - EVIDENCE FOR THE PRESENCE OF 3 P-2 RECEPTORS, ONLY ONE OF WHICH SUBSERVES STIMULATION OF NOREPINEPHRINE RELEASE

1. In pheochromocytoma PC12 cells ATP and, to a lesser extent, 2-methylthioATP stimulate phosphoinositide breakdown, release of intracellular calcium, and influx of external calcium, leading to stimulation of norepinephrine release. In contrast, although UTP also stimulates phosphoinositide breakdown, release of intracellular calcium, and influx of external calcium, there is no stimulation of norepinephrine release. 2. 2-MethylthioATP, presumably acting at P-2y receptors, and UTP, presumably acting at P-2u receptors, in combination elicit a phosphoinositide breakdown greater than that elicited by either alone. Intracellular levels of calcium measured with Fura-2 increase to greater levels with ATP than with UTP and are sustained, while the UTP intracellular levels of calcium rapidly return to basal values. Both ATP and UTP cause a similar influx of (45) Ca2+ presumably by stimulation of a P-2 receptor directly linked to a cation channel. 3. It is proposed that PC12 cells contain two distinct G protein-coupled P-2 receptors that activate phospholipase C and a P-2 receptor linked to a cation channel. The P-2y receptor sensitive to ATP (and to 2-methylthioATP) causes the depletion of a pool of intracellular calcium, sufficient to activate so-called ''receptor-operated calcium entry.'' The sustained elevation of intracellular calcium after ATP treatment is proposed to result in stimulation of norepinephrine release and activation of calcium-dependent potassium channels and sodium-calcium exchange pathways. 4. The P-2u receptor sensitive to UTP (and to ATP) causes only a transient elevation in levels of intracellular calcium, perhaps from a different pool, insufficient to activate so-called receptor-operated calcium entry. Further sequelae do not ensue, and the functional role of the UTP-sensitive P-2u receptor is unknown.