Salmonid microarrays identify intestinal genes that reliably monitor P deficiency in rainbow trout aquaculture.

Nutrient-responsive genes can identify important metabolic pathways and evaluate optimal dietary levels. Using a 16K Salmo salar microarray, we identified in rainbow trout (Oncorhynchus mykiss) 21 potential phosphorus (P)-responsive genes, mainly involved in immune response, proteolysis or transport, whose expression levels changed in the intestine after 5 days of feeding a low-P (LP) diet. Diet-induced changes in the expression levels of several genes in each fish were tightly correlated with changes in serum P, and the changes persisted for an additional 15 days after dietary P deficiency. We then evaluated these and previously identified P-responsive genes under simulated farm conditions, and monitored the intestinal gene expression from 6 h to 7 days after the trout were switched from a sufficient-P (SP) diet to a LP diet (SP-->LP), and from a LP diet to a SP diet (LP-->SP). After 7 days, mean serum P decreased 0.14 mM/day for SP-->LP and increased 0.10 mm/day for LP-->SP. The mRNA abundance of the metalloendopeptidase meprin 1alpha (MEP1alpha), the Na(+)-dependent phosphate co-transporter (NaPi2b,SLC34A2), the sulfotransferase SULT2beta1 and carbonic anhydrase XIII genes all increased after SP-->LP and decreased after LP-->SP, suggesting that adaptive expression is reversible and correlated with dietary P. The duration of change in gene expression in response to SP-->LP was generally shorter than that of LP-->SP, suggesting potentially different mechanisms of adaptation to deficiency as opposed to excess. Diet-induced changes in mRNA abundance of other genes were either transient or modest. We identified, by heterologous microarray hybridization, new genes sensitive to perturbations in dietary P, and then showed that these genes can reliably monitor P deficiency under field conditions. Simultaneous changes in the expression of these P biomarkers could predict either P deficiency (to prevent economic losses to the farmers) or P excess (to prevent inadvertent pollution of nearby waters).

Dietary P regulates phosphate transporter expression, phosphatase activity, and effluent P partitioning in trout culture.

Phosphate utilization by fish is an important issue because of its critical roles in fish growth and aquatic environmental pollution. High dietary phosphorus (P) levels typically decrease the efficiency of P utilization, thereby increasing the amount of P excreted as metabolic waste in effluents emanating from rainbow trout aquaculture. In mammals, vitamin D3 is a known regulator of P utilization but in fish, its regulatory role is unclear. Moreover, the effects of dietary P and vitamin D3 on expression of enzymatic and transport systems potentially involved in phosphate utilization are little known. We therefore monitored production of effluent P, levels of plasma vitamin D3 metabolites, as well as expression of phosphatases and the sodium phosphate cotransporter (NaPi2) in trout fed semipu diets that varied in dietary P and vitamin D3 levels. Mean soluble P concentrations varied markedly with dietary P but not with vitamin D3, and constituted 40-70% of total effluent P production by trout. Particulate P concentrations accounted for 25-50% of effluent P production, but did not vary with dietary P or vitamin D3. P in settleable wastes accounted for <10% of effluent P. The stronger effect of dietary P on effluent P levels is paralleled by its striking effects on phosphatases and NaPi2. The mRNA abundance of the intestinal and renal sodium phosphate transporters increased in fish fed low dietary P; vitamin D3 had no effect. Low-P diets reduced plasma phosphate concentrations. Intracellular phytase activity increased but brushborder alkaline phosphatase activity decreased in the intestine, pyloric caeca, and gills of trout fed diets containing low dietary P. Vitamin D3 had no effect on enzyme activities. Moreover, plasma concentrations of 25-hydroxyvitamin D3 and of 1,25-dihydroxyvitamin D3 were unaffected by dietary P and vitamin D3 levels. The major regulator of P metabolism, and ultimately of levels of P in the effluent from trout culture, is dietary P.

Dietary and developmental regulation of intestinal sugar transport.

The Na(+)-dependent glucose transporter SGLT1 and the facilitated fructose transporter GLUT5 absorb sugars from the intestinal lumen across the brush-border membrane into the cells. The activity of these transport systems is known to be regulated primarily by diet and development. The cloning of these transporters has led to a surge of studies on cellular mechanisms regulating intestinal sugar transport. However, the small intestine can be a difficult organ to study, because its cells are continuously differentiating along the villus, and because the function of absorptive cells depends on both their state of maturity and their location along the villus axis. In this review, I describe the typical patterns of regulation of transport activity by dietary carbohydrate, Na(+) and fibre, how these patterns are influenced by circadian rhythms, and how they vary in different species and during development. I then describe the molecular mechanisms underlying these regulatory patterns. The expression of these transporters is tightly linked to the villus architecture; hence, I also review the regulatory processes occurring along the crypt-villus axis. Regulation of glucose transport by diet may involve increased transcription of SGLT1 mainly in crypt cells. As cells migrate to the villus, the mRNA is degraded, and transporter proteins are then inserted into the membrane, leading to increases in glucose transport about a day after an increase in carbohydrate levels. In the SGLT1 model, transport activity in villus cells cannot be modulated by diet. In contrast, GLUT5 regulation by the diet seems to involve de novo synthesis of GLUT5 mRNA synthesis and protein in cells lining the villus, leading to increases in fructose transport a few hours after consumption of diets containing fructose. In the GLUT5 model, transport activity can be reprogrammed in mature enterocytes lining the villus column. Innovative experimental approaches are needed to increase our understanding of sugar transport regulation in the small intestine. I close by suggesting specific areas of research that may yield important information about this interesting, but difficult, topic.

Intestinal perfusion induces rapid activation of immediate-early genes in weaning rats.

C-fos and c-jun are immediate-early genes (IEGs) that are rapidly expressed after a variety of stimuli. Products of these genes subsequently bind to DNA regulatory elements of target genes to modulate their transcription. In rat small intestine, IEG mRNA expression increases dramatically after refeeding following a 48-h fast. We used an in vivo intestinal perfusion model to test the hypothesis that metabolism of absorbed nutrients stimulates the expression of IEGs. Compared with those of unperfused intestines, IEG mRNA levels increased up to 11 times after intestinal perfusion for 0.3-4 h with Ringer solutions containing high (100 mM) fructose (HF), glucose (HG), or mannitol (HM). Abundance of mRNA returned to preperfusion levels after 8 h. Levels of c-fos and c-jun mRNA and proteins were modest and evenly distributed among enterocytes lining the villi of unperfused intestines. HF and HM perfusion markedly enhanced IEG mRNA expression along the entire villus axis. The perfusion-induced increase in IEG expression was inhibited by actinomycin-D. Luminal perfusion induces transient but dramatic increases in c-fos and c-jun expression in villus enterocytes. Induction does not require metabolizable or absorbable nutrients but may involve de novo gene transcription in cells along the villus.

GLUT-5 expression in neonatal rats: crypt-villus location and age-dependent regulation.

The rat fructose transporter normally appears after completion of weaning but can be precociously induced by early feeding of a high-fructose diet. In this study, the crypt-villus site, the metabolic nature of the signal, and the age dependence of induction were determined. In weaning rats fed high-glucose pellets, GLUT-5 mRNA expression was modest, localized mainly in the upper three-fourths of the villus, and there was little expression in the villus base. When fed high-fructose pellets, GLUT-5 mRNA expression was two to three times greater in all regions except the villus base. Intestinal perfusion in vivo of a nonmetabolizable fructose analog, 3-O-methylfructose, tended to increase fructose uptake rate and moderately increased GLUT-5 mRNA abundance but had no effect on glucose uptake rates and SGLT1 mRNA abundance. Gavage feeding of high-fructose, but not high-glucose, solutions enhanced fructose uptake only in pups > or =14 days, suggesting that GLUT-5 regulation is markedly age dependent. Fructose or its metabolites upregulate GLUT-5 expression in all enterocytes, except those in the crypt and villus base and in pups <14 days old.

Lactase synthesis is pretranslationally regulated in protein-deficient pigs fed a protein-sufficient diet.

The in vivo effects of protein malnutrition and protein rehabilitation on lactase phlorizin hydrolase (LPH) synthesis were examined. Five-day-old pigs were fed isocaloric diets containing 10% (deficient, n = 12) or 24% (sufficient, n = 12) protein. After 4 wk, one-half of the animals in each dietary group were infused intravenously with [(13)C(1)]leucine for 6 h, and the jejunum was analyzed for enzyme activity, mRNA abundance, and LPH polypeptide isotopic enrichment. The remaining animals were fed the protein-sufficient diet for 1 wk, and the jejunum was analyzed. Jejunal mass and lactase enzyme activity per jejunum were significantly lower in protein-deficient vs. control animals but returned to normal with rehabilitation. Protein malnutrition did not affect LPH mRNA abundance relative to elongation factor-1alpha, but rehabilitation resulted in a significant increase in LPH mRNA relative abundance. Protein malnutrition significantly lowered the LPH fractional synthesis rate (FSR; %/day), whereas the FSR of LPH in rehabilitated and control animals was similar. These results suggest that protein malnutrition decreases LPH synthesis by altering posttranslational events, whereas the jejunum responds to rehabilitation by increasing LPH mRNA relative abundance, suggesting pretranslational regulation.

Chronic but not acute energy restriction increases intestinal nutrient transport in mice.

Chronic energy restriction (ER) dramatically enhances intestinal absorption of nutrients by aged mice. Do adaptations in nutrient absorption develop only after extended ER or immediately after its initiation? To determine the time course of adaptations, we measured rates of intestinal glucose, fructose and proline transport 1-270 d after initiation of ER (70% of ad libitum) in 3-mo old mice. Mice of the same age that consumed food ad libitum (AL) served as controls; a third group was starved for 1 or 2 d only, to distinguish the effects of acute ER from those of starvation. Acute ER of 1, 2 and 10 d had no effect on nutrient absorption. Starvation significantly decreased intestinal mass per centimeter, thereby reducing transport per centimeter and intestinal absorptive capacity without significantly altering transport per milligram of intestine. ER for 24 d enhanced only fructose uptake, whereas ER for 270 d enhanced uptake of all nutrients by 20-100%. Despite marked differences in body weights, the wet weights of the stomach, small intestine, cecum and large intestine were generally similar in AL and ER mice, suggesting that the gastrointestinal tract was spared during ER. In contrast, the wet weights of the lungs, kidneys, spleen, heart, pancreas and liver each differed by 40-120% between ER and AL mice. Intestinal transport adaptations develop gradually during ER, and the main mechanism underlying these adaptations is a dramatic increase in transport activity per milligram tissue.

Developmental reprogramming of rat GLUT-5 requires de novo mRNA and protein synthesis.

Fructose transporter (GLUT-5) expression is low in mid-weaning rat small intestine, increases normally after weaning is completed, and can be precociously induced by premature consumption of a high-fructose (HF) diet. In this study, an in vivo perfusion model was used to determine the mechanisms regulating this substrate-induced reprogramming of GLUT-5 development. HF (100 mM) but not high-glucose (HG) perfusion increased GLUT-5 activity and mRNA abundance. In contrast, HF and HG perfusion had no effect on Na(+)-dependent glucose transporter (SGLT-1) expression but increased c-fos and c-jun expression. Intraperitoneal injection of actinomycin D before intestinal perfusion blocked the HF-induced increase in fructose uptake rate and GLUT-5 mRNA abundance. Actinomycin D also prevented the perfusion-induced increase in c-fos and c-jun mRNA abundance but did not affect glucose uptake rate and SGLT-1 mRNA abundance. Cycloheximide blocked the HF-induced increase in fructose uptake rate but not the increase in GLUT-5 mRNA abundance and had no effect on glucose uptake rate and SGLT-1 mRNA abundance. In neonatal rats, the substrate-induced reprogramming of intestinal fructose transport is likely to involve transcription and translation of the GLUT-5 gene.

Intestinal transport during fasting and malnutrition.

Fasting or malnutrition (FM) has dramatic effects on small intestinal mucosal structure and transport function. Intestinal secretion of ions and fluid is increased by FM both under basal conditions and in response to secretory agonists. Intestinal permeability to ions and macromolecules may also be elevated by FM, which increases the potential for fluid and electrolyte losses and for anaphylactic responses to luminal antigens. Mucosal atrophy induced by FM reduces total intestinal absorption of nutrients, but nutrient absorption normalized to mucosal mass may actually be enhanced by a variety of mechanisms, including increased transporter gene expression, electrochemical gradients, and ratio of mature to immature cells. These observations underscore the value of enteral feeding during health and disease.

Dietary phosphorus regulates intestinal transport and plasma concentrations of phosphate in rainbow trout.

Intestinal inorganic phosphate transport and its regulation have not been studied in fish. In this study, we initially characterized the mechanisms of intestinal inorganic phosphate transport in rainbow trout (Oncorhynchus mykiss) then determined the effects of dietary phosphorus concentrations on intestinal inorganic phosphate uptake, plasma inorganic phosphate, and intestinal luminal inorganic phosphate concentrations. In 11-g trout, the saturable mechanism of brushborder inorganic phosphate uptake had a Kt= 1.2 mmol l(-1) and a Vmax = 0.22 nmol mg(-1) min(-1), while the diffusive component had a Kd = 0.012 min(-1). Similar kinetic constants were obtained from 51-g trout, suggesting that development or size had little effect on transport. Tracer inorganic phosphate (1.18 mmol l(-1)) uptake was almost completely inhibited (>95%) by 20 mmol l(-1) unlabeled inorganic phosphate. Inorganic phosphate uptake (0.2 mmol l(-1)) was strongly inhibited (approximately 75% inhibition) by phosphonoformic acid, a competitive inhibitor of mammalian inorganic phosphate transport, as well as by the absence of Na+ (approximately 90% inhibition). Northern blot and reverse transcription-polymerase chain reaction indicated that the intestinal inorganic phosphate transporter in trout is not related to the cloned Na+ inorganic phosphate-II transporter of winter flounder. Intestinal luminal and plasma inorganic phosphate concentrations each increased with dietary P concentrations. Intestinal inorganic phosphate, but not proline, absorption rates decreased with dietary phosphorus concentrations. As in mammals and birds, a Na-dependent inorganic phosphate carrier that is tightly regulated by diet is present in trout small intestine.

Dietary modulation of intestinal fructose transport and GLUT5 mRNA expression in hypothyroid rat pups.

BACKGROUND: Intestinal fructose transport rates or GLUT5 mRNA levels typically show a two- to threefold increase after weaning in rats allowed to wean normally but can be enhanced precociously by high-fructose diets during early weaning. Developmental increases in serum thyroxine levels coincide with the onset of weaning and have been linked to changes in intestinal sucrase and lactase activities. METHODS: Rat pups were made hypothyroid by giving the dam 0.01% propylthiouracil as drinking water from day 18 of gestation. The hypothyroid pups and age-matched euthyroid control pups were then fed high-fructose or high-glucose solutions by gavage, twice a day starting at 17 days of age for 3 days, and then killed at 20 days of age. RESULTS: Serum thyroxine levels were five times lower in the hypothyroid pups. Rates of intestinal fructose uptake in the proximal and middle small intestine were 2.0 to 2.5 times higher in the hypothyroid and euthyroid pups fed high-fructose solution than in littermates fed high-glucose solution or those allowed to wean normally with the dam. Intestinal glucose uptake also increased in hypothyroid but not in euthyroid pups fed high-fructose or high-glucose solutions. GLUT5 mRNA levels increased in euthyroid and hypothyroid pups fed high fructose and paralleled the increase in fructose uptake. CONCLUSION: During weaning, dietary fructose can precociously enhance intestinal fructose uptake and GLUT5 mRNA expression, independent of developmental increases in serum thyroxine levels. Modest changes in glucose transport rates indicate that nonspecific mechanisms may provide a minor contribution to diet-induced changes in nutrient absorption in hypothyroid pups.

Cholecalciferol modulates plasma phosphate but not plasma vitamin D levels and intestinal phosphate absorption in rainbow trout (Oncorhynchus mykiss).

Since the vitamin D endocrine system modulates phosphorus homeostasis and regulates inorganic phosphate (Pi) uptake by the small intestine in mammals and birds, we determined the effects of dietary cholecalciferol (vitamin D3) on Pi uptake by the small intestine, Pi concentrations in the plasma, Pi concentrations in the intestinal lumen, intestinal weights, liver weights, and concentrations of vitamin D metabolites in the plasma of rainbow trout (Oncorhynchus mykiss) fed phosphorus-sufficient (0.6 g/100 g) diets. Five groups of trout initially weighing 55.8 +/- 0.6 g were fed purified diets containing 0, 300, 2,500, 10,000, and 40,000 IU vitamin D3/kg diet over a 7- to 8-day feeding period. Plasma Pi concentration was higher in trout fed 2,500-40,000 IU/kg diet (8.26 +/- 0.27 mmol/L) than in those fed 0 and 300 IU/kg (6.99 +/- 0.30). Liver weights were 30-50% greater in fish fed 0 IU/kg than in those fed 300-40,000 IU/kg. There were no significant, diet-related differences in plasma levels of 25-hydroxycholecalciferol [25(OH)D3] and 1,25 dihydroxycholecalciferol [1,25(OH)2D3]. Increasing levels of dietary cholecalciferol also did not enhance in vitro Pi uptakes by the intestine (range of means: 0.22-0.29 nmol/mg tissue. min) and Pi concentrations in the intestinal lumen (8.5-13.5 mmol/L). Pi uptake did not differ among tissues incubated in vitamin D3, 25(OH)D3, or 1,25(OH)2D3. These results demonstrate that when fish are fed P-sufficient diets, dietary cholecalciferol increases plasma Pi concentrations but decreases liver weights, alterations which are not accompanied by changes in intestinal weight, Pi uptake by the intestine, Pi concentration in the intestinal lumen, and circulating metabolites of cholecalciferol.

Luminal fructose modulates fructose transport and GLUT-5 expression in small intestine of weaning rats.

In neonatal rats, precocious introduction of dietary fructose significantly enhances brush-border fructose transport rates and GLUT-5 mRNA levels during early weaning. In this study, these rates and levels were more than two times higher in the anastomosed intestine compared with those in the bypassed loop of weaning pups that underwent Thiry-Vella surgery and consumed high-fructose (HF) diets. In Thiry-Vella pups fed fructose-free (NF) diets, uptake rates and mRNA levels in the anastomosed intestine were very low and similar to those in the bypassed loop. In sham-operated littermates, transport rates and mRNA levels were similar between intestinal regions that corresponded to anastomosed and bypassed loops in Thiry-Vella pups and were two to three times greater in pups fed HF than in those fed NF diet. In contrast, rates of brush-border glucose transport and levels of SGLT-1 and of GLUT-2 mRNA were independent of diet and were similar between bypassed and anastomosed regions. Changes in GLUT-5 expression did not follow a distinct diurnal rhythm. When pups were fed HF diet after 12 h of starvation to empty the intestinal lumen, fructose transport rates increased with feeding duration and reached a plateau 12-24 h after feeding; in contrast, GLUT-5 mRNA levels were highest within 4 h after arrival of chyme in the jejunum and then decreased gradually and returned to baseline levels 24 h later. In littermates fed NF diet, mRNA levels and uptake rates were each independent of feeding duration. Luminal, and not endocrine, signals regulate GLUT-5 expression in weaning pups.

Effects of changes in calorie intake on intestinal nutrient uptake and transporter mRNA levels in aged mice.

In aged, chronically calorie-restricted (CR) mice, intestinal nutrient uptake is significantly higher than in same-age ad libitum controls. Can this chronic restriction-induced enhancement of uptake be reversed by ad libitum feeding? We addressed this question by switching 32-mo-old chronically CR mice to ad libitum feeding for 4 wk (CRAL). Intestinal transport rate and total intestinal absorptive capacity for D-sugars and several nonessential L-amino acids decreased significantly in CRAL mice. In contrast, switching CR mice to an ad libitum regimen for only 3 d had no effect on intestinal nutrient transport, indicating that the negative effects of ad libitum feeding require a duration longer than the 3-d lifetime of most enterocytes. Permeability of the intestinal mucosa to L-glucose was independent of the switches in diet. Levels of the brushborder glucose transporter SGLT1, brushborder fructose transporter GLUT5, and basolateral sugar transporter GLUT2 mRNA as determined by reverse transcriptase-polymerase chain reaction in 6-, 24-, and 32-mo-old mice were each apparently independent of caloric restriction and age. We conclude that the high rates of intestinal nutrient uptake exhibited by chronically CR mice can be reversed by ad libitum feeding of only 1 mo duration. These decreases in uptake were due mainly to specific decreases in transport per unit weight of intestine and not to nonspecific decreases in intestinal mass. Changes in rates of sugar uptake induced by chronic CR and age are apparently not accompanied by changes in steady-state levels of mRNA coding for those transporters.

Effect of aging and caloric restriction on intestinal sugar and amino acid transport.

The incidence of intestinal nutrient malabsorption increases with age. Therefore, an important question is whether there are age-related changes in intestinal nutrient absorption which may contribute to a decline in absorptive capacity. Sugar and amino acid transport per mg intestine generally decreases with age. The proximate mechanism underlying this age-related decrease in transport activity is a decrease in number of transporters per mg. This reduction in transporter number can be caused by age-related changes in cell proliferation rates which, in turn, can alter the ratio of absorptive to nonabsorptive cells. The age-related change in proliferation rates typically increases intestinal mass. There seems to be no age-related changes in the steady state levels of transporter mRNA. Aging also modestly impairs the ability of intestinal nutrient transport systems to adapt to changes in dietary conditions. Caloric restriction is the only procedure known to consistently increase the lifespan of mammals. Chronic caloric restriction markedly enhances intestinal nutrient transport per mg without affecting intestinal mass. Since body weight decreases with caloric restriction, there is a dramatic increase in intestinal absorptive capacity normalized to body weight. This suggests that an increase in intestinal nutrient absorption may be a critical adaptation to caloric restriction. There is a need to perform in vivo transport studies during senescence, to distinguish between acute and chronic effects of caloric restriction, and to identify hormones that may mediate aging and caloric restriction effects on intestinal nutrient transport.

Dietary fructose enhances intestinal fructose transport and GLUT5 expression in weaning rats.

Rates of fructose uptake by the small intestine of neonatal rats are typically very low from parturition through weaning but undergo a dramatic increase immediately after weaning is completed. In this study, we used intestinal fructose transport as a model to determine whether nutrient transport, normally enhanced only after completion of weaning, can be enhanced earlier during development. We found that ontogenetic changes in levels of GLUT5 mRNA correlate well with already known ontogenetic changes in rates of intestinal fructose transport: low levels and rates during suckling and weaning, and high levels and rates after weaning. In contrast, levels of GLUT2 and SGLT1 mRNA were relatively more elevated throughout the suckling and weaning periods. We then found that increased expression of GLUT5 mRNA caused by dietary fructose or sucrose paralleled diet-dependent increases in brush-border fructose uptake. Rates of brush-border glucose uptake and levels of SGLT1 and GLUT2 mRNA were not enhanced by dietary fructose, glucose, or sucrose. Finally, we found that rates of fructose uptake, levels of GLUT5 mRNA, and specific sucrase activity each increased with increasing concentrations of dietary fructose given precociously to midweaning rats. In contrast, brush-border glucose uptake was independent of dietary fructose concentration. Thus precocious introduction of dietary fructose causes enhanced expression of fructose transporters earlier during development. This effect is specific: only luminal fructose is effective, and only brush-border fructose transport can be modulated. These results unveil the potential for regulating nutrient transport early in development.

Precocious enhancement of intestinal fructose uptake by diet in adrenalectomized rat pups.

Intestinal fructose transport typically increases 3-fold after completion of weaning (> 28 d of age) in rats allowed to wean normally. Precocious enhancement of fructose transport has been demonstrated in rats fed high fructose diets during early weaning. To determine the role of corticosterone in the enhancement of fructose uptake by diet, we fed 17-d-old rat pups, previously adrenalectomized or sham-operated at 10 d of age, high (65%) fructose or fructose-free diets for 3 d. Corticosterone levels in 20-d-old sham-operated and unoperated controls were 2.2-3.3-fold higher than those in adrenalectomized littermates and in unoperated 10-d-old pups. Fructose uptake per mg and per cm were each 2.0-2.5-fold higher in adrenalectomized and sham-operated pups fed high fructose diets compared with those in adrenalectomized and sham-operated littermates fed fructose-free diets or to those in unoperated littermates allowed to wean normally with the dam. An increase in levels of GLUT5 mRNA in pups fed high fructose diets paralleled the increase in rates of fructose uptake. Intestinal glucose uptake was independent of corticosterone levels and of diet. Thus, the corticosterone surge is not necessary for the precocious enhancement of intestinal fructose transport and of GLUT5 mRNA expression by dietary fructose during weaning.

Intestinal absorption of water-soluble vitamins in rainbow trout (Onchorhynchus mykiss).

The intestinal uptake of water-soluble vitamins, nicotinamide, riboflavin, biotin and folic acid, was studied in isolated everted intestinal sleeves of the cold-water teleost rainbow trout (Onchorhynchus mykiss). The presence of a carrier-mediated transport mechanism was determined by competitive inhibition and by Michaelis-Menten kinetics. The uptake of riboflavin, biotin or folic acid was not only subject to competitive inhibition but also a saturable function of increasing vitamin concentration in the incubation medium. The kinetic constants of the saturable mechanism were for riboflavin: K(m), 2.32 +/- 0.76 microM; Vmax, 0.26 +/- 0.04 pmol/mg min; for biotin: K(m), 9.70 +/- 3.76 microM; Vmax, 0.31 +/- 0.07 pmol/mg min; and for folic acid: K(m), 32.9 +/- 21.2 microM; Vmax, 3.63 +/- 0.99 pmol/mg min. In contrast, the uptake of nicotinamide was not subject to competitive inhibition and was a linear function of concentration (Kd, 0.140 +/- 0.012 pmol/mg min microM). Folic acid was absorbed more rapidly than and was not inhibited by its derivative, 5-methyl-tetrahydrofolate. Thus, the intestinal uptake of riboflavin, biotin and folic acid is carrier-mediated while that of nicotinamide occurs by simple diffusion. These mechanisms are similar to those found in the channel catfish for the same vitamins, except for folic acid, which is absorbed by diffusion in this warm-water omnivorous species.

Regulation of intestinal sugar transport.

The recent surge in knowledge of cellular and molecular mechanisms of intestinal sugar transport has fueled an enormous interest in adaptive mechanisms regulating sugar transport. We first review several functional considerations that help us interpret the different patterns of adaptation for different nutrients. We then distinguish nonspecific adaptive mechanisms leading to parallel changes in transport of different nutrients from specific adaptive mechanisms only affecting the transport of a single nutrient. Nonspecific adaptive mechanisms include changes in mucosal surface area and in the ratio of transporting to nontransporting cells; specific mechanisms include changes in site density of transporters and in affinity constants. We also enumerate the patterns of regulation and describe how sugar transport is affected by changes in diet, energy budgets, and environmental salinity as well as by intestinal resection, starvation, stress, and age. We relate the various signals linking these stimuli to adaptive mechanisms and make predictions about the nature of these signals. Finally, we describe the significance of the interactions among sugar, fluid, and electrolyte transport mechanisms and of the paracellular pathway to transepithelial transport of sugars. We close by drawing attention to promising directions for future research.

Adaptations of intestinal nutrient transport to chronic caloric restriction in mice.

Lifelong caloric restriction increases median and maximum life span and retards the aging process in many organ systems of rodents. Because the small intestine absorbs a reduced amount of nutrients each day, does lifelong caloric restriction induce adaptations in intestinal nutrient transport? We initially compared intestinal transport of sugars and amino acids between 24-mo-old mice allowed free access to food [ad libitum (AL)] and those provided a calorically restricted [40% less than ad libitum (CR)] diet since 3 mo of age. We found that CR mice had significantly greater transport rates for D-glucose, D-fructose, and several amino acids and had significantly lower villus heights. Total intestinal absorptive capacities for D-glucose, D-fructose, and L-proline were each 40-50% greater in CR mice; absorptive capacity normalized to metabolic mass (body weight 0.75) was approximately 80% greater in CR mice. Comparison of uptakes in aged AL and CR mice with previously published results in young AL mice suggests that caloric restriction delays age-related decreases in nutrient transport. In contrast to published studies in hibernation and starvation, chronic caloric restriction enhances not only uptake per milligram but also uptake per centimeter. We then switched 24-mo-old AL mice to a calorie-restricted diet for 1 mo and found that short-term caloric restriction has no effect on intestinal nutrient transport, intestinal mass, and total absorptive capacity. Thus chronic but not short-term caloric restriction increases intestinal nutrient transport rates in aged mice, and the main mechanism underlying these increases is enhanced transport rates per unit intestinal tissue weight.