Kinetics of GTP-modulation of adenylate cyclase in gerbil cerebral cortex after bilateral ischemia

Changes in the sensitivity of adenylate cyclase and steady-state levels of cyclic AMP (adenosine 3',5'-monophosphate) occur in mammalian brain during ischemic episodes. In our previous investigation with the gerbil model of bilateral ischemia there was an indication that ischemic conditions produced an enhancement of GTP sensitivity of adenylate cyclase within the cerebral cortex. The present study employed a kinetic analysis to evaluate further the role of this GTP modulation of adenylate cyclase in the gerbil frontal cortex during periods of bilateral ischemia and recirculation. In general, after either 15-min (with or without 15-min reflow) or 60-min ischemia the Vmax to GTP (alone or with dopamine and norepinephrine) was increased. Under these conditions the ED50 for half-maximal enzyme activation was decreased, indicating a greater affinity of the transducer site for GTP during ischemia. However, if irreversible 60-min ischemia was followed by 15-min reflow the enzyme responses to GTP were now absent. An unexpected observation showed that the ED50 for GTP activation of cortical adenylate cyclase was likewise attenuated when sham-operated animals were compared to normal gerbils.