Abstract
Effects of ATP on the intracellular free calcium ion concentration ([Ca2+]i) in the rabbit eye suprachoroid were investigated by means of fura-2 microfluorophotometry. ATP application (10 to 100 µM) elicited a dose-dependent biphasic [Ca2+］i-increase: a fast phase typically peaking within
30 s and a following slow plateau phase, which lasted during the presence of ATP. The slow plateau phase was markedly diminished by removal of extracellular Ca2+, whereas the fast phase remained. An inhibitor of Ca2+ release from the sarcoplasmic reticulum (TMB-8), an endoplasmic Ca2+-ATPase inhibitor (thapsigargin) and a phospholipase C (PLC) inhibitor (U-73122) diminished the fast phase.
A P2 receptor antagonist (Suramin) inhibited the ATP-induced [Ca2+］i-response. The potency order of ATP and related substances in producing the [Ca2+］i-elevation was UTP≅ATP>ATP-γ-S>ITP>ADP. β,γ-MethyleneATP, 2-methylthioATP and UDP evoked no response. This order is consistent with the P2Y2 receptor characteristics.
Cross-desensitization between ATP and UTP excludes the co-existence of the other types of receptors. In conclusion, the ATP-induced [Ca2+］i-elevation in the rabbit eye suprachoroid was elicited by the Ca2+ release from the PLC-dependent, thapsigargin-sensitive intracellular Ca2+ storage sites by activating P2Y2 nucleotide receptors.
