ACTIVITY OF CA2+ CALMODULIN-DEPENDENT PROTEIN-KINASE-II FOLLOWING ISCHEMIA - A COMPARISON BETWEEN CA1 AND DENTATE GYRUS IN A HIPPOCAMPAL SLICE MODEL/

Both CA1 and dentate gyrus regions of the hippocampal slice exhibit an irreversible loss of synaptic transmission after exposure to in vitro ischemic conditions (buffer without oxygen and glucose). However, aft er shorter durations of ischemia (8-10 min) the CA1 region shows an ir reversible loss of synaptic responses, whereas the dentate gyrus regio n completely recovers synaptic responses upon reoxygenation. To determ ine biochemical mechanisms underlying this differential susceptibility , we have examined changes in Ca2+/calmodulin-dependent protein kinase II (CaM-KII) and cyclic AMP-dependent protein kinase activities in ho mogenates from CA1 and dentate gyrus regions of the hippocampal slice after increasing durations of in vitro ischemia. Time-dependent change s in CaM-KII activities were correlated with changes in electrophysiol ogical responses. CA1 homogenates from slices exposed to 1 min of isch emia showed significant increases in CaM-KII activity, whereas there w as no significant change in kinase activity in dentate homogenates aft er 1 min of ischemia. However, after longer durations of ischemia (5, 10, and 20 min) we found a time-dependent reduction in CaM-KII activit y in both CA1 and dentate gyrus regions, whereas no change was detecte d in cyclic AMP-dependent protein kinase activity. Irreversible depres sion of CaM-KII activity was seen at shorter durations of ischemia (10 min) in the CA1 region than in dentate region (20 min), which correla ted with irreversible effects on synaptic responses. Immunoblot analys is showed that the decrease in CaM-KII activity was not due to degrada tion of CaM-KII protein. However, the microtubule-associated protein M AP2, known to be a substrate for the Ca2+ -dependent proteases (calpai ns), showed extensive proteolysis evident after 90 min of reoxygenatio n after ischemia.