Purinergic regulation of cytosolic calcium and phosphoinositide metabolism in rat osteoblast‐like osteosarcoma cells

We have shown that ATP increases cytosolic Ca 2+ in UMR‐106 cells through P 2 ‐purinergic receptor stimulation (Calcif Tissue Int 45:251‐254). This response was further characterized using cells loaded with indo‐1/AM or prelabeled with [ 3 H]inositol. ATP elicited a rapid transient increase in Ca 2+ from 148 to 540 nM, followed by a biphasic decline (first rapid and then slower) to basal within 1 minute and then a late slow rise to 200 nM by 4 minutes. ADP also elicited a rapid transient increase, but this was followed by a second smaller transient and a later, slow increase above basal Ca 2+ . These transient increases in Ca 2+ induced by ATP and ADP were dose dependent, detected at 10 −6 M ATP and 10 −7 M ADP, and saturated at 10 −4 M with both nucleotides. The maximum increase in Ca 2+ was 20% greater with ATP than ADP. EGTA chelation of extracellular Ca 2+ abolished the biphasicity of the ATP‐induced Ca 2+ transient, the second ADP‐induced transient, and all late slower increases in Ca 2+ . Desmethoxyverapamil pretreatment attenuated the biphasicity of the ATP‐induced transient and the second peak elicited by ADP. Elevated extracellular Ca 2+ (5 mM) prevented the return to the basal level that normally follows the ATP‐induced Ca 2+ transient and amplified the sustained increase in Ca 2+ but had little effect on the response to ADP. IP 3 and IP 4 increased rapidly after addition of ATP, with I(1,4,5)P 3 increasing before I(1,3,4)P 3 . These data indicate that P 2 ‐purinergic stimulation of UMR‐106 cells causes three consecutive responses in cytosolic Ca 2+ : (1) a transient increase due to IP 3 ‐mediated mobilization of intracellular Ca 2+ ; (2) a transient increase due in part to influx, probably associated with a Ca 2+ channel; and (3) a later sustained increase that requires extracellular calcium.