Potentiation of the hypoglycemic effect of insulin by thiorphan, an enkephalinase inhibitor

Insulin administration to rats produced a dose-related hypoglycemia. When insulin and the enkephalinase (Enk'ase) inhibitor thiorphan (30 or 100 mg/kg s.c.) were co-administered, there was a potentiation of the hypoglycemic response to insulin; these doses of thiorphan alone had no significant effect on plasma glucose. When tested in vitro against isolated Enk'ase, both insulin and its beta-chain inhibited the catabolism of [Met5]enkephalin. Theoretically, thiorphan blocked the catabolism of insulin by inhibiting Enk'ase. Alternatively, thiorphan acted as an inhibitor of another insulin-catabolizing enzyme having similar substrate requirements as Enk'ase.