Abundance of intestinal Na+/glucose cotransporter (SGLT1) in roe deer (Capreolus capreolus)

Introduction Based on morphological and physiological differences in their digestive systems H ofmann and S tewart (1972) classified ruminants into three feeding types: concentrate selectors (CS), grass and roughage eaters (GR) and intermediate, opportunistic, mixed feeders (IM). In evolutionary terms CS most closely resemble the original ruminants (H ofmann 1989). They have a relatively small forestomach with large openings between the different parts of the stomach. The retention time of ingesta is short and therefore less suited to optimizing cellulose digestion. They have relatively large saliva glands, producing high volumes of saliva (F ickel et al. 1998), which may facilitate the passage of soluble nutrients down their highly developed ventricular groove. The CS ruminants choose easily digestible forage, especially dicotyledons, which are rich in soluble plant cell contents (PCC, T ixier et al. 1997; D uncan et al. 1998). With evolution, the forestomach of the ruminants became larger relative to body size, and also more subdivided. These evolutionary adaptations optimized the selective retention of feed particles and increased retention time, leading to improvement of cellulose digestion. Large GR are well adapted to digest plant cell walls (PCW), i.e. structural carbohydrates (cellulose); they consequently feed mainly on monocotyledons. Roe deer (Capreolus capreolus) are very small ruminants which select concentrated feed. As energy requirements increase proportional to metabolic body weight (K leiber 1961) roe deer require relatively more energy per unit body mass for maintenance compared to that required by large ruminants (V an S oest 1994; D uncan et al. 1998). Easily digestible forage is not sufficient to satisfy the energy requirements of roe deer. It is proposed therefore that these animals may have other methods of avoiding energy losses due to microbial fermentation, either bypassing nutrients down the ventricular groove (rumen bypass) or by ruminal escape of unfermented and/or partially fermented nutrients (H ofmann 1989). The relatively high content of polyunsaturated fatty acids present in the depot fat of roe deer due to only partial rumen biohydrogenation (M eyer et al. 1998) supports the alternative hypothesis of rumen bypass or ruminal escape. Once reaching the small intestine, partially fermented or bypassed soluble carbohydrates can be further hydrolysed by maltase and sucrase to monosaccharides. These monosaccharides are then available for absorption. The intestinal sodium-dependent glucose cotransporter (SGLT1) in the brush border membrane (BBM) of the enterocytes is a stable indicator for the presence of sugars in the small intestine. The expression and activity of SGLT1 protein are regulated by the level of sugars reaching the small intestine (F erraris and D iamond 1989). Infusion of d-glucose into the intestinal lumen of ruminant sheep for 2 h stimulates the expression of SGLT1 which can be detected 4 days later in the newly formed mature enterocytes (S hirazi-B eechey et al. 1995). When dietary carbohydrate is removed, it takes 3 days before the brush border glucose uptake capacity decreases significantly from control values (F erraris 1994). This reflects the time of physiological cell renewal. In GR, such as sheep, the capacity of the intestine to absorb sugars decreases up to 500-fold during development (S hirazi-B eechey et al. 1995). This decrease in the intestinal absorptive capacity is a reflection of the decline in SGLT1 transport activity and protein abundance. It is not due to modifications in the structure of the absorptive surface and cannot be explained in terms of an age-related process (S hirazi-B eechey et al. 1991). It has been shown that there is a direct correlation between the level of monosaccharides reaching the lumen of the small intestine and the expression of functional SGLT1 protein (D yer et al. 1997). Therefore in this study we aimed to investigate the presence of functional SGLT1 in the small intestine of roe deer as a stable indicator of the availability of glucose in the luminal content of the small intestine. The presence of SGLT1 protein in the small intestine will support the hypothesis of rumen bypass and/or ruminal escape of nutrients. Furthermore we measured the maltase activities in the BBM, as part of the carbohydrates reach the intestinal lumen as polysaccharides and have to be broken down to monosaccharides before absorption.