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We have previously demonstrated that intake of fat as well as carbohydrate affects the activity and immunoreactive amount of sucrase-isomaltase (S-I) in rat jejunum. To examine whether diet-related changes in sucrase and isomaltase activities are accompanied by the variations of sucrase-isomaltase mRNA levels, 7-week-old rats were fed either a high-long-chain triacylglycerols diet (73 energy% as corn oil), a high-medium-chain triacylglycerols (MCT) diet (66 energy% as MCT, 7 energy% as corn oil) or a high-carbohydrate diet (70 energy% as corn starch) for 7 days. Northern blot analysis revealed that S-I mRNA levels were abundant in the jejunum of rats fed the high-MCT diet; the levels were similar to those in the rats fed the high-carbohydrate diet. Force-feeding a high-sucrose diet (40 energy% as sucrose) brought about a parallel rise in both S-I mRNA and sodium/D-glucose cotransporter (SGLT1) mRNA levels within 12 h. Force-feeding the high-MCT diet also produced an elevation of S-I mRNA and SGLT1 mRNA. However, force-feeding a diet containing alpha-methylglucoside, a non-metabolizable but actively transported sugar, did not increase S-I mRNA or SGLT1 mRNA level; sucrase activity was nevertheless elevated by feeding alpha-methylglucoside diet. These results suggest that not only carbohydrate intake but also MCT intake might influence S-I mRNA and SGLT1 mRNA levels in the jejunum, presumably through common metabolite(s) of carbohydrates and MCT, and that carbohydrate may play another role in enhancement of the sucrase activity through modulation of translation and/or posttranslational modifications of the sucrase-isomaltase complex.

Cellular retinol-binding protein II (CRBP II) is an abundant cytosolic protein of intestinal absorptive cells. In this study, we examined whether dietary fat modulates the expression of CRBP II in the small intestine. In the rats fed a diet rich in long-chain triacylglycerols (LCT), both CRBP II mRNA and CRBP II protein levels in the jejunum were more than two-fold greater than in the rats fed a low fat diet and a diet rich in medium-chain triacylglycerols (MCT). The mRNA abundance of a retinoid X receptor (RXR alpha), which is thought to interact with the cis-element located in the CRBP II promoter, was elevated in the jejunum of rats fed high-LCT and high-MCT diets as compared with that of animals fed a low-fat diet, but the levels of RXR alpha mRNA of the LCT diet group was similar to that of MCT diet group. These results suggest that the expression level of the CRBP II gene is not directly related to the RXR alpha expression, and that it might be modulated by long-chain fatty acids or their metabolites.