Compartmental modeling of whole-body vitamin A kinetics in unsupplemented and vitamin A-retinoic acid-supplemented neonatal rats.

Little is known about the contribution of different tissues to whole-body vitamin A (VA) kinetics in neonates. Here, we have used model-based compartmental analysis of tissue tracer kinetic data from unsupplemented (control) and VA-retinoic acid (VARA)-supplemented neonatal rats to determine VA kinetics in specific tissues under control and supplemented conditions. First, compartmental models for retinol kinetics were developed for individual tissues, and then an integrated compartmental model incorporating all tissues was developed for both groups. The models predicted that 52% of chylomicron (CM) retinyl ester was cleared by liver in control pups versus 22% in VARA-treated pups, whereas about 51% of VA was predicted to be extrahepatic in 4- to 6-day-old unsupplemented neonatal rats. VARA increased CM retinyl ester uptake by lung, carcass, and intestine; decreased the release into plasma of retinol that had been cleared by liver and lung as CM retinyl esters; stimulated the uptake of retinol from plasma holo-retinol binding protein into carcass; and decreased the retinol turnover out of the liver. Overall, neonatal VA trafficking differed from that previously described for adult animals, with a larger contribution of extrahepatic tissues to CM clearance, especially after VA supplementation, and a significant amount of VA distributed in extrahepatic tissues.

Retinol kinetics in unsupplemented and vitamin A-retinoic acid supplemented neonatal rats: a preliminary model.

Vitamin A (VA) metabolism in neonates is virtually uncharacterized. Our objective was to develop a compartmental model of VA metabolism in unsupplemented and VA-supplemented neonatal rats. On postnatal day 4, pups (n = 3/time) received 11,12-[(3)H]retinol orally, in either oil (control) or VA combined with retinoic acid (VARA) [VA (∼6 mg/kg body weight) + 10% retinoic acid]. Plasma and tissues were collected at 14 time points up to 14 days after dose administration. VARA supplementation rapidly, but transiently, increased total retinol mass in plasma, liver, and lung. It decreased the peak fraction of the dose in plasma. A multi-compartmental model developed to fit plasma [(3)H]retinol data predicted more extensive recycling of retinol between plasma and tissues in neonates compared with that reported in adults (144 vs. 12-13 times). In VARA pups, the recycling number for retinol between plasma and tissues (100 times) and the time that retinol spent in plasma were both lower compared with controls; VARA also stimulated the uptake of plasma VA into extravascular tissues. A VARA perturbation model indicated that the effect of VARA in stimulating VA uptake into tissues in neonates is both dramatic and transient.

The neural signature of satiation is associated with ghrelin response and triglyceride metabolism.

Eating behavior is guided by a complex interaction between signals conveying information about energy stores, food availability, and palatability. How peripheral signals regulate brain circuits that guide feeding during sensation and consumption of a palatable food is poorly understood. We used fMRI to measure brain response to a palatable food (milkshake) when n=32 participants were fasted and fed with either a fixed-portion or ad libitum meal. We found that larger post-prandial reductions in ghrelin and increases in triglycerides were associated with greater attenuation of response to the milkshake in brain regions regulating reward and feeding including the midbrain, amygdala, pallidum, hippocampus, insula and medial orbitofrontal cortex. Satiation-induced brain responses to milkshake were not related to acute changes in circulating insulin, glucose, or free fatty acids. The impact of a meal on the response to milkshake in the midbrain and dorsolateral prefrontal cortex differed depending upon whether meal termination was fixed or volitional, irrespective of the amount of food consumed. We conclude that satiation-induced changes in brain response to a palatable food are strongly and specifically associated with changes in circulating ghrelin and triglycerides and by volitional meal termination.

Streptococcus pneumoniae-induced pneumonia and Citrobacter rodentium-induced gut infection differentially alter vitamin A concentrations in the lung and liver of mice.

In the developing world, vitamin A (VA) deficiency is endemic in populations that are also at great risk of morbidity and mortality because of pneumococcal pneumonia and enteric infections. To better understand how lung and gastrointestinal pathogens affect VA status, we assessed VA concentrations in serum, lung, and liver during an invasive pneumonia infection induced by Streptococcus pneumoniae serotype 3, and a noninvasive gut infection induced by Citrobacter rodentium, in vitamin A-adequate (VAA) and vitamin A-deficient (VAD) mice. For pneumonia infection, mice were immunized with pneumococcal polysaccharide serotype 3 (PPS3), or not (infected-control), 5 d prior to intranasal inoculation with S. pneumoniae. Two days post-inoculation, immunization was protective against systemic infection regardless of VA status as PPS3 immunization decreased bacteremia compared with infected-control mice (P < 0.05). Retinol concentrations in the lung were higher in infected-control VAA mice (15.7 nmol/g: P < 0.05) compared with PPS3-immunized mice (8.23 nmol/g), but this was not associated with increased lung bacterial burden. VAA mice had reduced severity of C. rodentium-induced gut infection as measured by fecal bacterial shedding compared with VAD mice (P < 0.05). Liver retinol and retinyl ester concentrations in VAA mice decreased at the peak of infection (retinol, 8.1 nmol/g; retinyl esters, 985 nmol/g; P < 0.05, compared with uninfected mice; retinol, 29.5 nmol/g; retinyl esters, 1730 nmol/g), whereas tissue VA concentrations were low in VAD mice during both infections. Colonic mucin gene expression was also depressed at peak infection compared with uninfected mice (P < 0.05). Overall, pneumonia had less effect on VA status than gastrointestinal infection, predominantly owing to reduced hepatic VA storage at the peak of gut infection.

Lipid emulsion administered intravenously or orally attenuates triglyceride accumulation and expression of inflammatory markers in the liver of nonobese mice fed parenteral nutrition formula.

The accumulation of hepatic TG and development of hepatic steatosis (HS) is a serious complication of the use of parenteral nutrition (PN) formulas containing a high percentage of dextrose. But whether fat emulsions or other nutrients can ameliorate the induction of HS by high-carbohydrate diets is still uncertain. We hypothesized that administration of a lipid emulsion (LE; Intralipid) and/or the vitamin A metabolite retinal (RAL) will reduce hepatic TG accumulation and attenuate indicators of inflammation. C57BL/6 male mice were fed PN formula as their only source of hydration and nutrition for 4-5 wk. In Expt. 1, mice were fed PN only or PN plus treatment with RAL (1 µg/g orally), LE (200 µL i.v.), or both LE and RAL. In Expt. 2, LE was orally administered at 4 and 13.5% of energy to PN-fed mice. All PN mice developed HS compared with mice fed normal chow (NC) and HS was reduced by LE. The liver TG mass was lower in the PN+LE and PN+RAL+LE groups compared with the PN and PN+RAL groups (P < 0.01) and in the 4% and 13.5% PN+LE groups compared with PN alone. Hepatic total retinol was higher in the RAL-fed mice (P < 0.0001), but RAL did not alter TG mass. mRNA transcripts for fatty acid synthase (Fasn) and sterol regulatory element-binding protein-1c (Srebpf1) were higher in the PN compared with the NC mice, but FAS protein and Srebpf1 mRNA were lower in the PN+LE groups compared with PN alone. The inflammation marker serum amyloid P component was also reduced. In summary, LE given either i.v. or orally may be sufficient to reduce the steatotic potential of orally fed high-dextrose formulas and may suppress the early development of HS during PN therapy.

Oral vitamin A and retinoic acid supplementation stimulates antibody production and splenic Stra6 expression in tetanus toxoid-immunized mice.

Coadministration of retinoic acid (RA) and polyinosinic acid:polycytidylic acid (PIC) has been shown to cooperatively enhance the anti-tetanus toxoid (anti-TT) vaccine response in adult mice. Germinal center formation in the spleen is critical for a normal antibody response. Recent studies have identified Stimulated by Retinoic Acid-6 (Stra6) as the cell membrane receptor for retinol-binding protein (RBP) in many organs, including spleen. The objectives of the present studies were to test whether orally administered vitamin A (VA) itself, either alone or combined with RA, and/or treatment with PIC regulates Stra6 gene expression in mouse spleen and, concomitantly, antibody production. Eight-week-old C57BL/6 mice were immunized with TT. In an initial kinetic study, oral VA (6 mg/kg) increased anti-TT IgM and IgG production as well as splenic Stra6 mRNA expression. In treatment studies that were analyzed 9 d postimmunization, retinoids including VA, RA, VA and RA combined, and PIC significantly increased plasma anti-TT IgM and IgG (P < 0.05) and splenic Stra6 mRNA (P < 0.05). Treatments that included PIC elevated plasma anti-TT IgM and IgG concentrations >20-fold (P < 0.01). Immunohistochemistry of STRA6 protein in mouse spleen confirmed its increase after immunization and retinoid treatment. In conclusion, retinoid treatments that included VA, RA, VA and RA combined, and the combination of retinoid and PIC stimulated the expression of Stra6 in spleen, which potentially could increase the local uptake of retinol. Concomitantly, these treatments increased the systemic antigen-specific antibody response. The ability of oral retinoids to stimulate systemic immunity has implications for public health and therapeutic use of VA.

Perinatal exposure to vitamin A differentially regulates chondrocyte growth and the expression of aggrecan and matrix metalloprotein genes in the femur of neonatal rats.

Vitamin A (VA) and its active form, retinoic acid (RA), are regulators of skeletal development. In the present study, we investigated if maternal VA intake during pregnancy and lactation, as well as direct oral supplementation of neonates with VA + RA (VARA) in early life, alters neonatal bone formation and chondrocyte gene expression. Offspring of dams fed 3 levels of VA (marginal, adequate, and supplemented) for 10 wk were studied at birth (P0) and postnatal day 7 (P7). One-half of the newborns received an oral supplement of VARA on P1, P4, and P7. Tissues were collected on P0 and 6 h after the last dose on P7. Pup plasma and liver retinol concentrations were increased by both maternal VA intake and VARA (P < 0.01). Although maternal VA did not affect bone mineralization as assessed by von Kossa staining, newborn femur length was increased with maternal VA (P < 0.05). VARA supplementation of neonates increased the length of the hypertrophic zone only in VA-marginal pups, close to that in neonates from VA-adequate dams, suggesting VARA caused a catching up of growth that was limited by low maternal VA intake. Maternal diet did not alter type X nor type II collagen mRNA. However, VARA-treated pups from VA-supplemented dams had reduced mRNA for aggrecan, a major component of cartilage matrix, and increased mRNA for matrix metalloproteinase (MMP)13, which catalyzes the degradation of aggrecan and collagens. These results suggest that moderately high maternal VA intake combined with neonatal VARA supplementation can reduce the ratio of aggrecan:MMP, which may unfavorably alter early bone development.

Cortical and trabecular bone, bone mineral density, and resistance to ex vivo fracture are not altered in response to life-long vitamin A supplementation in aging rats.

High vitamin A (VA) intakes have been correlated with increased risk of bone fracture. Over 50% of the U.S. adult population reports use of dietary supplements, which can result in VA intakes > 200% of the RDA. In this study, 2 experiments were designed to determine the effect of dietary VA on cortical and trabecular bone properties and resistance to ex vivo fracture. In Expt. 1, we investigated whether orally administered VA accumulates in bone. Seven-week-old rats were treated daily with VA (6 mg/d for 14 d). Total retinol increased in both the tibia and femur (P < 0.01). In Expt. 2, we conducted a longitudinal study in which rats were fed 1 of 3 levels of dietary VA (marginal, adequate, and supplemented, equal to 0.35, 4, and 50 µg retinol/g diet, respectively) from weaning until the ages of 2-3 mo (young), 8-10 mo (middle-age), and 18-20 mo (old). Tibial trabecular and cortical bone structure, bone mineral density, and resistance to fracture were measured using micro-computed tomography and material testing system analysis, respectively. The VA-marginal diet affected measures of cortical bone dimension, suggesting bone remodeling was altered. VA supplementation increased medullary area and decreased cortical thickness in young rats (P < 0.05), but these changes were not present during aging. VA supplementation did not affect resistance to fracture or bone mineral content in old rats. From these results, we conclude that VA-marginal status affects trabecular bone more than cortical bone, and VA supplementation at a moderate level over the lifetime is unlikely to increase the risk of age-related bone fracture in rats.