Involvement of nitric oxide in the deregulation of cytosolic calcium in cerebellar neurons during combined glucose-oxygen deprivation

Nitric oxide (NO) has been proposed as a neuronal messenger molecule in hypoxic/ischemic cell injury (Nowicki et al., 1991; Trifiletti, 1992). We conducted studies in a model of combined glucose-oxygen deprivation using cultured rat cerebellar granule cells. Experiments were designed to test the hypothesis that sustained elevation of cytosolic calcium ([Ca²⁺]_i) and NO generation act in concert to trigger neuronal injury after anoxic insult. A hypoxic state was achieved by perfusing the cells with medium pre-equilibrated with argon gas. [Ca²⁺]_i was monitored using digital-imaging fluorescence microscopy in cells loaded with fura-2 AM. Under short-term hypoxic conditions, cells displayed a progressive and sustained, moderate increase of [Ca²⁺]_i, which returned to near basal levels on restoration of O₂-containing medium. Prolonged hypoxic conditions (>60 min) caused irreversible elevation of [Ca²⁺]_i followed by disruption of cell membrane integrity, as indicated by severe swelling, loss of regular cell shape and processes, leakage of dye fura-2, and propidium iodide uptake ("point of no return"). Pretreatment with N^G-nitro-l-arginine methyl ester (l-NAME, 100 μM), a specific NO synthase inhibitor, markedly delayed the onset of intensity of the rise of [Ca²⁺]_i. The hypoxia-induced elevation of [Ca²⁺]_i was also greatly attenuated if l-NAME (100 μM) was added to the argon-perfused medium before the cells demonstrated signs of irreversible injury. Prolonged or repeated hypoxic conditions, however, caused a rapid and intense increase of [Ca²⁺]_i, which could not be blocked by inhibition of NO synthase (NOS). In addition, reoxygenation after the "point of no return", as characterized above, greatly potentiated [Ca²⁺]_i overload and facilitated the process of cell injury. The potentiation and facilitation of cell damage, as demonstrated by rapid massive increase of [Ca²⁺]_i and subsequent cell death, was not blocked by NOS inhibitor, l-NAME.