Ischemia of Rat Brain Decreases Pertussis Toxin-Catalyzed [32P]ADP Ribosylation of GTP-Binding Proteins (Gi1 and G0) in Membranes

As an approach to understanding the molecular basis of the pathophysiology of cerebral ischemia, we examined qualitative and quantitative changes in pertussis toxin substrates, G i1 and G 0 , in the membrane of rat cerebral cortex after decapitation. Within 1 min after decapitation, the extent of pertussis toxin-catalyzed [ 32 P]ADP ribosylation of the G proteins in the cerebral cortex membrane was significantly decreased and the magnitude of the decrease became slightly larger upon further incubation of the decapitated brain. Addition of guanine nucleotides, GTP and GDP, or the purified βγ subunits of transducin to the membranes of control and ischemic cerebral cortex stimulated [ 32 P]ADP ribosylation of the G proteins. The stimulation of [ 32 P]ADP ribosylation in the control situation by guanine nucleotides was almost to the same extent as that in ischemia. However, the stimulation by transducin βγ subunits was different; the control stimulation was greater than that in ischemia. In immunoblots probed with antibodies against G i1α G 0α and T β , the immunoreactivity of the corresponding proteins in ischemia was similar to that in control, suggesting that the amounts of G proteins were not changed in ischemia. These results suggest that ischemia accelerates the dissociation of α–GDP–βγ to α–GDP and free βγ and causes the denaturation of the dissociated α–GDP, thereby decreasing [ 32 P]ADP ribosylation.