Binding of
cyclic AMP to the regulatory subunit of
cyclic AMP-dependent protein kinase is an essential step in
cyclic AMP-mediated intracellular signal transduction. In the present study, the binding capacity of
cyclic AMP-dependent 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
suture method. The binding capacity of
cyclic AMP-dependent protein kinase and local cerebral blood flow were assessed by the in vitro [3H]
cyclic AMP binding and the [14C]
iodoantipyrine methods, respectively. At 3 h of occlusion, a significant reduction in the binding of
cyclic AMP-dependent protein kinase to
cyclic AMP was already noted in the lateral region of the caudate-putamen and the parietal cortex. Between three and five hours of occlusion, the area with reduced
cyclic AMP binding was significantly expanded to the peri-ischemic regions including the frontal cortex and the medial region of the caudate-putamen. The threshold in local cerebral blood flow for reduced
cyclic AMP binding was clearly noted at 5 h of
ischemia, and was 45 ml/100 g per min in the cerebral cortices, and 38 ml/100 g per min in the caudate-putamen, respectively. No threshold was noted at 3 h of
ischemia, since
cyclic AMP binding showed a large variation ranging from reduced to normal 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
cyclic 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 AMP binding was not due to loss or degradation of the
subunit protein of
cyclic AMP-dependent protein kinase, and may therefore have resulted from conformational changes in the
protein. A significant increase in
cyclic AMP binding was noted after recirculation in the non-ischemic regions such as the frontal and the cingulate cortices on the occluded side and in the contralateral cortices. These data indicate that
cyclic AMP-mediated signal transduction in the brain tissue may be very susceptible to ischemic stress, and the region of disrupted 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 cerebral circulation is fully recovered after short-term
ischemia, suggesting that a regional vulnerability to ischemic stress may also exist in
cyclic AMP-mediated signal transduction. A significant increase in
cyclic AMP binding after recirculation in regions outside of ischemic area may be closely related with the protective mechanisms of brain tissue, since
cyclic AMP has been reported to exert various neuroprotective actions.