Overexpression of apolipoprotein A-IV enhances lipid transport in newborn swine intestinal epithelial cells

Apolipoprotein A-IV (apoA-IV) has myriad functions, including roles as a post-prandial satiety factor and lipid antioxidant. ApoA-IV is expressed in mammalian small intestine and is up-regulated in response to lipid absorption. In newborn swine jejunum, a high fat diet acutely induces a 7-fold increase in apoA-IV expression. To determine whether apoA-IV plays a role in the transport of absorbed lipid, swine apoA-IV was overexpressed in a newborn swine enterocyte cell line, IPEC-1, followed by analysis of the expression of genes related to lipoprotein assembly and lipid transport, as well as quantitation of lipid synthesis and secretion. A full-length swine apoA-IV cDNA was cloned, sequenced, and inserted into a Vp and Rep gene-deficient adeno-associated viral vector, containing the cytomegalovirus immediate early promoter/enhancer and neomycin resistance gene, and was used to transfect IPEC-1 cells. Control cells were transfected with the same vector minus the apoA-IV insert. Using neomycin selection, apoA-IV-overexpressing (+AIV) and control (-AIV) clones were isolated for further study. Both undifferentiated (-D) and differentiated (+D) +AIV cells expressed 40- to 50-fold higher levels of apoA-IV mRNA and both intracellular and secreted apoA-IV protein compared with -AIV cells. Expression of other genes was not affected by apoA-IV overexpression in a manner that would contribute to enhanced lipid secretion. +D +AIV cells secreted 4.9-fold more labeled triacylglycerol (TG), 4.6-fold more labeled cholesteryl ester (CE), and 2-fold more labeled phospholipid (PL) as lipoproteins, mostly in the chylomicron/very low density lipoprotein (VLDL) density range. ApoA-IV overexpression in IPEC-1 cells enhances basolateral TG, CE, and PL secretion in chylomicron/VLDL particles. This enhancement is not associated with up-regulation of other genes involved in lipid transport. ApoA-IV may play a role in facilitating enterocyte lipid transport, particularly in the neonate receiving a diet of high fat breast milk.     

Mediated transport and metabolism of lactate in rat aorta.

1. Under appropriate conditions L- and D-lactate enter the cells of rat aorta and are metabolized. Oxidation of lactate to CO2 occurs under aerobic conditions. 2. L- and D-lactate are taken up into the cells when oxygen, glucose, or both oxygen and glucose are present in the incubation medium. Both L- and D-lactate are excluded from the cells when neither oxygen nor glucose is present. 3. D,L-Glyceraldehyde prevents the uptake of L-lactate. The effect is apparently not due to the inhibition of glucose metabolism by L-glyceraldehyde. 4. L-lactate (20 mM) markedly inhibits the uptake of 5 mM D-lactate, but 20 mM D-lactate fails to inhibit the uptake of 5 mM L-lactate. 5. Raising the pH of the incubation medium markedly depresses the uptake of L-lactate. 6. The results provide evidence that L- and D-lactate enter the cells of rat aorta by a mediated transport system.     

Auxin transport: a field in flux

Polar auxin transport is crucial for plant growth and development. Auxin moves between plant cells through a combination of membrane diffusion and carrier-mediated transport. Several classes of membrane proteins that facilitate auxin uptake and efflux have recently been identified in Arabidopsis. The relative contribution to auxin transport made by the different facilitators and by membrane diffusion is unclear. In this Opinion article, we assess the significance of auxin diffusion versus carrier-mediated transport and then discuss the physiological importance of the transport facilitators within the context of the multiple trans-cellular auxin fluxes recently described in the Arabidopsis root apex.     

Convective oxygen transport and fatigue.

During exercise, fatigue is defined as a reversible reduction in force- or power-generating capacity and can be elicited by "central" and/or "peripheral" mechanisms. During skeletal muscle contractions, both aspects of fatigue may develop independent of alterations in convective O(2) delivery; however, reductions in O(2) supply exacerbate and increases attenuate the rate of accumulation. In this regard, peripheral fatigue development is mediated via the O(2)-dependent rate of accumulation of metabolic by-products (e.g., inorganic phosphate) and their interference with excitation-contraction coupling within the myocyte. In contrast, the development of O(2)-dependent central fatigue is elicited 1) by interference with the development of central command and/or 2) via inhibitory feedback on central motor drive secondary to the peripheral effects of low convective O(2) transport. Changes in convective O(2) delivery in the healthy human can result from modifications in arterial O(2) content, blood flow, or a combination of both, and they can be induced via heavy exercise even at sea level; these changes are exacerbated during acute and chronic exposure to altitude. This review focuses on the effects of changes in convective O(2) delivery on the development of central and peripheral fatigue.     

Skin-flap metabolism in rats: oxygen consumption and lactate production

Overall glucose metabolism was evaluated by measuring the rate of oxygen consumption (QO2) and lactate production in the pedicle skin flaps of rats. Skin flaps exhibited increases in QO2 and lactate production in vitro. The distal portion of the flap is characterized by a greater deposition of glucose to lactate during the initial 3 days following flap elevation. The contribution of glycolysis and of the oxidative pathways to glucose metabolism in skin flaps approximates that in normal skin on day 7 postoperatively.     

The mechanism of inhibition by acidosis of gluconeogenesis from lactate in rat liver.

1. Gluconeogenesis from lactate or pyruvate was studied in perfused livers from starved rats at perfusate pH7.4 or under conditions simulating uncompensated metabolic acidosis (perfusate pH6.7-6.8). 2. In 'acidotic' perfusions gluconeogenesis and uptake of lactate or pyruvate were decreased. 3. Measurement of hepatic intermediate metabolites suggested that the effect of acidosis was exerted at a stage preceding phosphoenolpyruvate. 4. Total intracellular oxaloacetate concentration was significantly decreased in the acidotic livers perfused with lactate. 5. It is suggested that decreased gluconeogenesis in acidosis is due to substrate limitation of phosphoenolypyruvate carboxykinase. 6. The possible reasons for the fall in oxaloacetate concentration in acidotic livers are discussed; two of the more likely mechanisms are inhibition of the pyruvate carboxylase system and a change in the [malate]/[oxaloacetate] ratio due to the fall in intracellular pH.     

Water column oxygen demand and sediment oxygen flux: patterns of oxygen depletion in tidal creeks

Low dissolved oxygen (DO) levels often occur during summer in tidal creeks along the southeastern coast of the USA. We analyzed rates of oxygen loss as water-column biochemical oxygen demand (BOD₅) and sediment oxygen flux (SOF) at selected tidal creek sites monthly over a 1-year period. Ancillary physical, chemical and biological data were collected to identify factors related to oxygen loss. BOD₅ rates ranged from 0.0 mg l^?1 to 7.6 mg l^?1 and were correlated positively with organic suspended solids, total suspended solids, chlorophyll a concentrations, temperature, and dissolved oxygen, and negatively with pH and nitrate + nitrite. SOF rates ranged from 0.0 to 9.3 g O₂ m^?2 d^?1, and were positively correlated with temperature, chlorophyll a, and total suspended solids, but negatively with dissolved oxygen. Both forms of oxygen uptake were seasonally dependent, with BOD₅ elevated in spring and summer and SOF elevated in summer and fall. Average oxygen loss to sediments was greater and more variable than oxygen loss in the water column. Oxygen deficits at three of five locations were significantly related to BOD₅ and SOF, but not at two sites where ground water discharges were observed. Correlation and principal component analyses suggested that BOD₅ and SOF responded to somewhat different suites of environmental variables. BOD₅ was driven by a set of parameters linked to warm season storm water inputs that stimulated organic seston loads, especially chlorophyll a, while SOF behaved less strongly so. Runoff processes that increase loads of organic material and nutrients and ground water discharges low in dissolved oxygen contribute to occurrences of low dissolved oxygen in tidal creeks.     

Adrenergic involvement in blood oxygen transport and acid-base balance during hypercapnic acidosis in the Rainbow Trout,Salmo gairdneri

Summary Rainbow trout (Salmo gairdneri) were subjected to 12 h of external hypercapnia (1% CO₂ in air) during - and/or -adrenoceptor blockade in order to assess the importance of adrenergic responses in modulating blood oxygen transport and acid-base balance during an acute acidotic stress. External hypercapnia caused an elevation of blood carbon dioxide tension and a reciprocal decrease in whole blood pH. A gradual elevation of blood bicarbonate levels caused whole blood pH to increase toward pre-hypercapnic values throughout the hypercapnic period. Pre-treatment of fish with propranolol (a -adrenoceptor antagonist) or phentolamine (an -adrenoceptor antagonist) did not affect their ability to regulate extracellular acid-base status during hypercapnia. On the other hand, adrenergic responses were essential in the maintenance of arterial blood oxygen content during hypercapnia despite the severe extracellular acidosis and a marked Root effect in trout blood, in vitro. Important adrenergic responses included pronounced increases in haematocrit (an -adrenergic effect) and arterial oxygen tension (- and -adrenergic effects) as well as partial regulation of red blood cell pH (a -adrenergic effect). Although pre-treatment of fish with either propranolol or phentolamine caused a reduction in blood oxygen content during hypercapnia, fish died only during complete adrenoceptor blockade, presumably due to severe hypoxemia.Symbols and abbreviations total concentration of oxygen or carbon dioxide, respectively - hct haemotocrit - rbc red blood cell     

Regulation of glucose transport in chick fibroblasts: bicarbonate, lactate and ascorbic acid.

The rate at which chick embryo fibroblasts in primary or secondary culture transport glucose or 3-O-methyl glucose is strongly influenced by the presence of bicarbonate ion in the culture medium. Cells growing or maintained on glucose at physiologic concentration (5.5 mM) have an 8 to 10 fold higher rate of glucose uptake than their counterparts cultivated without bicarbonate. These cells also produce more lactate as a consequence of their more rapid intake of glucose. The hydrogen acceptors, methylene blue and dehydroascorbate added to the culture medium reduce the cell capacity to transport glucose and 3-O-methyl glucose to levels obtaining in the bicarbonate-free medium. There is a concomitant reduction in glucose utilized by cells during 24 hours and further reduction in lactate formed per molecule of glucose metabolized.     

Red blood cell lactate transport in sickle disease and sickle cell trait.

This study determined and compared rates and mechanisms of lactate transport in red blood cells (RBCs) of persons with 1) sickle cell disease (HbSS), 2) sickle cell trait (HbAS), and 3) a control group (HbAA). Blood samples were drawn from 30 African-American volunteers (10 HbSS, 10 HbAS, 10 HbAA). Lactate influx into RBCs was measured by using [14C]lactate at six (2, 5, 10, 15, 25, and 40 mM) unlabeled lactate concentrations. The monocarboxylate transporter pathway was blocked by p-chloromercuriphenylsulfonic acid to determine its percent contribution to total lactate influx. Generally, total lactate influx into RBCs from the HbSS group was significantly greater than influx into RBCs from HbAS or HbAA, with no difference between HbAS and HbAA. Faster influx into HbSS RBCs was attributed to increased monocarboxylate transporter activity [increased apparent Vmax (V'max)]. V'max (4.7 +/- 0.6 micromol x ml(-1) x min(-1)) for HbSS RBCs was significantly greater than V'max of HbAS RBCs (2.9 +/- 1.5 micromol x ml(-1) x min(-1)) and HbAA RBCs (2.0 +/- 0.5 micromol x ml(-1) x min(-1)). Km (42.8 +/- 8 mM) for HbSS RBCs was significantly greater than Km (27 +/- 12 mM) for HbAA RBCs. We suspect that elevated erythropoietin levels in response to chronic anemia and/or pharmacological treatment (erythropoietin injections, hydroxyurea ingestion) is the underlying mechanism for increased lactate transport capacity in HbSS RBCs.