Elevated Serum Growth Hormone Accelerates Gastric Tumorigenesis in F344 Rats after Treatment with N‐Methyl‐N‐nitrosourea in Their Drinking Water

We examined the effects of growth hormone on tumorigenesis in F344 rats treated with N‐methyl‐N‐nitrosourea (MNU). Four‐week‐old male F344 rats were exposed to 100 ppm MNU in their drinking water for 15 weeks. Thereafter Group II animals received 100 μCi/100 g body weight of (131)I (radiothyroidectomy, Tx) injected i.p. and Group III rats were implanted with pituitary tumors (MtT) secreting growth hormone while Group I received no further treatment after MNU. Non‐carcinogen control animals received MtT, Tx or no treatment. Animals were killed at 39 weeks after starting MNU administration. Gastric tumors were present in 13 of 31 (43%), 15 of 32 (47%) and 17 of 32 (53%) rats in Groups I to III, respectively. All tumors were of well‐differentiated type. Spinal cord tumors appeared in 15 of 31 (47%) in Group I, 10 of 32 (32%) in Group II and 10 of 32 (32%) in Group III, most being malignant schwannomas. Thymic lymphornas also appeared in 10 of 31 (32%), 5 of 32 (16%) and 6 of 32 (19%) animals in Groups I to III, respectively. There were no significant differences among the groups. However, tumors in Group III developed significantly earlier than in Groups I or II. This was mainly due to gastric tumors, and cumulative incidence curves for spinal cord tumors or thymic lymphomas were similar in all groups. The results indicate that gastric tumors induced by MNU in F344 male rats are influenced by elevated levels of growth hormone.