| Over the last few years, new scientific approaches have unraveled significant physiological roles for angiotensin in the brain. We asked the question if exogenous angiotensin and the subsequent activation of the AT2 receptor could create a transient ionic preconditioned neuron, which could potentially protect neurons from hypoxic injury? In an effort to answer this question we determined that 10 mM sodium azide (hypoxia) treatment for 5 min induces apoptosis in mouse primary cortical neuronal cultures. Further more, we showed that the NMDA receptor and the activation of caspase-3, was required for sodium azide-induced apoptosis. In an effort to confirm the initiation of a caspase-3 apoptotic cascade, we measured cytoplasmic cytochrome-c release and caspase-3 activation. Sodium azide was shown to induce both cytochrome-c release and caspase 3 activation. We further showed that angiotensin, through activation of the AT2 receptor, could protect these neuronal cultures from hypoxic injury. We also determined that the AT2 receptor could prevent the activation of caspase-3. This was the first time anyone has shown that angiotensin could modulate a known apoptotic pathway in neurons.; In order to determine the mechanism in which the AT2 receptor protected cortical neuronal cultures from sodium azide-induced apoptosis. We show that K+ channels are required for AT2 receptor neuroprotection and more specifically the actions of the delayed rectifier K+ channel. We suggest that the yielding ionic effect resulting from the activation of the delayed rectifier K+ channel, was a transiently ionic preconditioned neuron, which could temporarily withstand the ionic stress, generated during hypoxia. We confirmed this theory by showing that the neuroprotective actions of the AT2 receptor are lost when the Na+/Ca+2 exchanger or the Na+/K + ATPase were blocked. In summary, our findings suggest that angiotensin's protective effect against chemical hypoxia results, at least in part, from ionic pre-conditioning and appears to be coupled to activation of the delayed rectifier K+ channel. However the Na+/Ca +2 exchanger and the Na+/K+ ATPase may also be directly modulated by angiotensin. Further understanding the role of ionic regulation during and after ischemia, and the potential actions of angiotensin, should provide insights into new therapeutic strategies for the treatment and/or the prevention of stroke injury. |
