Reduced Protein Kinase C Activity in Ischemic Spinal Cord

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Abstract
Protein phosphorylation was evaluated in a rabbit spinal cord ischemia model under conditions where cyclic AMP‐dependent protein kinase (PK‐A) and calcium/phos‐pholipid‐dependent protein kinase (PK‐C) were activated. One hour of ischemia did not affect PK‐A activity significantly; however, PK‐C activity was reduced by more than 60%. In vitro phosphorylation of endogenous proteins by endogenous PK‐C revealed that eight particulate and five cytosolic proteins showed stimulated phosphorylation by PK‐C activators in control tissue, although this stimulation was virtually absent in ischemic samples. When control and ischemic particulate fractions were combined, the endogenous protein phosphorylation pattern under PK‐C‐activating conditions was similar to the ischemic sample, which suggests that inhibitory molecules may be present in the ischemic particulate fraction. In vitro phosphorylation of endogenous proteins under PK‐A‐activating conditions in ischemic tissue was similar to that in control tissue. The results suggest that the PK‐C phosphorylation system is selectively impaired in ischemic spinal cord. In addition to reduced PK‐C‐dependent phosphorylation, an Mr 64,000 protein was phosphorylated in ischemic cytosolic samples, but not in control samples. The phosphorylation of the Mr 64,000 protein was neither PK‐C‐dependent nor PK‐A‐dependent. These altered phosphorylation reactions may play critical roles in neuronal death during the course of ischemia.