Technical note: pig model for studying nutrient assimilation by the intestine and colon.

We have developed a system for chronically catheterizing 10- to 25-d-old pigs that permits stable isotope tracer studies of intestinal or colonic assimilation of nutrients. This model also can be used to ensure constant enteral feeding or to assess the rate of entry into the terminal ileum of carbohydrates, fats, and amino acids. A plastic cannula with a luminal flange can be surgically placed in the stomach for tracer studies of sugar digestion or for controlled infusion of any formula diet. A similar cannula can be placed in the cecum for infusion of tracer and(or) substrates for studies of fermentation. The cannula has been machined so that a washer and nut can be threaded onto it, allowing the entire apparatus to be fixed to the abdominal wall. The distal end protruding above the skin was tapered to fit standard i.v. extension tubing. A carotid arterial catheter was used to sample substrates for isotopic enrichment measurements.

Measurement of the rate of entry of intact colon-derived lactose into the circulation: a model for assessing gut uptake of molecules not endogenously synthesized.

BACKGROUND: Results of in vitro studies have documented colonic absorption of lactose in the newborn. A stable isotope model was developed for assessing the entry rate of intact lactose into the portal circulation in newborn piglets. METHODS: In experiment 1, unlabeled and [D-1-(13C)]-lactose were infused into two separate mesenteric veins, and in experiment 2, labeled lactose was infused into a mesenteric vein and unlabeled lactose was infused into the colon. The 13C-enrichment of plasma lactose was assessed by high performance liquid chromatography gas chromatography combined with mass spectrometry. RESULTS: The isotopic estimate of the mesenteric venous infusion rate of lactose was 91% of the theoretical. In the second experiment 13% of the unlabeled lactose infused into the colon reached the portal circulation. CONCLUSIONS: The current study provides the first, direct, in vivo confirmation of colon absorption of intact lactose. The tracer model could be used to evaluate intestinal or colonic absorption of other organic compounds not endogenously synthesized, including vitamins or drugs.

Lactose flux occurs by differing mechanisms in the colon and jejunum of newborn piglets.

The unidirectional flux of 10- and 40-mM lactose was studied in newborn porcine jejunum and colon mounted in Ussing chambers. Polyethylene glycol 400 was used to measure passive paracellular permeability. The mucosal-to-serosal flux and the tissue accumulation of labeled lactose from the colon was similar to that of lactose-derived glucose from the jejunum. However, only jejunum showed a lactose-stimulated increase in short-circuit current. In jejunum, glucose was the sole sugar identified in the serosal bath, whereas in colon, only intact lactose was identified. Despite colonic lactose flux, polyethylene glycol oligomers were not found in the serosal bath, suggesting that they do not share the same route of absorption. Colonic lactose transport was nonsaturable between 1 and 40 mM. Under nongradient conditions, no net colonic lactose transport was observed. Cumulatively, these data suggest that the colon, unlike the jejunum, does not contain a glucose-galactose sodium cotransporter. The colon of the newborn piglet transports intact lactose at a flux equal to that of lactose-derived glucose by the small intestine, but by a different mechanism that is as yet undefined.

Absorption of lactose from colon of newborn piglet.

Piglets in three age groups (1-3, 9-11, and 16-25 days after birth) were used for in vivo colonic perfusions. Studies compared an isosmolar (312 mosM) with a high osmolar (551 mosM) solution and two equimolar substrates (with hexose concentrations of 73.1 mM), lactose and glucose-galactose. From the isosmolar perfusates, lactose absorption was 0.43 +/- 0.04 in the 18-20 day olds and 1.04 +/- 0.2 mumol.cm-1.min-1 in the 1-3 day olds; absorption from the glucose-galactose solution was negligible in all age groups (less than 0.05 +/- 0.05 mumol.cm-1.min-1). From the high osmolar perfusate, lactose absorption also exceeded that of glucose and galactose. In a third set of perfusion studies, the concentration of lactose was varied between 15 and 240 mM perfusate. Five-day-old animals absorbed 67% more lactose than 18-day-old animals; the right colon absorbed 57% more than the left. Lactose absorption, correlated with its concentration in the perfusate (r = 0.99), was nonsaturable at concentrations up to 240 mM, and was correlated with the uptake both of sodium (r2 = 0.59 for young and 0.64 for older neonates) and of chloride (r2 = 0.55 for young and 0.31 for older neonates). The results suggest that lactose may be removed from the colon without apparent cleavage by beta-galactosidase.

Failure to conserve lactose and glucose polymers during frozen storage of fecal specimens: methods for preservation.

Freezing is often used to retard bacterial enzymatic activity in fecal specimens collected to quantify specific carbohydrates. The effectiveness of freezer storage on preservation of lactose and glucose polymers was assessed. The data showed that more than 50% of lactose that was added to fecal supernatants that were stored without treatment for more than 50 days at -20 degrees C was lost. Adjustment of pH with HCl (pH 4.9), with HgCl2 (pH 6.3 or 5.85), or with NaOH (pH 10) improved carbohydrate preservation (P less than 0.0004). Storage of the supernatants of fecal homogenates lessened the loss of carbohydrate compared with the total homogenates (P less than 0.001). In supernatants, degradation occurred via simple hydrolysis; in homogenates, degradation occurred by hydrolysis and fermentation to a variety of end-products. Unprocessed fecal specimens that were frozen for months, then retrieved and incubated with lactose or glucose polymers showed extensive fermentative capacity. Cumulatively, the data indicate that enzymatic activity in feces is not halted by storage in the freezer, even if bacteria have been filtered from the stool.

Absorption of glucose polymers from canine jejunum deprived of pancreatic amylase.

We evaluated the absorption of glucose polymers in canine jejunal Thiry-Vella fistulas proven to be free of pancreatic amylase. Medium-length oligomers with degrees of polymerization of 6 through 10 glucose units (DP 6-10) and long-chain material (DPavg23) were isolated from a cornstarch hydrolysate. We perfused 90, 180, and 360 mg/dl solutions of glucose, DP 6-10, and DPavg23 at 0.4, 1.9, and 3.4 ml/min. At all perfusion rates carbohydrate absorption decreased as the chain length of the oligomers increased, and these differences persisted even at the slowest perfusion rate employed. In two additional animals the fistulas were perfused at 3.4 ml/min with the three test carbohydrates at concentrations of 90, 180, 225, 270, 315, 360, 405, and 450 mg/dl. At this flow rate, the assimilative process of DP 6-10 and the long-chain fraction appeared to be saturated at carbohydrate concentrations above 360 mg/dl, whereas the absorption of glucose was linearly related to concentration throughout the range studied. With both groups of polymers, the fluid emerging from the fistula was virtually free of glucose, a finding that suggests that polymer digestion, not glucose absorption, is the rate-limiting step for polymer assimilation.

The jejunal absorption of glucose oligomers in the absence of pancreatic enzymes.

We compared the absorption of carbohydrate from solutions of glucose oligomers and glucose in jejunal Thiry-Vella fistulae, a preparation deprived of pancreatic secretions. The studies were performed with two concentrations (90 and 360 mg/dl) of both glucose and the glucose oligomers. Carbohydrate absorption from glucose solutions (33.1 +/- 2.8, 115.9 +/- 8.9 micrograms/cm/min) was significantly greater (P less than 0.025; P less than 0.005) than that from oligomer solutions (26.6 +/- 2.1 and 92.4 +/- 9.0 micrograms/cm/min). Thin-layer analyses of the perfusates demonstrate digestion of oligomers with a chain length up to eleven and suggest digestion of oligomers of even greater chain length. Atrophy of the jejunal mucosa occurred over the course of the study as evidenced by a decrease in the ratio of villous height to crypt depth from 3.8 to 0.3, and by a 80% decrease in the activity of maltase, sucrase, and lactase. Atrophy was accompanied by a significant decline in the absorption of both glucose oligomers (P less than 0.005) and glucose (P less than 0.01) from the more concentrated solutions but the decrement in absorption of both carbohydrates was similar: glucose oligomers, 79.3 +/- 19.4 micrograms/cm/min; and glucose, 69.8 +/- 14 micrograms/cm/min (P greater than 0.20). Water absorption was enhanced by both carbohydrates, but there was no demonstrable difference between solutions of glucose and glucose oligomers. The osmolality of the solutions clearly influenced water absorption (P less than 0.025) but failed to effect the absorption of carbohydrates.