INTRODUCTION

TOP ABSTRACT INTRODUCTION PATIENTS AND METHODS RESULTS DISCUSSION REFERENCES

THE HUMAN SMALL INTESTINE extends from the pylorus to the ileocecal valve and measures ~5 m (3.5-6.5 m) in length. It is divided into three regions: the duodenum occupies the first 25 cm, and the rest is divided between the jejunum (the proximal 40%) and the ileum (the distal 60%). The ligament of Trietz marks the junction between the duodenum and the jejunum, but there is no recognized line of division between the jejunum and the ileum. However, the tissues are different: as well as a diminution of outside diameter distally, the circular mucosal folds become less prominent, and the frequency and degree of aggregation of lymphatic follicles increase. The villi also differ, being large, broad, and leaf-shaped in the duodenum; tall, thin, and foliate in the jejunum; and short, broad, and finger-like in the ileum. The rates of epithelial cell renewal are different, being faster proximally, and it is likely that rates of protein turnover show a similar pattern, as observed in animals.

Recently there has been a considerable interest in human gastrointestinal protein metabolism (1, 4, 5, 9, 11), and work in animal preparations suggests that the small bowel contributes significantly to the whole body protein turnover despite its relatively small contribution to whole body protein mass. No data are available comparing the rates of mucosal protein synthesis between the various regions of the human small intestine, although we hypothesize that the rates of cell renewal and protein turnover will show a pattern similar to that in animals.

We previously developed a safe, reproducible, and reliable method to measure protein synthesis in the gastrointestinal mucosa using [¹³C]leucine (10) and have now used it to answer the question, is the rate of mucosal protein synthesis different in jejunum and ileum? To do this we took advantage of the fact that it is possible to gain access to the distal ileum in patients who had an ileostomy. The jejunum was accessed via the mouth using a Crosby-Kugler capsule.

	PATIENTS AND METHODS

TOP ABSTRACT INTRODUCTION PATIENTS AND METHODS RESULTS DISCUSSION REFERENCES

Patients. Six subjects with ileostomy, three men and three women aged 27-72 yr weighing 54-84 kg, were studied. Five of them had undergone total colectomy for ulcerative colitis between 5 and 15 yr previously; one patient had colonic inertia and still had her colon in situ, although she did have an ileostomy, which had been constructed 6 mo previously. All the patients studied were in good health; they were not receiving any nutritional supplements, including iron and vitamins, or medications, such as steroids or antibiotics. Results obtained from a group of eight normal volunteer subjects [44 ± 15 (SD) yr, 71 ± 12 kg, 171 ± 11 cm] and previously reported here (10) were used for comparison.

Methods. L-[1-¹³C]leucine (99 atoms %) was obtained from MassTrace (Woburn, MA). Immediately before administration, the tracer was dissolved in sterile, nonpyrogenic 0.9% NaCl solution (150 mmol/l, Baxter Healthcare, Thetford, Norfolk, UK) and was sterilized by passage through a 0.22-µm filter (Acrodisc-DLL, Pall Gelman Sciences, Port Washington, NY).

1

1

	RESULTS

TOP ABSTRACT INTRODUCTION PATIENTS AND METHODS RESULTS DISCUSSION REFERENCES

In all subjects studied, plasma leucine and KIC enrichment reached plateau values within 1 h of intravenous L-[1-¹³C]leucine infusion and remained so until biopsies were obtained. The rates of jejunal and ileal mucosal protein synthesis calculated on the basis of the intracellular free leucine labeling and the incorporation into mucosal proteins (Table 1) were 2.14 ± 0.20 and 1.20 ± 0.20 %/h, respectively, P < 0.001 (Fig. 1). Duodenal mucosal protein synthesis in the control subjects was 2.5 ± 0.25 %/h (Fig. 1). The ratios of protein to DNA (an indicator of cell size) and RNA per protein (an indicator of the capacity of protein synthesis per cell) were also significantly (P < 0.05) higher in the jejunum than the ileum, and for both measures the values tended to be lower than in the duodenum of normal subjects (Fig. 2).

View larger version (15K): [in this window] [in a new window]   Fig. 1.   Mucosal protein synthesis (Prot Synth) in jejunum (Jejun) and ileum from ileostomy patients and in duodenum (Duoden) from normal subjects. Values are means ± SE. P values show probabilities of significance of differences between values.

View larger version (24K): [in this window] [in a new window]   Fig. 2.   Composition of gut mucosa in ileostomy patients (jejunum and ileum) and in normal subjects (duodenum). Prot, protein. Values are means ± SE. * Significant differences at 5% level.

The plasma KIC ¹³C enrichment was higher in the ileostomy patients than in the previously reported normal subjects [5.32 ± 0.59 vs. 3.82 ± 0.53 atoms % excess (APE)]. This indicated a significantly lower (P < 0.01) rate of whole body protein breakdown, calculated as 145 ± 1.5 µmol · kg¹ · h¹ compared with 196 ± 4.4 µmol · kg¹ · h¹ in normal subjects (10).

	DISCUSSION

TOP ABSTRACT INTRODUCTION PATIENTS AND METHODS RESULTS DISCUSSION REFERENCES

This study shows that the fractional rate of mucosal protein synthesis is substantially higher in the jejunum than in the ileum in patients with ileostomies. The compositional results suggest that cells are bigger in the jejunum and contain more ribosomes per unit of protein (reflecting a high rate of protein synthesis; see discussion on pages 461-467 in Chapter 14 of Ref. 16), and these results match the known histological and cytological features of the mucosae in each tissue (17).

How physiologically representative are the results likely to be? How useful are they in constructing a general view of the rates of protein turnover along the human small intestine? The answers very much depend on how metabolically normal the patients were and to what extent their small intestinal physiology was intact. Histologically, their jejunal and ileal biopsies were normal, with no evidence of stunting or elongation of villi and no evidence of bacterial overgrowth. Furthermore, the patients reported not having made any major modifications to their diet as a result of the ileostomies. They were considered to be of normal nutritional status, although their body mass indexes were slightly less than those in the normal control subjects studied (22.7 vs. 24.3, P < 0.05). However, as a group they exhibited a marked reduction in the whole body protein turnover. The reason for this is not understood; it is difficult to believe that it could be related to reduced lean tissue mass after colectomy, for reasons we shall explain. The results also suggest that the rate of protein synthesis in the ileal mucosa is half that in the duodenal mucosa of normal subjects (10) but still very much faster than the rates observed, for example, in human skeletal muscle, which are normally ~0.04%/h (14), and faster than in the colon (4, 5).

The contribution of the intestinal mucosa (excluding the stomach) to whole body protein turnover may be worked out approximately from the values presented here, the colonic turnover rates reported by Heys et al. (4), and the mucosal masses reported by James et al. (6). We emphasize that the following calculation, although normalized for a 70-kg adult (near to averages of 69 and 71 kg for our subects), is approximate only: it uses average rate data from our patients and normal subjects (the former of which had 75% of the normal rate of whole body protein turnover), data from patients with cancer of the colon, although the tissue reported was not cancerous, and mucosal mass data from the postmortem study reported by James et al. However, systematic rather than biological errors are unlikely to be more than ±15% relative or 25% absolute. In a 70-kg person, the masses of the duodenal, jejunal, ileal, and total large bowel mucosa are 34, 149, 295, and 121 g, respectively; if the mucosal protein concentration per gram wet weight is 12% (an average value we routinely find in these studies), and turnover rates (%/h) are, respectively, 2.5, 2.1, 1.2, and 0.4, then the amount of protein synthesized per hour would be (34 × 0.12 × 0.025) + (149 × 0.12 × 0.021) + (295 × 0.12 × 0.012) + (121 × 0.12 × 0.004) g, i.e., 0.96 g protein/h, or ~24 g/day. Independently, both Fauconneau and Michel (3) and Waterlow et al. (chapter 14, page 470, in Ref. 16) calculated the replacement requirement of protein lost into the gut as sloughed epithelial cells as 50-60 g protein/day. (Fauconneau and Michel calculated that ~7 g extra would need to be added for export protein secretions, most of which would have been measured by our technique in 4 h.) Thus our estimate, based on direct measurements, suggests that the contribution of the gut to whole body protein synthesis is substantially less than previously thought, both absolutely and relatively. In a table comparing nitrogen exchange in the digestive tract and turnover of body protein of a 70-kg man, Fauconneau and Michel suggest that mucosal replacement would be 50 g out of 327 g/day of whole body protein synthesis, i.e., 15%. The figure for whole body synthesis quoted was derived using outdated methods, and we have the opportunity to use more reliable values obtained with the leucine/KIC technique applied here. In our subjects, the rate of whole body turnover of protein was 400 g/day in the ileostomates and 544 g/day in the normal subjects (calculated with a figure of 131 for leucine molecular weight and protein being 8% of leucine and body weights of 70 kg). The amount of 24 g/day we calculate is equivalent to either 6 or 4%/day. Because the amount of the flux accounted for by disappearance of leucine into protein is normally ~85-90%, this suggests that gut mucosa contributes <10% to whole body protein turnover.

These calculations also suggest that the colectomy can have had little effect on the whole body protein turnover, although the calculations here do not take into account the serosal tissues, which are ~66% of the mass of the gut (7). In animals the serosa turns over much less quickly than the mucosa (7), but even if in human beings it turned over as quickly as mucosa, the contribution would still be small, only ~1 µmol leucine · kg¹ · h¹.

We know little about the effects of physiological and pathophysiological factors that might affect the rates of protein turnover in the human mucosa, simply because so few studies have been carried out. The work of Heys and colleagues (4, 5) demonstrates that various gastrointestinal pathologies can stimulate colonic mucosal protein synthesis, and we have found that celiac disease (9) and Crohn's disease (unpublished data) increase the rate of small intestinal protein synthesis markedly; we have also found that alcoholic liver disease markedly depresses jejunal protein synthesis (9a). Given the obvious adaptability of the gut mucosa to pathophysiological changes, it is the more remarkable that short-term feeding and fasting (1) or amino acid infusion (11) has so little effect on the rates of protein turnover in human small intestinal mucosa.

The results presented here add to our knowledge of the protein economy of the gut and its contribution to the whole body. The techniques used may be of value in determining the effects of interventions that are known to alter the function and histology of the small intestine, such as ileorectal anastomosis and pelvic pouch formation.