Duodenal mucosal reductase in wild-type and Hfe knockout mice on iron adequate, iron deficient, and iron rich feeding

BACKGROUND

Genetic haemochromatosis is a common hereditary iron loading disorder in humans. The disease is associated with loss of function mutations in the HFE gene. This is thought to change iron stores via increased iron absorption.

AIMS

In this study we investigated how adaptation of mucosal reductase activity is engaged in this process and how the changes compare with adaptation seen when an iron deficient diet is fed.

METHODS

Duodenal mucosal surface reductase was measured with nitroblue tetrazolium in age matched groups of male Hfe knockout mice (Hfe) and wild- type mice fed a purified diet containing normal (iron adequate), high (iron rich), or low (iron deficient) iron concentrations.

RESULTS

Reductase activity increased when mice were fed an iron deficient diet and decreased when they were fed an iron rich diet. Total villus activity, as measured by the average area under the activity curve along the crypt-villus axis, was increased 2.8-2.9-fold by iron deficiency in both genotypes. Approximately half of this difference was attributable to the significantly increased length of the villi in mice on an iron deficient diet (p<0.05). Hfe knockout did not affect villus length but increased mucosal reductase activity near the villus tips. Similar increases (1.3-1.6-fold) were seen on all diets but the increase was significant for iron deficient and iron loaded diets only (p<0.05).

CONCLUSION

Hfe gene product and dietary iron downregulate villus reductase activity in mice.

Decomposition of cocoa procyanidins in the gastric milieu

There is considerable interest in the bioavailability of flavonoids and phenolic components of the diet and their bioactivity in vivo. However, little is known of pre-absorption events in the gastric lumen. The effects of the acidic environment, as found in the gastric milieu, on procyanidin oligomers of catechin polyphenols has been investigated. The results show that under these conditions the procyanidin oligomers (trimer to hexamer) are hydrolysed to mixtures of epicatechin monomer and dimer, thus enhancing the potential for their absorption in the small intestine.

Interactions of the flavonoid naringenin in the gastrointestinal tract and the influence of glycosylation

We have studied interactions in the gastrointestinal tract of flavonoids and the influence of glycosylation on their subsequent metabolism by examining the urinary recoveries of the flavonoid naringenin-7-glucoside and its aglycone, in the conscious rat model, after oral and intravenous administration. Absorption studies were also conducted using an in vitro isolated rat jejunum. The results show that ca. 10% of the administered dose of naringenin was recovered after oral dosing, the majority as naringenin glucuronide, whereas, after intravenous administration, the recovery of the glucuronide was ca. 20%. In contrast, after oral dosing of naringenin-7-glucoside, its hydrolysis product naringenin (0.5%) and naringenin glucuronide (12.7%) were detected. After intravenous dosing the majority of that identified in the urine was as the native glucoside. These findings suggest that, via the oral route, the glycoside group is cleaved by an intestinal enzyme prior to glucuronidation within the epithelium. This is substantiated by the urinary elimination of the native glucoside and the lack of detection of glucuronide after intravenous administration. Transport studies with isolated intestine showed that neither unchanged naringenin nor the 7-glucoside was absorbed in significant quantities across the gut wall. The major metabolite detected in both cases was naringenin glucuronide, thus supporting the notion that glucuronidation as well as hydrolysis can occur at the intestinal epithelium.

Urinary excretion of hydroxycinnamates and flavonoids after oral and intravenous administration

The urinary recoveries of the hydroxycinnamates, ferulic acid (3-methoxy, 4-hydroxy cinnamic acid), and chlorogenic acid (the quinic acid ester of 3,4-dihydroxycinnamic acid), and three structurally related flavonoids were studied in the rat. For the latter, the aglycone quercetin was compared with its 3-glucoside (isoquercitrin) and 3-rhamnoglucoside (rutin). Doses of 50 mg/kg were administered via the oral and intravenous routes and urine collected over the subsequent 24-h period. Reverse phase HPLC with photo-diode array detection was used to analyze the unchanged compound and their metabolites excreted in the urine. Ferulic acid and isoquercitrin were orally absorbed (5.4 and 0.48% of administered dose, respectively) and are therefore bioavailable. In contrast, neither unchanged chlorogenic acid, rutin, quercetin, nor the conjugated metabolites in the form of glucuronide or sulphate were detected in the urine after oral dosing. All the flavonoids studied produced low total urinary recoveries after intravenous administration, 9.2% for quercetin-3-rhamnoglucoside, 6.7% for the 3-glucoside, and 2.4% for the aglycone, indicating that extensive metabolism to low molecular weight compounds or excretion via other routes may be occurring. Overall it can be stated that renal excretion is not a major pathway of elimination for intact flavonoids and hydroxycinnamates in the rat.

Transcriptional and translational control over sodium-glucose-linked transporter (SGLT1) gene expression in adult rat small intestine

We have measured SGLT1 mRNA content and SGLT1-mediated glucose transport at different positions along the small intestine of control and streptozotocin diabetic rats and shown both parameters to be similar but higher in jejunal compared with ileal tissue. No such correlation was seen when comparing measurements of SGLT1 mRNA along jejunal villi with previous estimates of SGLT1 protein and SGLT1-mediated glucose transport [Debnam et al., Eur. J. Physiol. 430 (1995) 151-159]. This is the first time it has been possible to directly relate these three aspects of SGLT1 gene expression in a single species. Results are discussed in terms of a possible time rather than positional control over translation of SGLT1 mRNA.