Polyamine metabolism in reversible cerebral ischemia.

Synthesis of the polyamines putrescine, spermidine, and spermine is controlled by the activity of the key enzymes ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (SAMDC). Beside their function in cellular growth processes, polyamines and particularly putrescine play a role in calcium-related events at the cell membrane, coupling an extracellular stimulus to an intracellular response (second messenger-like reactions), modulate the calcium-buffering capacity of mitochondria (spermine), and, if present in the extracellular compartment, modulate the activity of the N-methyl-D-aspartate receptor (spermidine and spermine). Reversible cerebral ischemia triggers pathological disturbances in polyamine metabolism that are characterized by a sharp increase in ODC synthesis, even in the most vulnerable hippocampal CA1 subfield in which overall protein synthesis is severely depressed at the same time, and a marked suppression of SAMDC synthesis in parallel with the inhibition of overall protein synthesis. ODC immunohistochemistry has revealed that the observed changes are neuronal responses to reversible ischemia. These changes in enzyme activities result in an overshoot in the formation of putrescine, the product of ODC activity. Spermine levels are significantly reduced in vulnerable brain structures after prolonged recirculation. In addition, evidence is accumulating that polyamines may be released from the cell during ischemia and after prolonged recirculation at a time when cell necrosis is apparent. This review will summarize the major features of ischemia-induced disturbances in polyamine metabolism and the possible consequences for the cells involved, taking into account that the underlying changes may be indicative of either the activation of a recovery process of neurons from the metabolic stress produced by reversible ischemia or pathological disturbances resulting in the manifestation of neuronal necrosis. Elucidating the mechanisms responsible for the postischemic disturbances in polyamine metabolism may lead to a better understanding of the molecular mechanisms involved in the development of neuronal necrosis after different pathological stimuli.