Cerebral ischemia and trauma lead to rapid increases in cerebral concentrations of cyclic AMP and dehydroascorbic acid (DHAA; oxidized vitamin C), depletion of intracellular ascorbic acid (AA; reduced vitamin C), and formation of reactive astrocytes. We investigated astrocytic transport of AA and DHAA and the effects of cyclic AMP on these transport systems. Primary cultures of astrocytes accumulated millimolar concentrations of intracellular AA when incubated in medium containing either M or DHAA. AA uptake was Nat-dependent and inhibited by 4,4 '-diisothiocyanostilbene-2,2 '-disulfonic acid (DIDS), whereas DHAA uptake was Na~-independent and DIDS-insensitive. DHAA uptake was inhibited by cytochalasin B, D-glucose, and glucose analogues specific for facilitative hexose transporters. Once inside the cells, DHM was reduced to AA. DHAA reduction greatly decreased astrocytic glutathione concentration. However, experiments with astrocytes that had been previously depleted of glutathione showed that DHM reduction does not require physiological concentrations of glutathione. Astrocyte cultures were treated with a permeant analogue of cyclic AMP or forskolin, an activator of adenylyl cyclase, to induce cellular differentiation and thus provide in vitro models of reactive astrocytes. Cyclic AMP stimulated uptake of M, DHAA, and 2-deoxyglucose. The effects of cyclic AMP required at least 12 h and were inhibited by cycloheximide, consistent with a requirement for de novo protein synthesis. Uptake and reduction of DHM by astrocytes may be a recycling pathway that contributes to brain M homeostasis. These results also indicate a role for cyclic AMP in accelerating the clearance and detoxification of DHAA in the brain.

Skeletal development and bone remodeling depend on the coordinated activity of osteoblasts and osteoclasts, which are responsible for bone formation and resorption, respectively. Mature osteoclasts result from the fusion of precursor cells, and they are large, multinucleated, highly specialized cells. Cellular release of ATP and UTP occurs in response to a variety of stimuli including mechanical stimulation, which occurs in the bone environment. ATP and UTP or their metabolites can then act on P2 receptors in the plasma membrane to induce various responses in bone cells. The influence of these receptors on osteoclast physiology and bone physiology in general is beginning to be understood, but much work is still required. This review focuses on P2 receptors in osteoclasts, their expression, signaling and function in the regulation of osteoclast formation, resorptive activity and survival.