Alteration of Cyclic Adenosine Monophosphate Binding in Ischemic Brain: Sensitive Metabolic Marker for Early Ischemic Tissue Damage

Abstract

Binding of cyclic adenosine monophosphate (cAMP) to the regulatory subunit of cAMP-dependent protein kinase (PKA) is an essential step for cAMP-mediated signal transduction in neuronal cells. Various neuroprotective effects of activated PKA have recently been demonstrated. In the acute phase of cerebral ischemia, the cAMP content of the brain tissue has been found to increase markedly. In spite of these facts, the alterations in the binding capacity of PKA with cAMP have not yet been examined in cerebral ischemia. We, therefore, undertook an autoradiographic study of the binding capacity of PKA and the local cerebral blood flow (lCBF) during cerebra] ischemia using [3H]cAMP and [14C] iodoantipyrine, respectively. After occlusion of the common carotid artery in the gerbil, a significant reduction in cAMP binding began to manifest itself in the dendritic subfields in the hippocampus CA1 at 15 min, then proceeded centrally to the pyramidal cell bodies. In contrast, other brain regions did not reveal any significant changes in cAMP binding until 30 min of ischemia. Between 30 min and 6 h of ischemia, the cAMP binding decreased progressively in various regions in response to a reduction in CBF. The ischemic CBF threshold for a reduction in cAMP binding in the hippocampus CA1 was significantly higher than that in other regions at each time point. The level of the above threshold increased progressively during the time course of ischemia. Westernblot analysis of PKA revealed that each subunit protein of PKA was preserved even at 6 h of ischemia. In focal ischemia, induced by occlusion of the middle cerebral artery in the rat, the reduction in cAMP binding in the cerebral cortex and striatum in the ischemic hemisphere at 5 h after occlusion was extensive and severe, but restricted to localized parts of the striatum and the temporal cerebral cortex at 3 h. However, each subunit protein of PKA was preserved in the ischemic tissue even after 5 h of ischemia. These data suggest that reductions in binding capacity of PKA with cAMP in the acute phase of cerebral ischemia may be caused by conformational changes of the PKA protein, and may precisely reflect the ischemic vulnerability of each brain region. The duration and degree of ischemia exert a definite influence on the cAMP binding, suggesting that the binding capacity of PKA with cAMP represents a good indicator of the viability of ischemic neuronal cells. The implementation of measures to maintain the binding ability of PKA with cAMP is expected to help cAMP to exert its neuroprotective effects in acute cerebral ischemia.