Diadenosine pentaphosphate increases levels of intracellular calcium in astrocytes by a mechanism involving release from caffeine/ryanodine- and IP₃-sensitive stores

Diadenosine polyphosphates (Ap(n)As, n = 2 to 6 phosphate groups) activate P2-type cell-surface adenine nucleotide purinoreceptors, increase the influx of calcium into neural cells, and modulate the binding of ryanodine to ryanodine receptor-regulated intracellular calcium release channels. In this study, we tested the hypothesis, using single cell fluorescence techniques and cultured human fetal astrocytes, that P¹, P⁵- di(adenosine-5') pentaphosphate (Ap₅A)-induced increases in levels of intracellular calcium ([Ca²⁺](i)) resulted from release of calcium from intracellular pools. Basal [Ca²⁺](i) were 141 ± 12 nM anct Ap₅A increased [Ca²⁺](i) to 980 ±150 nM. The effect of Ap₅A on [Ca²⁺](i) was mediated in part through activation of purinoceptors and influx of extracellular calcium because the purinoceptor antagonist pyridoxal-phosphate-6-azophenel- 2', 4'-disuphonic acid blocked by 52%, and chelation of extracellular calcium with EGTA prevented, almost completely, Ap₅A-induced increases in [Ca²⁺](i). Implicating calcium release from IP₃- and ryanodine-regulated pools of intracellular calcium were findings that Ap₅A-induced increases in [Ca²⁺](i) were blocked, at least in part, by thapsigargin, ryanodine, caffeine, and xestospongin, and Ap₅A increased by 2-fold the production of IP₃. Release of calcium from IP₃- and ryanodine-regulated intracellular pools may be an important signaling event in neural cells that are exposed to Ap₅A.