Temperature effects during and after the diethylnitrosamine treatment on liver tumorigenesis in the fish, Oryzias latipes

Abstract

Four experimental groups of the medaka ( Oryzias latipes) were treated with diethylnitrosamine (DENA) at the concentration of 100 ppm for 8 weeks. Two of them were kept at a high temperature ( 22 ± 2°C) and others at a low temperature ( 5–6°C). After the DENA treatment, one of the high temperature groups was kept again at a high temperature in normal tap water (H.H-group) and the other was transferred to a low temperature (H.L-group). In the similar way, L.H- and L.L-groups were made. Eight and twelve weeks after the beginning of the experiment, about 10 fish in each group were sacrificed for histological observation. Both in H.H- and H.L-groups, many tumor nodules were found in their livers. But the number and the size of tumor nodules were larger in H.H-group than in H.L-group. On the contrary, no tumor nodules were observed in L.H- and L.L-groups which were treated with DENA at a low temperature. From these results, an appropriate high temperature is considered to be necessary in the formation and the growth of tumor nodules.