Changes in brain calpain activity as a result of in vitro ischemia and pH alterations

Calpains and calpastatin in the brain of the rabbit were examined in experimental situations that could mimic some features of brain ischemia. Incubations of bisected brains in saline at 39 degrees C for 0.5, 1, or 1.5 h resulted in a decreased calpain I activity in the cytosol and in an increased hydrophobicity of cytosolic calpain II activity. Incubation of brain homogenates at different pH levels demonstrated an almost-complete transfer of calpains from the cytoplasmic compartment to the membranes when pH was lowered from 6 to 5. At pH values lower than 5, the total calpain activity (soluble plus membrane-bound) markedly decreased. No significant changes of calpastatin activity or its subcellular distribution was found following incubation of the homogenates at different pH levels.