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Enhancement by methionine- and leucine-enkephalin of gastric carcinogenesis induced by N-methyl-N'-nitro-N-nitrosoguanidine in Wistar rats.

The effects of the opioid receptor agonists methionine-enkephalin (Met-ENK) and leucine-enkephalin (Leu-ENK) on gastric carcinogenesis induced by N-methyl-N'-nitro-N-nitrosoguanidine were investigated in Wistar rats. After 25 weeks of oral treatment with the carcinogen, the rats received subcutaneous injections of Met-ENK (20 micrograms/kg) or Leu-ENK (20 micrograms/kg) once every 2 days. The prolonged administrations of Met-ENK and Leu-ENK significantly increased the incidence of gastric cancers in week 52. Treatments with these opioid receptor agonists significantly increased the labeling index of the antral mucosa. These findings indicate that opioids enhance gastric carcinogenesis and suggest that their effects may be related to their influence on increasing proliferation of the antral epithelial cells.