Fructose transport mechanisms in humans.

BACKGROUND & AIMS: The possible mechanisms of fructose transport are diffusion, a disaccharidase-related transport system, and glucose-facilitated fructose transport. However, these mechanisms in the human small intestine have not been systematically examined. This study was designed to investigate the mechanisms of fructose transport in the human duodenojejunum. METHODS: A triple-lumen tube was fluoroscopically positioned in the duodenojejunum of 7 men. Nine carbohydrate-electrolyte solutions were perfused at the rate of 15 mL/min. Acarbose and lactulose were used to examine the disaccharidase-related transport system and glucose-facilitated fructose transport, respectively. RESULTS: Fructose absorption was greater (P < 0.05) from fructose-glucose (FruGlu) and fructose-glucose-acarbose (FruGluA) solutions than from fructose-mannitol (FruMann) and fructose-mannitol-acarbose (FruMannA) solutions, but there was no difference between FruGlu and FruGluA solutions. A sucrose solution produced greater (P < 0.05) sucrose absorption than a sucrose-acarbose solution. Lactulose absorption (0.016-0.039 mmol.h-1.cm-1) was observed from solutions containing glucose or sucrose. Water absorption was not different among sucrose, FruGlu, and glucose solutions. FruMann solution produced net water secretion. These data suggest that free fructose and glucose transport were not inhibited by acarbose and that the presence of glucose induced lactulose absorption and enhanced fructose absorption. CONCLUSIONS: Fructose is transported transcellularly by facilitated diffusion and paracellularly (based on lactulose transport) via glucose-activated solution drag. In the human small intestine, free fructose and glucose transport does not occur via the disaccharidase system.

Upper limit for intestinal absorption of a dilute glucose solution in men at rest.

We studied gastric and intestinal function by gastric intubation/intestinal perfusion in six healthy male volunteers to evaluate optimal use of a 6% glucose-electrolyte (GES) solution. Gastric volume, residual volume, emptying rate, and secretion were measured for an initial 763 +/- 19 ml gastric load of GES and at the beginning and end of four additional gastric loads (2.2 ml.kg-1; approximately 180 ml) given at 10-min intervals. The relatively high gastric (713 +/- 58 ml) and residual (507 +/- 26 ml) volumes maintained a high gastric emptying rate (19.5 +/- 1.4 ml.min-1). Composition of the GES emptied into the duodenum was also measured in this first experiment. In a second experiment, this modified solution was infused (triple lumen tube) into the duodenum at a rate equal to gastric emptying rate, or at 38 or 77% greater rates. Absorption of water (11.3-12.9 ml.h-1.cm-1) and glucose 4.3-5.6 mmol.h-1.cm-1) were similar at all perfusion rates during the second experiment. We conclude that duodenojejunal segmental absorption rates of water and glucose produced by a rapid, sustained gastric emptying rate cannot be increased by delivering a greater load of glucose and water by intestinal perfusion.

Effects of dietary calcium and phosphorus on vitamin D metabolism and calcium absorption in hamster.

We studied the following responses to restriction of dietary calcium and phosphorus in the growing hamster: (i) serum concentrations of calcium, inorganic phosphorus, magnesium, and vitamin D metabolites; and (ii) calcium transport by ileum. Diets fed were normal calcium with normal or low phosphorus or low calcium with normal or low phosphorus. We found serum 1 alpha,25-dihydroxycalciferol (1,25-[OH]2D) concentration did not differ significantly among the diet groups. Calcium absorption, measured as serosal/mucosal calcium concentration ratio produced by everted ileal sac, was greater in the low calcium, normal phosphorous group than in all other groups. The other groups did not differ from one another in calcium absorption. Feeding the low calcium, normal phosphorus diet increased inorganic phosphorus and magnesium but decreased calcium concentration in serum in comparison with the three other diets. Both low phosphorus diets were without effect on serum calcium, but the low calcium, low phosphorus diet increased serum inorganic phosphorus and magnesium above that of the normal calcium, low phosphorus diet. Ileal calcium absorption in hamster (i) was independent of serum 1,25-(OH)2D concentration; (ii) increased in response to low dietary calcium if dietary phosphorus was normal; and (iii) was independent of dietary calcium, if dietary phosphorus was low. Despite increased calcium absorption, serum calcium was decreased in the low calcium-normal phosphorus group as compared with all other groups. Feeding low calcium diets increased serum inorganic phosphorus and magnesium as compared with feeding the corresponding normal calcium diets (i.e., independently of whether dietary phosphorus content was normal or low). These studies demonstrate that the interrelationships between calcium absorption and vitamin D and mineral metabolism in hamster differ from other mammals.

Physical determinants of solute and water clearance by the perfused dog jejunum.

We systematically compared the individual effects of infusion rate, solution osmolality, and temperature on plasma solute secretion during luminal perfusion of an isolated 90-cm jejunal segment in the conscious and otherwise normal dog. The control state was perfusion at a rate of 3 ml/min of a 25 degrees C temperature solution containing (mM): NaCl, 40; NaHCO3, 10; and mannitol, 220 (320 mosm/kg). Increasing infusion rate from 3 to 9 ml/min increased secretion of Na-, K+, and urea (P < 0.05); further elevation of infusion rate to 18 ml/min increased only Cl- and urea secretion (P < 0.05). Thus, increasing infusion rate of the control solution had only a limited role in promoting solute secretion. In contrast, perfusion solutions elevated in osmolality to 600 and 1200 mosm/kg by mannitol progressively augmented the secretions of water, Na-, K+, Cl-, HCO3(-), Ca2+, and urea (P < 0.05). Perfusion of the 1200 mosm/kg solution produced solute secretions either equivalent to or greater than secretions obtained during 18-ml/min perfusion with the control solution, despite the fact that 1200 mosm/kg perfusion yielded about one-third the effluent volume. Warming of the control solution to progressively increase infusion temperatures from 29 to 41.9 degrees C increased (P < 0.05) water, Na+, K+, and Cl- secretion and HCO3 absorption linearly. Urea secretion increased (P < 0.05) above a temperature threshold of 38-38.9 degrees C. In conclusion, perfusion solution infusion rate, osmolality, and temperature are unique independent determinants of plasma solute secretion by jejunum. These data are the basis for optimizing the physical determinants of jejunal clearance for applying jejunal perfusion to treatment of renal failure.

Simultaneous determination of gastric emptying and intestinal absorption during cycle exercise in humans.

Because fluid absorption values derived from intestinal perfusion may not represent intestinal absorption of the same solution following its oral ingestion, the present study measured intestinal absorption following oral ingestion of a beverage. To do so required the simultaneous determination of gastric emptying. Seven males positioned a nasogastric tube in the gastric antrum and a multilumen tube in the duodenum under fluoroscopic guidance. Gastric emptying (GE) and intestinal water flux (WF) were measured during 85 min of cycle exercise at 60.6 +/- 3.7% VO2max (x +/- SE) in a 22 degrees C environment. Subjects ingested a total of 23 ml.kg-1 body weight (2005 +/- 187 ml) of a 6% isotonic carbohydrate-electrolyte solution by drinking 396 +/- 34 ml 5 min prior to exercise followed by 198 +/- 17 ml every 10 min during exercise. Mean stomach volume (312 +/- 80 ml) and GE (19.7 +/- 2.0 ml.min-1) did not change significantly after the initial 35 min equilibration period. Mean WF during oral ingestion of the solution (19.5 +/- 2.6 ml.cm-1.h-1) did not differ significantly from mean WF (16.4 +/- 1.9 ml.cm-1.h-1) during perfusion of the same solution directly into the duodenum at a rate equal to each subject's GE rate. Total solute flux (mmol.cm-1.h-1) was not different between drinking (4.1 +/- 1.3) and infusion (3.8 +/- 1.0) trials, nor were the changes in plasma volume. Urine production immediately following the exercise bout was unchanged between drinking (89.1 +/- 27.5 ml) and perfusion (88.5 +/- 24.2) experiments. These data indicate that: 1) relatively constant stomach volumes can be maintained over a prolonged period of time and can produce relatively constant GE rates, and 2) intestinal absorption of an isotonic carbohydrate-electrolyte beverage can be accurately determined by a modified segmental perfusion technique employing ingestion rather than intestinal perfusion.

Effects of carbohydrate type and concentration and solution osmolality on water absorption.

We studied intestinal absorption of solutions containing either one (glucose, Glu, or maltodextrin, Mal) or two (fructose, Fru, and Glu or sucrose, Suc) transportable carbohydrate (CHO) substrates using segmental perfusion technique in eight healthy male subjects. These CHO were either free or directly transportable monosaccharides (Glu, Fru), bound as the disaccharide (sucrose, Suc), or as oligomers (maltodextrins, Mal). [CHO] was varied from 6% to 8% (120-444 mmol.1(-1)). All solutions contained low [Na+] (15-19 mEq) and [K+] (3-4 mEq). Solutions osmolalities varied from 165 to 477 mOsm.kg(-1). Osmolalities in the test segment ranged from 268 to 314 mOsm.kg(-1). The regression line of osmolality with water absorption differed for single as compared with multiple substrate solutions. The significantly different intercepts of these two regression lines suggest that solutions with multiple substrates produce greater water absorption at a given osmolality than those with one. Comparing all solutions, test segment solute flux (partial r = 0.69) was more important than mean osmolality (partial r = 0.32). In conclusion, solutions with multiple substrates stimulate several different solute absorption mechanisms yielding greater water absorption than solutions with only one substrate.

Effect of sodium concentration in a carbohydrate-electrolyte solution on intestinal absorption.

Intestinal absorption during infusion (15 ml.min-1) of a 6% carbohydrate (2% glucose and 4% sucrose) solution containing either 0, 25 or 50 mEq.l-1 Na+ was measured by segmental perfusion with a triple lumen tube in males (age 24.3 +/- 1.6) at rest in a neutral environment (22 degrees C). Infusion of the 25 and 50 mEq.l-1 Na+ solutions was performed using a balanced design on the same day, separated by a 1-hr period of no infusion. Infusion of 0 mEq.l-1 Na+ solution was performed on a separate day. A 45-min equilibration period preceded a 90-min test session. Water and solute fluxes were determined from differences in concentration of polyethylene glycol and solute across a 40-cm intestinal segment of the duodenojejunum. A two-factor repeated measures ANOVA indicated no differences (P > 0.1) over time for water, Na+, or glucose flux for all solutions. Plasma volume increased (P < 0.01) approximately 5% over time for all solutions. We conclude that Na+ concentrations of 0, 25, or 50 mEq.l-1 in a 6% carbohydrate solution have similar effects on the absorption of water, Na+, and glucose from the duodenojejunum. Glucose in the infusion solution is the more important factor determining intestinal water absorption than Na+. This study suggests that adding Na+ to fluid replacement beverages may not be a factor in fluid absorption.

Effects of diabetes on calcium uptake by rat brush border membrane vesicles.

1. We investigated the mechanism of decreased transmucosal calcium transport in the gut of the diabetic rat by comparing calcium uptake by brush border membrane vesicles from control and streptozotocin diabetic rats at 5 days. Brush border calcium uptake consists of saturable and non-saturable components. Saturable uptake is mediated by a specific mobile carrier mechanism and is defined by Vmax (saturable uptake of calcium at infinite medium calcium concentration) and KT (calcium concentration at Vmax/2). Non-saturable uptake is defined by kD (rate constant for non-saturable uptake per unit calcium concentration), and comprises both diffusive and surface binding components of calcium uptake. 2. We found both saturable and non-saturable calcium uptake to be decreased (P < 0.05) in diabetes. Comparing control and diabetic, Vmax was 247 compared to 152 (data are pmol/mg protein per 3 s); kD was 285 compared to 172 (data are pmol/mg protein per 3 s at 1 mmol/L calcium); and KT (mmol/L) did not differ between groups, 0.070 compared to 0.057. 3. The decreased Vmax in the setting of unchanged KT in vesicles from diabetics is consistent with decreased calcium transporter specific activity, rather than with altered transporter function. 4. Since (i) Vmax is decreased by vitamin D deficiency in the normal rat, and (ii) circulating 1 alpha, 25-dihydroxycholecalciferol is decreased in the diabetic rat, decreased Vmax in the diabetic may be related to the low 1 alpha,25-dihydroxycholecalciferol.(ABSTRACT TRUNCATED AT 250 WORDS)

Surface charge, fluidity, and calcium uptake by rat intestinal brush-border vesicles.

Biological membrane outer surfaces are negatively charged and interact with positively charged calcium ion during calcium uptake. Positively charged polycations such as polyarginine bind to membranes with high affinity, displacing bound calcium from the membrane. We tested the effect of polyarginine on uptake of calcium by brush-border membrane vesicles and examined the responses in terms of membrane fluidity by electron paramagnetic resonance (EPR). Polyarginine inhibited the saturable component of calcium uptake by a mechanism combining inhibition characteristics of strontium (competitive) and magnesium (non-competitive). Unlike the inhibition of non-saturable calcium uptake by strontium and magnesium, polyarginine increased kD, the rate constant for non-saturable calcium uptake, by a concentration dependent mechanism. These effects of polyarginine on calcium uptake were associated with decreased membrane fluidity at the uptake temperature. These findings are consistent with a role for surface negative charge in determining both saturable and non-saturable calcium uptake. Increased membrane fluidity is associated with decreased saturable and increased non-saturable calcium uptake. Although increased fluidity might be involved in the increased kD for non-saturable uptake, the concentration-specific stimulating effect of polyarginine suggests a gating mechanism.

Deoxycholate effects on plasma solute clearance by gut perfusion in dog.

In vivo luminal perfusion of in situ intestinal segments isolated from continuity with the alimentary tract has been used to treat uremia. One prior study showed that 2 mM deoxycholate markedly increased clearances of inorganic phosphorus, urate, urea, and creatinine. The other investigation failed to confirm the increased inorganic phosphorus or urate clearance. Because of the clinical potential of the findings, we systematically examined the effects of perfusing deoxycholate (2, 7, 15, and 30 mM) on clearance of plasma solutes by a 90-cm jejunal Thiry-Vella loop in the conscious dog. During control perfusion, clearance of all solutes was in a steady state in the final five of seven successive 30-min samples. During deoxycholate perfusion, all solutes except inorganic phosphorus and urate also attained a steady state of clearance in these final five samples. Inorganic phosphorus and urate clearances were elevated several orders of magnitude above control, but only during the first two 30-min collection periods. This increase was deoxycholate concentration-dependent and progressively declined during the subsequent five 30-min collection periods (P < 0.0001). Steady-state clearances of sodium, potassium, calcium, chloride, and creatinine were increased (P < 0.05) by 2 mM deoxycholate. Clearance of urea was decreased by deoxycholate. In summary, (a) deoxycholate increased clearances of most solutes, but steady-state increases were modest; (b) clearances tended to increase further with increasing deoxycholate; (c) the markedly increased clearances in a prior study were probably non-steady state; (d) urea clearance was decreased by deoxycholate. We conclude that detergents such as deoxycholate cause only modest increases in steady-state gastrointestinal clearance of plasma solutes.

Effects of solution osmolality on absorption of select fluid replacement solutions in human duodenojejunum.

These experiments examined relationships between initial osmolality and carbohydrate (CHO) composition of an infused solution and osmolality and water and CHO absorption in a test segment. A triple-lumen tube with a 10-cm mixing segment and a 40-cm test segment was passed into the duodenojejunum. The infusion port was approximately 10 cm beyond the pyloric sphincter. Perfusion solutions were hypotonic (186 mosmol/kg; solution A), isotonic (283 mosmol/kg; solution B), and hypertonic (403 mosmol/kg; solution C). All solutions contained 18 meq Na+ and 3 meq K+. In the mixing segment, osmolality increased 83 mosmol/kg and decreased 90 mosmol/kg for solutions A and C, respectively. Corresponding changes in the test segment were an increase of 60 mosmol/kg and a decrease of 34 mosmol/kg. The osmolality of solution B did not change. In the test segment, mean osmolality and water and total solute fluxes were not significantly different among solutions, but solution C produced 27% greater fluid absorption than did solution A. When net fluid movement from mixing and test segments was determined, solution A produced 17% greater fluid absorption than did solution C. The mean increases in plasma and urine volumes over the 80-min test period were not significantly different. In the test segment, water flux correlated with CHO and Na+ fluxes but not with osmolality. In conclusion, 1) significant differences in solution osmolality were eliminated within the proximal duodenum and 2) perfusing 6% CHO solutions with osmolalities ranging from 186 to 403 mosmol/kg did not produce significant differences in fluid homeostasis (plasma volume) at the end of an 80-min test period.

Vitamin D and enterocyte brush border membrane calcium transport and fluidity in the rat.

Prior studies of vitamin D repletion showed a threefold increase in the maximum rate (Vmax) for calcium uptake by brush border membrane vesicles, but did not differentiate saturable and nonsaturable uptake components. We studied the calcium uptake and fluidity response of intestinal brush border vesicles to vitamin D by treatment with 1 alpha,25-dihydroxy-24,24-difluorocholecalciferol (24,24-F-1,25-(OH)2D3). Treatment responses were measured by effects on (1) saturable and nonsaturable initial uptake rates of calcium by rat proximal small intestinal brush border membrane vesicles; (2) transmucosal calcium transport by everted duodenal sac; and (3) fluorescence anisotropy. Treatment of vitamin D-depleted weanlings increased the Vmax by 50% (P < .05) in vesicles from the proximal 12 cm of small intestine from rats injected with disodium ethane-1-hydroxy-1,1-diphosphonate (EHDP), but there was no response in rats not injected with EHDP or in vesicles from the proximal 30 cm of small intestine. Vitamin D-depleted weanlings were D-deficient based on serum 25-hydroxycalciferol(25-OH-D) concentration, but to produce 1 alpha,25-dihydroxycalciferol [1,25-(OH)2D] depletion, EHDP injection was required. Treatment of vitamin D-replete adult rats caused a 20% (P < .05) increase in Vmax. Treatment did not affect the calcium concentration at half-Vmax (KT), the rate constant for nonsaturable uptake (KD), or vesicle fluidity measured as fluorescence anisotropy. Contrasting with these minimal effects of treatment on brush border Vmax, treatment increased transmucosal calcium transport by everted duodenal sac almost threefold in vitamin D-depleted weanlings administered EHDP. Thus, vitamin D actions on enterocyte calcium transport (1) at the brush border increase saturable but not nonsaturable uptake, and (2) produce the major transport response distal to the brush border. Despite previously described changes in membrane lipid, brush border fluidity is unaffected by vitamin D treatment.

Intestinal absorption during rest and exercise: implications for formulating an oral rehydration solution (ORS). Proceedings of a roundtable discussion. April 21-22, 1993.

Formulation of oral rehydration solutions (ORS) is reviewed in the context of methods for measuring absorption of water and component substrates, transport mechanisms of substrates and water, requirements of the athlete, and effects of exercise on absorption. The triple lumen tube intubation perfusion method is the optimal technique for obtaining absorption data from the human small intestine during rest and exercise. Factors that must be considered when interpreting absorption data obtained by this technique include the role of the mixing segment in altering composition of the infused solution, defining optimal segment length, effects of ORS osmolality, and absorption of "nonabsorbed" indicators. Absorption data are applicable only to the test segment and may lack relevance to ORS transport proximal and distal to the test segment. Absorption rate of an ORS measured by perfusion may not correlate with absorption rate following ingestion. Transport of water, electrolytes, carbohydrates, and other solutes including glutamine and amino acids is considered in relation to ORS formulation. Factors affecting absorption of an ORS including the unstirred layer, motility, intestinal blood flow, and maximal absorptive capacity of the alimentary tract are considered. Exercise per se at 30-70% VO2max for 60-90 min probably has minimal effects in limiting absorption of an ORS. Consideration relevant to supplying needs of the athlete during prolonged exercise in relation for ORS formulation are discussed.

Brush border calcium uptake in short-bowel syndrome in rats.

Resection of distal small intestine causes calcium malabsorption in humans and in a rat model of 50% distal resection. We tested the hypothesis that this calcium malabsorption is caused in the rat model by a brush border defect. We compared brush border membrane vesicles from the proximal small intestine of control (transection and anastomosis at mid-small intestine) with distally resected rats. Mucosal protein was 25% greater in the resected group and the vesicles were enriched 37-fold in sucrase activity when compared with homogenate. Kinetic constants Vmax (maximal initial rate of saturable calcium uptake at infinite concentration), kT (calcium concentration for saturable calcium uptake rate at half Vmax), and KD (rate constant for nonsaturable calcium uptake per unit concentration) were slightly but not significantly greater in the resected as compared with the transected group, ruling out the brush border as the cause for decreased transmucosal calcium transport.

Intestinal water absorption from select carbohydrate solutions in humans.

Eight men positioned a triple-lumen tube in the duodenojejunum. By use of segmental perfusion, 2, 4, 6, or 8% solutions of glucose (111-444 mM), sucrose (55-233 mM), a maltodextrin [17-67 mM, avg. chain length = 7 glucose units (7G)], or a corn syrup solid [40-160 mM, avg. chain length = 3 glucose units (3G)] were perfused at 15 ml/min for 70 min after a 30-min equilibration period. All solutions were made isotonic with NaCl, except 6 and 8% glucose solutions, which were hypertonic. An isotonic NaCl solution was perfused as control. Water absorption (range: 9-15 ml.h-1.cm-1) did not differ for the 2, 4, and 6% CHO solutions but was greater (P < 0.05) than absorption from control (3.0 +/- 2.2 ml.h-1.cm-1). The 8% glucose and 3G solutions reduced (P < 0.05) net water flux compared with their 2, 4, and 6% solutions, but 8% sucrose and 8% 7G solutions promoted water absorption equivalent to lower CHO concentrations. Water absorption was independent of [Na+] in the original solution. In the test segment, 1) Na+ flux correlated with net water flux (r = 0.72, P < 0.01), K+ (r = 0.78, P < 0.01), and [Na+] (r = 0.68, P < 0.001); 2) Na+ absorption occurred at luminal [Na+] as low as 50 mM; 3) glucose transport increased linearly over the luminal concentration range of 40-180 mM; and 4) net water flux was similar over a range of glucose-to-Na+ concentration ratios of 0.4:1 to 3.5:1.(ABSTRACT TRUNCATED AT 250 WORDS)

Transport properties of an in situ jejunal reservoir in dogs.

To evaluate partial or total replacement of renal function using gut, we measured in vivo transport of nitrogen metabolites, electrolytes, and water into a jejunal segment configured as a continent reservoir in the dog. Reservoir contents were sampled and analyzed at serial time intervals during a 3-h period after instillation of solution containing (in mM) 40 NaCl, 10 NaHCO3, 220 mannitol, pH 8.5, without or with added urease. At 10 min postinstillation, the amount of urea in the solution without added urease was 3-5 times greater than in the presence of added urease, but accumulation of NH4+ was 14-21 times greater in the solution containing added urease, giving a luminal NH4+ concentration up to 10,000 times that of plasma. In the absence of urease, HCO3- concentration fell to 0, and pH declined to 6 at 3 h; in the presence of urease, HCO3- concentration was 4.5 mM, and pH was 7.8 at 3 h. We conclude 1) urea is secreted by the reservoir; 2) H+ is secreted and/or formed in the reservoir; 3) in the presence of urease, urea hydrolyzed to NH3 is converted to NH4+ by H+ and trapped in the lumen; and 4) in the urease solution, H+ binding by NH3 preserves luminal HCO3-, maintaining the initial pH. Thus the continent jejunal reservoir may supplement or replace impaired renal function.

Effects of cycle exercise on intestinal absorption in humans.

Intestinal absorption was measured in six trained male cyclists during rest, exercise, and recovery periods with the segmental perfusion technique. Each subject passed a multilumen tube into the duodenojejunum. The experiments consisted of 1) a sequence of 1-h bouts of cycling exercise at 30, 50, and 70% maximal O2 uptake (Vo2max) separated by 1-h rest periods or 2) a 90-min bout at 70% VO2max. The cycling was performed on a constant-load Velodyne trainer. Absorption of water and a 6% carbohydrate-electrolyte (2% glucose, 6% sucrose, 20 meq Na+, 2.6 meq K+) solution (both perfused at 15 ml/min) were compared. The effects of perfusing an isotonic electrolyte solution during mild (30% VO2max) exercise were also studied. Fluid was sampled every 10 min from ports 10 and 50 cm distal to the infusion site. Water flux was determined by differences in polyethylene glycol concentration across the 40-cm test segment. Results showed 1) no difference in water or electrolyte absorption rates among rest, exercise, and recovery periods; 2) no difference in absorption rates among the three exercise intensities or different exercise durations; and 3) significantly greater fluid absorption rates from the carbohydrate-electrolyte (CE) solution than from water. Water flux during rest, exercise, and recovery was about sixfold greater from the CE solution than from the isotonic solution without carbohydrate. We conclude that 1) exercise has no effect on water or solute absorption in the duodenojejunum, 2) fluid absorption occurs significantly faster from a CE solution than from water, and 3) fluid absorption is increased sixfold by addition of carbohydrate to an electrolyte solution.

Metabolism of aspartame and its L-phenylalanine methyl ester decomposition product by the porcine gut.

The intestinal metabolism of aspartame (N-L-alpha-aspartyl-L-phenylalanine methyl ester; APM) and its L-phenylalanine methyl ester (PME) decomposition product was evaluated in six young pigs. Equimolar doses (2.5 mmol/kg body weight) of APM, PME, and L-phenylalanine (PHE) administered to the proximal jejunum produced similar increases in portal blood PHE concentrations. Methanol, nondetectable in portal blood after PHE ingestion, increased markedly after administration of either APM or PME. Portal blood aspartate concentrations were unchanged after PME and PHE administration, but increased significantly after APM administration. Increases in portal blood PHE concentrations were significantly greater than were increases in aspartate concentrations following APM administration. Neither APM, PME, nor aspartyl-phenylalanine (AspPhe) were detected in portal or vena caval blood following administration of any test compound. Steady-state perfusion of the small intestine with APM showed a net intraluminal appearance rate of AspPhe at 36% of the disappearance rate of APM. During steady-state PME perfusion, PHE had a significantly greater net appearance rate than during APM perfusion. Methanol appearance rates were slightly, but not significantly, higher during PME than during APM perfusions. The data suggest that (1) APM is hydrolyzed to AspPhe and both APM and PME are hydrolyzed to their constituent amino acids and and methanol prior to entering the portal circulation; (2) AspPhe is an important intraluminal intermediate in aspartame metabolism; and (3) aspartate is rapidly metabolized by the enterocyte.

Effects of resection on absorption and secretion of divalent cations by small intestine of rat.

Resection increases villus height and crypt depth of remaining intestine. We examined functional consequences of resection by measuring absorption of strontium and secretion of calcium and magnesium by proximal and distal segments remaining after resecting 70% of mid small intestine. Compared with the transected control group, resection decreased strontium absorption per unit weight of mucosa (specific absorption) in the proximal segment. The decreased specific absorption was compensated by increased mucosal growth in the resected group so that absorption per unit length of segment (per cm) was the same in both groups. Resection increased secretion of calcium and magnesium by 66% per unit weight of mucosa and by 145%/cm in the distal segment. Comparing proximal with distal segments in the resected group, secretion was greater in distal for calcium and in proximal for magnesium. Intestinal resection causes responses in absorption and secretion of divalent cations important in mineral homeostasis.

Human intestinal water absorption: direct vs. indirect measurements.

Distilled water, a carbohydrate-electrolyte (CE; 4% sucrose, 2% glucose, 17.2 meq/l NaCl, and 2.8 meq/l KCl) solution, or a 10% glucose solution, all containing the nonabsorbed indicator polyethylene glycol (PEG) and deuterium oxide (D2O, 30 ppm), were infused (15 ml/min) into the duodenojejunum of seven men by using the triple lumen technique. Net water absorption was determined directly from the change in PEG concentration and was calculated from plasma D2O derived from D2O in the perfusion solutions. The protocol included a 45-min equilibration period followed by a 90-min test period. Intestinal samples were drawn at 10-min intervals from 15 to 45 min and at 15-min intervals thereafter. Blood was drawn at 45, 50, 55, 60, 75, 90, 105, 120, and 135 min. Intestinal samples were analyzed for D2O, Na+, K+, osmolality, PEG, and glucose; blood was analyzed for D2O. Results (+/- SE; positive values secretion, negative values absorption) showed net fluid absorption from distilled water (-9.40 +/- 1.28 ml.h-1.cm-1) and the CE (-13.30 +/- 1.22 ml.h-1.cm-1) solution, but net secretion (4.40 +/- 1.25 ml.h-1.cm-1) from the 10% glucose solution. All values were significantly (P less than 0.05) different from each other. Perfusing the CE solution caused net Na+ and K+ absorption, whereas perfusing the 10% dextrose solution caused net electrolyte secretion. Rates of D2O accumulation in the plasma were independent of the solutions perfused.(ABSTRACT TRUNCATED AT 250 WORDS)