Inhibition of cyclic AMP-dependent protein kinase in the acute phase of focal cerebral ischemia in the rat

Binding of cyclic AMP to the regulatory subunit of cyclic AMP-dependent pro tein kinase is an essential step in cyclic AMP-mediated intracellular signa l transduction. In the present study, the binding capacity of cyclic AMP-de pendent protein kinase for cyclic AMP was examined by autoradiography with local cerebral blood flow in focal cerebral ischemia in the rat, which was induced by occlusion of the middle cerebral artery using the intraluminal s uture method. The binding capacity of cyclic AMP-dependent protein kinase a nd local cerebral blood flow were assessed by the in vitro [H-3]cyclic AMP binding and the [C-14]iodoantipyrine methods, respectively. At 3 h of occlu sion, a significant reduction in the binding of cyclic AMP-dependent protei n kinase to cyclic AMP was already noted in the lateral region of the cauda te-putamen and the parietal cortex. Between three and five hours of occlusi on, the area with reduced cyclic AMP binding was significantly expanded to the peri-ischemic regions including the frontal cortex and the medial regio n of the caudate-putamen. The threshold in local cerebral blood flow for re duced cyclic AMP binding was clearly noted at 5 h of ischemia, and was 45 m l/100 g per min in the cerebral cortices, and 38 ml/100 g per min in the ca udate-putamen, respectively. No threshold was noted at 3 h of ischemia, sin ce cyclic AMP binding showed a large variation ranging from reduced to norm al values even when local cerebral blood flow was below 20 ml/100 g per min . Recirculation for 3.5 h following 1.5 h of ischemia restored the normal c yclic AMP binding in the cerebral cortices, but failed to normalize cyclic AMP binding in the caudate-putamen despite good recovery of local cerebral blood flow. Western blot analysis suggested that this reduction in cyclic A MP binding was not due to loss or degradation of the subunit protein of cyc lic AMP-dependent protein kinase, and may therefore have resulted from conf ormational changes in the protein. A significant increase in cyclic AMP bin ding was noted after recirculation in the non-ischemic regions such as the frontal and the cingulate cortices on the occluded side and in the contrala teral cortices. These data indicate that cyclic AMP-mediated signal transduction in the bra in tissue may be very susceptible to ischemic stress, and the region of dis rupted signal transduction may expand progressively from the ischemic core to peri-ischemic regions in the acute phase of ischemia. Such impairment of signal transduction may not be restored in the caudate-putamen even when c erebral circulation is fully recovered after short-term ischemia, suggestin g that a regional vulnerability to ischemic stress may also exist in cyclic AMP-mediated signal transduction. A significant increase in cyclic AMP bin ding after recirculation in regions outside of ischemic area may be closely related with the protective mechanisms of brain tissue, since cyclic AMP h as been reported to exert various neuroprotective actions. (C) 1999 IBRO. P ublished by Elsevier Science Ltd.