Hypoxia stimulates Ca2+ release from intracellular stores in astrocytes via cyclic ADP ribose-mediated activation of ryanodine receptors

The ability of O2 levels to regulate Ca2+ signalling in non-excitable cells is poorly understood, yet crucial to our understanding of Ca2+-dependent cell functions in physiological and pathological situations. Here, we demonstrate that hypoxia mobilizes Ca2+ from an intracellular pool in primary cultures of cortical astrocytes. This pool can also be mobilized by bradykinin, which acts via phospholipase C and inositol trisphosphate production. By contrast, hypoxic Ca2+ mobilization utilizes ryanodine receptors, which appear to be either present on the same intracellular pool, or on a separate but functionally coupled pool. Hypoxic activation of ryanodine receptors requires formation of cyclic ADP ribose, since hypoxic Ca2+ mobilization was fully prevented by nicotinamide (which inhibits ADP ribosyl cyclase) or by 8-Br-cADP ribose, an antagonist of cyclic ADP ribose. Our results demonstrate for the first time the involvement of cyclic ADP ribose in hypoxic modulation of Ca2+ signalling in the central nervous system, and suggest that this modulator of ryanodine receptors may play a key role in the function of astrocytes under conditions of fluctuating O2 levels.