Effect of Brain Ischemia on Protein Kinase C

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Abstract
We examined the influence of brain ischemia on the activity and subcellular distribution of protein kinase C (PKC). Two different models of ischemic brain injury were used: postdecapitative ischemia in rat forebrain and transient (6‐min) cerebral ischemia in gerbil hippocampus. In the rat forebrain model, at 5 and 15 min postdecapitation there was a steady decrease of total PKC activity to 60% of control values. This decrease occurred without changes in the proportion of the particulate to the soluble enzyme pools. Isolated rat brain membranes also exhibited a concomitant decrease of [3H]phorbol 12,13‐dibutyrate ([3H]PDBu) binding with an apparent increase of the ligand affinity to the postischemic membranes. On the other hand, the ischemic gerbil hippocampus model displayed a 40% decrease of total PKC activity, which was accompanied by a relative increase of PKC activity in its membrane‐bound form. This resulted in an increase in the membrane/total activity ratio, indicating a possible enzyme translocation from cytosol to the membranes after ischemia. Moreover, after 1 day of recovery, a statistically significant enhancement of membrane‐bound PKC activity resulted in a further increase of its relative activity up to 162% of control values. In vitro experiments using a synaptoneurosomal particulate fraction were performed to clarify the mechanism of the rapid PKC inhibition observed in cerebral tissue after ischemia. These experiments showed a progressive, Ca2+‐dependent, antiprotease‐insensitive down‐regulation of PKC during incubation. This down‐regulation was significantly enhanced by prior phorbol (PDBu) treatment. It can be postulated that in the ischemic gerbil hippocampus model as well as during phorbol stimulation of the synaptoneurosomal fraction in vitro, the common phenomenon was an initial PKC activation (translocation) followed by a subsequent enhancement of Ca2+‐dependent enzyme inhibition. Experimental data suggest that the inhibition is localized in the membrane compartment.