To evaluate the role of NK cells during gastric carcinogenesis, especially in early stage of tumorigenesis, the changes of NK activity was examined. Rats were given N-methylnitrosourea (MNU) at a concentration of 100 ppm in their drinking water for 15 weeks. Rats were sacrificed sequentially on week 15, 18, 20, and 40 of the experimental period. Histological changes such as mild erosion, regenerative changes, focal or severe atypical lesions and invasive adenocarcinoma were observed sequentially in the pyloric region. Adenomatous hyperplasia was induced in majority of the rat stomach in MNU-treated group and incidence of adenocarcinoma was 20% in 40 weeks of MNU-treated group. There was no difference in NK activity until week 20, however, it was increased in MNU-treated rats at 40 week, when compared to that of untreated control group. From week 15, the ratio of pepsinogen altered pyloric gland (PAPG) between untreated control and MNU-treated rats was progressively increased, but there was no significant increment in the number of PAPG in MNU-treated rats after 20 weeks. NK activity was increased in MNU-treated rats, when compared to that of untreated control group. These results suggest that PAPG is a relatively good marker for the evaluation of progression of gastric carcinogenesis and increased NK activity is shown, especially in early stage of gastric carcinogenesis.

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