运动诱导的低铁状态大鼠骨髓细胞铁摄入的变化

本文观察了运动性低铁状态大鼠骨髓细胞转铁蛋白 (Tf)结合铁和非Tf结合铁摄入的变化。大鼠随机分为 6个月的运动组 (EG)和对照组 (SG)。SG平均每个幼红细胞Tf受体数为 890 15 0± 16 4849个 ,而在EG为 2 17536 0± 46 2 737个 (P <0 0 5 ) ,但受体的解离常数不受运动影响。EG中Tf的内吞平台和胞内铁聚积速度显著高于SG ,胞浆和胞内膜性成分中Tf结合铁和Fe(Ⅱ )摄入增加。EG的胞浆内Fe(Ⅱ )摄入的米氏常数值降低 ;细胞膜性成分中Fe(Ⅱ )摄入的最大速度增加。上述结果表明 ,运动不仅通过增加Tf受体的表达促进Tf结合铁的内吞 ,而且增强非Tf结合铁的内吞途径。尽管这些变化的机制尚不清楚 ,但它们有利于运动时血红素的合成     

Increased expression of transferrin receptor on membrane of erythroblasts in strenuously exercised rats.

This study investigated the effects of strenuous exercise on transferrin (Tf)-receptor (TfR) expression and Tf-bound iron (Tf-Fe) uptake in erythroblasts of rat bone marrow. Female Sprague-Dawley rats were randomly assigned to either an exercise or sedentary group. Animals in the exercise group swam 2 h/day for 3 mo in a glass swimming basin. Both groups received the same amount of handling. At the end of 3 mo, the bone marrow erythroblasts were freshly isolated for Tf-binding assay and determination of Tf-Fe uptake in vitro. Tissue nonheme iron and hematological iron indexes were measured. The number of Tf-binding sites found in erythroblasts was approximately 674,500 +/- 132,766 and 1,270,011 +/- 235,321 molecules/cell in control and exercised rats, respectively (P < 0. 05). Total Fe and Tf uptake by the cells was also significantly increased in the exercised rats after 30 min of incubation. Rates of cellular Fe accumulation were 5.68 and 2.58 fmol. 10(6) cells(-1). min(-1) in the exercised and control rats, respectively (P < 0.05). Tf recycling time and TfR affinity were not different in exercised and control rats. Increased cellular Fe was mainly located in the stromal fraction, suggesting that most of accumulated Fe was transported to the mitochondria for heme synthesis. The findings demonstrated that the increased cellular Fe uptake in exercised rats was a consequence of the increased TfR expression rather than the changes in TfR affinity and Tf recycling time. The increase in TfR expression and cellular Fe accumulation, as well as the decreased serum Fe concentration and nonheme Fe in the liver and the spleen induced by exercise, probably represented the early signs of Fe deficiency.     

Increased nitric oxide is one of the causes of changes of iron metabolism in strenuously exercised rats

This study was carried out to investigate the possible role of increased nitric oxide (NO) production in the development of the low iron status in strenuously exercised rats. Female Sprague-Dawley rats were randomly assigned to four groups: sedentary (S1), sedentary + nitro-L-arginine methyl ester (L-NAME; S2), exercise (E1), and exercise + L-NAME (E2). Animals in the E1 and E2 groups swam for 2 h/day for 3 mo. L-NAME in the drinking water (1 mg/ml) was administrated to rats in the S2 and E2 groups for the same period. At the end of third month, hematological indexes and nitrite and nitrate (NOx) contents in the plasma and non-heme iron and NOx levels in the liver, spleen, and bone marrow cells were measured. Three months of exercise induced a significant increase in NOx content and a decrease in iron level both in plasma and tissues. Treatment with L-NAME, an inhibitor of NO synthase (NOS), led to a significant decrease in NOx and an increase in iron level both in plasma and tissues in the exercised rats. The E2 group had a significantly lower NOx content as well as a higher iron level both in plasma and tissues than the E1 group. However, the iron contents in the plasma and tissues of the E2 group were still significantly lower than those found in S1. No difference was found in NOx levels between E2 and S1. These findings showed that exercise was associated with elevation in NOx and reduction in iron in plasma and the tissues. Treatment with L-NAME was able to completely inhibit the effect of exercise on NOx as well as partly recover the decreased iron contents in plasma and tissues resulting from exercise. This suggests that the increased production of NO might be one of the causes of the lower iron status in exercised rats.     

Molecular analysis of increased iron status in moderately exercised rats

Although iron plays a critical role in exercise, the regulatory mechanism of iron metabolism remains poorly understood. The aims of the present study were to investigate the effects of different intensity exercise on body iron status and the regulatory mechanism of duodenal iron absorption. Thirty female Sprague-Dawley rats (90–100 g) were randomly divided into three groups: a control group (remained sedentary, CG), a moderately exercised group (swam 1.5 h/day, MG) and a strenuously exercised group (swam with different load, SG). Serum iron status, serum ferritin and Hct were examined after 10 weeks of swimming. Western blot was performed to detect the expression of iron transport proteins: divalent metal transporter1 (DMT1) and ferroportin 1 (FPN1) in duodenal epithelium. The expression of hepcidin mRNA in liver was examined by RT-PCR. The results showed: (1) the body iron status in MG was kept at a high level compared to that of CG and SG, (2) Western blot showed DMT1 with iron responsive element (IRE) and FPN1 in duodenal epithelium which were higher in MG than that of CG and (3) the expression of hepatic hepcidin mRNA was down regulated in MG (p < 0.05). The data suggested that moderate exercise improved iron status and that was likely regulated by increased DMT1 with IRE and FPN1 expression. Hepcidin signaling pathway may involve in the regulation of duodenal iron absorption proteins. Xiang Lin Duan and Yan Zhong Chang share Senior Authorship     

Demonstration of a nuclear androgen receptor in erythroblasts obtained by culture of human bone marrow

The new techniques of culture of bone marrow have shown that androgens and 5 beta steroids exert a direct effect on erythroid precursor cells from human or animal bone marrow. By contrast, the mechanisms of the intracellular actions of those compounds are poorly understood. Tritiated methyltrienolone (R 1881), a synthetic androgen that highly binds to androgen receptor, has been used for the study of a binding activity in nuclear extracts of cultured erythroblasts from human bone marrow. The nuclear extracts contain binding sites saturable at low concentrations of 3H-R 1881 (20-30 nM). Scatchard analysis revealed that the R 1881-nuclear complex has a dissociation constant (Kd) of 25-50 nM, and the number of binding sites was 235-370 fmoles/mg protein. The complex sedimented on 5-20% sucrose gradient in the 3.9 S region and 5 beta dihydrotestosterone compete strongly with R 1881 for binding sites. This binding component has characteristics of high affinity, low-capacity, sedimentation behaviour, and specificity commonly attributed to "androgen receptors".     

The effect of heterologous transferrin on the uptake of iron and haem synthesis by bone marrow cells

The initial process of transfer of extracellular iron to the haem-synthesizing mitochondria of immature erythroid cells is the association of iron-transferrin with the cell membrane. When rat bone marrow cells were incubated in the presence of iron bound to rat transferrin, iron uptake was higher than in the presence of iron bound to heterologous transferrin. The relative activities of the various isolated transferrins towards rat transferrin were found to be approximately 0.3, 0.8, 0.1 and 0.04 for rabbit, human, bovine and fish (tench, Tinca tinca) transferrin, respectively, and 0.7, 0.7 and 0.15 for mouse, guinea pig and calf serum, respectively, as compared with rat serum. Although great difference exist in cellular uptake of iron bound to different species of transferrin, the subcellular distribution of ⁵⁹Fe was quite similar. In all cases about 60% of the radioactivity taken up by the cells could be recovered in the haemin fraction and only about 15% in each the membrane and the non-haem soluble cell fraction. Similar results were obtained with guinea pig bone marrow cells.From the results of the experiments presented it might be concluded that the species of transferrin plays an important role during the initial stages of iron uptake by bone marrow cells, whereas the intracellular iron transfer process is not influenced by the species of transferrin.     

Mobilization of ferritin iron in erythroblasts by chelating agents

Intracellular ferritin in newt (Triturus cristatus) erythroblasts was accessible to the chelating effects of EDTA and pyridoxal phosphate. EDTA (0.5-1 mM) promoted release of radioactive iron from ferritin of pulse-labelled erythroblasts during chase incubation, but its continuous presence was not necessary for ferritin iron mobilization. Brief exposure to EDTA was sufficient to release 60-70% of ferritin 59Fe content during ensuing chase in EDTA-free medium. EDTA also suppressed cellular iron uptake and utilization for heme synthesis, but these activities were restored upon its removal. Pyridoxal-5'-phosphate (0.5-5 mM) also stimulated loss of radioactive iron from ferritin; however, ferritin iron release by pyridoxal phosphate required its continued presence. Unlike EDTA, pyridoxal phosphate did not interfere with iron uptake or its utilization for heme synthesis. Chelator-mobilized ferritin iron accumulated initially in the hemolysate as a low-molecular-weight component and appeared to be eventually released into the medium. No radioactive ferritin was found in the medium of chelator-treated cells, indicating that secretion or loss of ferritin was not responsible for decreasing cellular ferritin 59Fe content. Moreover, there was no transfer of radioactive iron between the low-molecular-weight component released into the medium and plasma transferrin. These results indicate that chelator-released ferritin iron is not available for cellular utilization in heme synthesis and that ferritin iron released by this process is not an alternative or complementary iron source for heme synthesis. Correlation of these data with effects of succinylacetone inhibition of heme synthesis and with previous studies indicates that the main role of erythroid cell ferritin is absorption and storage of excess iron not used for heme synthesis.     

Plasma nitric oxide and iron concentrations in exercised rats are negatively correlated

The aim of this study was to investigate the effect of strenuous exercise on plasma nitric oxide and iron (PI) concentrations in rats. The rats were divided into six groups: 3, 6 and 12 months of the exercise (swimming) groups and their corresponding controls. At the end of experimental periods, blood samples were collected to measure plasma NOx (nitrate and nitrite) and iron concentrations and other hematological indices. The correlative analysis of plasma NOx with PI in the exercised and the control rats was performed. The results showed that plasma NOx concentration was significantly greater and PI lower in the 3, 6, and 12 months of the exercise groups compared to their sedentary controls (p < 0.01). However, the duration of strenuous exercise had no significant effect on plasma NOx or PI contents. A negative correlation between plasma NOx and PI levels was found in all three exercise groups (r = -0.750, -0.578, and -0.808 and p < 0.01, 0.05, 0.01 respectively), but not in the sedentary control groups. These results imply that strenuous exercise may lead to an increase in plasma NOx concentration as well as a low iron level. They also suggest the possibility that the increased NO production might be associated with the development of the lower iron status in exercise.     

Influence of clenbuterol on bone metabolism in exercised or sedentary rats.

This paper reports that the selective beta(2)-adrenergic receptor agonist clenbuterol affects bone metabolism in growing 3-mo-old male Wistar rats treated over 8 wk. Thirty-two 3-mo-old growing Wistar rats weighing 234 +/- 2 g were assigned to a progressive isometric force, strength-training exercise program plus oral clenbuterol (2 mg x kg body wt(-1) x day(-1)) for 5 days each week, exercise program without clenbuterol 5 days each week, no exercise program plus oral clenbuterol (2 mg x kg(-1) x day(-1)) for 5 days each week, or no exercise without clenbuterol 5 days each week. At the end of 8 wk, lean mass, fat mass, and right total femoral, distal metaphyseal femoral, and diaphyseal femoral bone mineral density were measured by Hologic QDR 4,500 dual X-ray absorptiometry (DEXA) technique. Left femoral bones were harvested after death on day 58, and femoral resistance was determined by three-point bending testing. We found that fat mass was decreased in rats given strength training exercise and decreased further in rats treated with clenbuterol. Lean mass was increased in clenbuterol-treated animals. Strength-training exercise appeared to have no effect on bone mineral density, serum osteocalcin, or urinary deoxypyridinoline. However, clenbuterol treatment decreased femoral length, diameter, bone mineral density, and mechanical resistance. Clenbuterol had no effect on osteocalcin but increased urinary deoxypyridinoline. We concluded that clenbuterol treatment decreased bone mineral density and increased bone resorption independent of the level of exercise rats were given.     

Chylomicron metabolism. Chylomicron uptake by bone marrow in different animal species

Previously it was shown in rabbits that 20-40% of the injected dose of chylomicrons was cleared from the plasma by perisinusoidal bone marrow macrophages. The present study was undertaken to determine whether the bone marrow of other species also cleared significant amounts of chylomicrons. Canine chylomicrons, labeled in vivo with [14C]cholesterol and [3H] retinol, were injected into marmosets (a small, New World primate), rats, guinea pigs, and dogs. Plasma clearance and tissue uptake of chylomicrons in these species were contrasted with results obtained in rabbits in parallel studies. The chylomicrons were cleared rapidly from the plasma in all animals; the plasma clearance of chylomicrons was faster in rats, guinea pigs, and dogs compared with their clearance from the plasma of rabbits and marmosets. The liver was a major site responsible for the uptake of these lipoproteins in all species. However, as in rabbits, the bone marrow of marmosets accounted for significant levels of chylomicron uptake. The uptake by the marmoset bone marrow ranged from one-fifth to one-half the levels seen in the liver. The marmoset bone marrow also took up chylomicron remnants. Perisinusoidal macrophages protruding through the endothelial cells into the marrow sinuses were responsible for the accumulation of the chylomicrons in the marmoset bone marrow, as determined by electron microscopy. In contrast to marmosets, chylomicron clearance by the bone marrow of rats, guinea pigs, and dogs was much less, and the spleen in rats and guinea pigs took up a large fraction of chylomicrons. The uptake of chylomicrons by the non-human primate (the marmoset), in association with the observation that triglyceride-rich lipoproteins accumulate in bone marrow macrophages in patients with type I, III, or V hyperlipoproteinemia, suggests that in humans the bone marrow may clear chylomicrons from the circulation. It is reasonable to speculate that chylomicrons have a role in the delivery of lipids to the bone marrow as a source of energy and for membrane biosynthesis or in the delivery of fat-soluble vitamins.     

Ferritin in bone marrow and serum in iron deficiency and iron overload

Nonheme iron and ferritin in the bone marrow and serum ferritin was investigated in patients with iron deficiency anaemia or iron overload. As controls served patients without any disturbance of the iron metabolism. There is a precise correlation between the nonheme iron and ferritin in the bone marrow of patients with and without disturbance of iron metabolism. A correlation was also found between the ferritin in the bone marrow and the serum. Nonheme iron and ferritin in the bone marrow and serum ferritin was decreased in patients with iron deficiency anaemia. Conversely, the same parameters were increased in patients with iron overload.