SELECTIVE TIME-DEPENDENT EFFECTS OF INSULIN ON BRAIN PHOSPHOINOSITIDE METABOLISM

The effect of insulin on phosphoinositide metabolism in the cerebral cortex was examined using P-32 as precursor. A maximal increase was detected as early as 15 s; phospholipid labeling declined after this initial peak but then increased to another maximum at 30 min. The levels of these phospholipids were unchanged at the earliest time examined, but at 30 min insulin caused an increase in the content of all phospholipids tested. In pulse-chase experiments, insulin stimulated. depletion of P-32-labeled phosphoinositides only at 15 s. On the other hand, insulin treatment caused a biphasic diacylglycerol (DAG) production. We conclude that in cerebral cortex, insulin has a dual mechanism of action on phosphoinositide metabolism. First, insulin causes a rapid but transient hydrolysis of phosphoinositides by a phospholipase C-dependent mechanism, followed by subsequent resynthesis; thereafter, insulin increases de novo phospholipid synthesis.