In order to compare the ability to utilize carbohydrate of carp Cyprinus carpio, red sea bream Chrysophrys major, and yellowtail Serbia quinqueradiata (which were reared on the diets with 0 %, 10%, and 40% dextrin for 30 days), they were subjected to a glucose tolerance test. The fishes showed a higher response of blood sugar and a lower response of insulin after the glucose admin istration than the responses of normal human beings. The glucose tolerance of yellowtail was lowest, followed by red sea bream, and carp in increasing order. The fishes exhibited a similar pattern of change in plasma insulin levels after the administration of glucose. However, the maxi mum level of plasma insulin and a ratio of the maximum plasma insulin level to the maximum blood sugar level (Max. IRI/Max. BS) was lowest in yellowtail, followed by red sea bream and carp in increasing order. Furthermore, in all species, the 40 % dextrin diet groups showed a higher response of blood sugar, a lower response of insulin, and a lower value of Max. IRI/Max. BS than those of the 0 % and 10 % groups.In the previous paper1), the optimum level of dietary dextrin for maximum growth and feed efficiency was highest in carp, followed by red sea bream and yellowtail in decreasing order, and these species showed comparably high percent absorp tion of dextrin regardless of dietary dextrin levels. In another experiment with red sea bream2), it was found that the elevation of blood sugar level following glucose administration was depressed by the injection of insulin. From these results, au thors supposed that the difference in the optimum level of dietary dextrin noticed between the fish species may be attributable to the difference in the carbohydrate utilizability resulting from the dif ference in the secretion of insulin after feeding. To confirm this presumption, the change in blood sugar and plasma insulin levels of.fishes which were subjected to the glucose tolerance test was exa mined in the present paper. Materialsand Methods

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In order to investigate the changes in activities of hepatic glycolytic and gluconeogenic en zymes after feeding and the responses of these enzymes to insulin injection , carp Cyprinus carpio, red sea bream Chrysophrys major, and yellowtail Serbia quinqueradiata were subjected to glucose tolerance and insulin-glucose tolerance tests after a 30-day feeding trial with diets containing 0% and 10% dextrin. Activities of glycolytic enzymes (hexokinase and PFK) and gluconeogenic enzymes (G6Pase and FDPase) increased after the administration of glucose in all species . The enzyme activities were intensified or restrained by the insulin injection. No significant differences between the fish species were recognized for hexokinase and G6Pase activities before or after the administration of glucose, nor for the response of the enzymes to the insulin injection. PFK ac tivity before and after the administration of glucose was lowest and its response to the insulin injection was highest in yellowtail, in which the optimum level of dietary dextrin was the lowest of the species tested. PFK of carp showed the highest activity and FDPase of carp showed the lowest activity before and after the administration of glucose.

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