Metabolism of inositol phosphates in cerebral ischemia.

摘要

Cerebral ischemia resulting from reduction of blood flow in the brain is known to cause progressive tissue damage leading to delayed cell death. Among other causes, changes in receptor-mediated phosphoinositide (PI) signaling and metabolism of the second messenger, inositol 1,4,5-trisphosphate (Ins(1,4,5)P{dollar}sb3{dollar}), may play a crucial role in the pathophysiology of ischemia-induced tissue damage. Studies described here were directed toward the development of in vivo protocols to examine the PI signaling activity in brain and to analyze activities of enzymes for the metabolism of Ins(1,4,5)P{dollar}sb3.{dollar} These protocols then were used to study the effects of agonist stimulation, ethanol, and various models of cerebral ischemia on brain poly-phosphoinositide turnover. The levels of energy metabolites under these conditions as well as effects of extracellular ATP acting on the P{dollar}sb{lcub}rm 2U{rcub}{dollar} purinergic receptor in a neuroblastoma X glioma hybrid (NG108-15) cell line were also examined.; Global cerebral ischemia induced in mouse brain by decapitation resulted in time-dependent sequential appearances of Ins(1,4,5)P{dollar}sb3,{dollar} inositol 1,4-bisphosphate and inositol 4-monophosphate (Ins(4)P) but not inositol 1-monophosphate (Ins(1)P), indicating the preferred route of Ins(1,4,5)P{dollar}sb3{dollar} metabolism under energy deficiency. Pretreatment of mice with lithium resulted in large increases in levels of both Ins(1)P and Ins(4)P, indicating its effectiveness in blocking the inositol monophosphatase in brain. To examine events related to reperfusion injury, metabolism of inositol phosphates was examined in a rat focal cerebral ischemia model induced by ligation of the right middle cerebral artery. The levels of Ins(1,4,5)P{dollar}sb3{dollar} were reduced during the ischemic insult and the ability to recover from these changes after reperfusion was dependent on the duration of the ischemic insult. Among several brain regions, activities of Ins(1,4,5)P{dollar}sb3{dollar} 3-kinase and 5-phosphatase were found to be highest in hippocampus and cerebellum, respectively. Although the 5-phosphatase activity was not altered, a time-dependent and irreversible decrease in the 3-kinase activity was observed after both decapitation and focal cerebral ischemic insult. The irreversible alteration of the Ins(1,4,5)P{dollar}sb3{dollar} 3-kinase due to prolonged ischemic insult may lead to an altered metabolism of Ins(1,4,5)P{dollar}sb3{dollar} which, in turn, perturbs the calcium homeostasis in these cells.