Stimulation of the P2Y Purinergic Receptor on Type 1 Astroglia Results in Inositol Phosphate Formation and Calcium Mobilization

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Abstract
Cultured astroglia express purinergic receptors that initiate phosphoinositide metabolism and calcium mobilization. Experiments were conducted to characterize the purinergic receptor subtype on type 1 astroglia responsible for stimulating these second‐messenger systems. Inositol phosphate (IP) accumulation and calcium mobilization were measured after stimulation with ATP or purinergic receptor subtype‐selective ATP analogues. ATP (10–5M) increased IP accumulation severalfold. Dose–effect assays monitoring astroglial IP accumulation revealed the order of potency that defines the P2Y receptor: 2‐methylthioadenosine 5′‐triphosphate > ATP > αβ‐methyleneadenosine 5′‐triphosphate > βγ‐methyleneadenosine 5′‐triphosphate. The influence of ATP on intracellular calcium levels in individual type 1 astroglia was examined using the calcium indicator dye, fura‐2. Dose–effect experiments indicated that ATP was equally potent for generating inositol phosphates and increasing cellular calcium. The most prevalent response (87% of total responses) to ATP consisted of a rapid increase in calcium to a peak level that was approximately five times greater than the prestimulation level. This peak was followed by a decline to a plateau level that was significantly above baseline. This plateau phase of the calcium increase was maintained for at least 5 min in the presence of ATP and was dependent on external calcium. Many (23%) astroglia exhibited spontaneous calcium oscillations whose frequency and magnitude increased after the addition of 10–5M ATP. Immunocytochemical staining indicated that the responses occurred in glial fibrillary acidic protein positive cells. We conclude that type 1 astroglia express the P2Y purinergic receptor which regulates IP production and calcium mobilization.

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