Mechanisms underlying ATP

Mechanisms underlying ATP-induced Ca²⁺Oscillations
	The role of extracellular Ca²⁺ ATP induced Ca²⁺ oscillations in epithelial cells in the absence of extracellular Ca²⁺ (top left). However, these oscillations would run-down following prolonged expose of the cells to Ca²⁺-free solutions. The washing of the cells with the Ca²⁺-free solutions alone did not induce the oscillations (bottom left). These results indicate that internal stores of Ca²⁺ are primarily used to generate Ca²⁺ oscillations.
The role of phospholipase C (PLC) The addition of the PLC inhibitor, U73122 (10 uM) inhibited the ATP-induced Ca²⁺ oscillations (right, top). By contrast, the inactive sister compound, U73343, had no effect on the oscillations. These results suggest that continued PLC activity and the production of IP₃ is is required for the maintenance of ATP-induced Ca²⁺ oscillations.
	The role of internal Ca²⁺ stores The addition of thapsigargin (1 uM), an inhibitor of the SERCA pumps of the endoplasmic reticulum, inhibited ATP-induced Ca²⁺ oscillations (top left). As the Ca²⁺ oscillations were inhibited, the basal Ca²⁺ level became elevated, a result consistent with the gradual emptying of Ca²⁺ stores. DMSO, the solvent for thapsigargin, had no effect on the oscillations (bottom left). These results indicate that internal Ca²⁺ stores need to be replenished to maintain Ca²⁺ oscillations.
Conclusions These results indicate that ATP-induced Ca²⁺ oscillations require the repetitive filling and release of Ca²⁺ from intracellular Ca²⁺ stores that involves IP₃ and its receptor.