In recent years, intestinal transport processes have been studied in detail regarding both, functional and structural aspects. For monosaccharides different systems have been demonstrated for apical uptake: this includes the high-affinity SGLT1 as a distinct d-glucose system and GLUT5 for fructose. Specifically in pigs a low affinity, high-capacity system for d-glucose and d-mannose with no preference for Na+ over K+ and a very low affinity system are suggested as further uptake systems. As in other species, basolateral extrusion is mediated by GLUT2. The distributions of monosaccharide transport along the gastrointestinal axis as well as the potential role of paracellular monosaccharide absorption have not yet been clarified.
Amino acids can principally be absorbed by the paracellular and transcellular pathway whereas transcellular transport can either be mediated by facilitated diffusion or secondary active Na+-coupled transport. This includes different transport systems for neutral, anionic and cationic acids. In addition, the presence of the di-/tripeptides transport system PEPT1 which depends on an inwardly directed H+-gradient has also been confirmed for the pig small intestine, its quantitative proportion is still under debate.
Short chain fatty acids (SCFA) are the major end products of microbial carbohydrate fermentation which occurs along the gastrointestinal tract with the highest production rates in the large intestines. At least two uptake mechanisms have to be assumed, i.e., non-ionic diffusion and anionic exchange via SCFA−/HCO3−-exchange. Controversial views still exist to what extent SCFA are metabolized within the epithelial tissue.
Segmental differences between small and large intestines have been demonstrated for Na+ absorption. Whereas in the small intestines the major part of Na+ absorption is mediated by coupled nutrient transport systems, aldosterone sensitive Na+ channels and Na+/H+-exchange are the dominant mechanisms in the hindgut. For Cl− paracellular transport and anionic Cl−/HCO3−-exchange are the major absorptive mechanisms. Cl− secretion is mediated by apical channels which may be activated by toxins of different origin. Different types of Cl− channels have been identified, such as Cystic Fibrosis Transmembrane Regulator (CFTR), Ca-activated Cl− channels (CLCA) and Outwardly Rectifying Cl− Channels (ORCC). Whereas CFTR has clearly been shown for jejunal and colonic epithelial and goblet cells controversy still exists on the relevance of CLCA and ORCC in pigs.
For Ca2+ there is evidence that both recently published channels TRPV5 and TRPV6 are also expressed in pig intestinal tissues, however, this has not yet been shown on protein level. From several functional approaches it was demonstrated that phosphate uptake can be mediated by both, a Na+-dependent transcellular component and paracellularly. On a molecular basis it is uncertain whether the transport protein of transcellular mechanism belongs to the NaPi-IIb cotransporter family. 相似文献
Abstract Element deficiencies, such as molybdenum (Mo), iron (Fe), calcium (Ca), lead to disturbances of morphological and physiological processes of tobacco (Nicotiana tabacum L.). On exposure to nutrient solution without Mo, leaf morphology was significantly affected, whereas photosynthetic processes conserved their normal activities. The decreased Mo, Fe, and Ca concentrations in tobacco were associated with loss of pigments and photosynthetic disturbances. In Ca‐deficient plants a reduction in the rate constant of energy trapping by PSII centers and a physical dissociation of LHC from PSII core, were observed. The poisonous action of Fe and Ca deficiencies focused mainly in the decreased proportion of active chlorophyll associated with the reaction center (RC) of PSII (decreased Fv/F0) and in the declined total number of electrons that have gone through the RCs (decreased Area/Fm ? F0). In parallel, the non‐photochemical quenching coefficient was significantly enhanced in Fe and Ca‐deficient plants, but remained unchanged in Mo‐deficient plants. It is obvious that nutrient deficiency may trigger some protective mechanisms in order to PSII could maintain its activity under that type of stress. 相似文献
AbstractLeaf sheaths of rice plants are known to temporarily accumulate starch prior to heading, which is subsequently remobilized and transported into the panicle after heading. We investigated the time course for both carbohydrate content and steady state mRNA levels of enzymes related to starch and sucrose metabolism in the rice leaf sheath (Oryza sativa L. cv. Nipponbare). Leaf sheaths from the second leaf below the flag leaf accumulated high levels of starch before heading but they rapidly decreased after heading. In contrast, the flag leaf sheath did not accumulate as much starch. In the second leaf sheath, the mRNA levels of enzymes involved in starch synthesis, ADP glucose pyrophosphorylase (EC 2. 7. 7. 27), soluble starch synthase (EC 2. 4. 1.21) and branching enzyme (EC 2. 4. 1. 18) were high before heading, which coincided with rapid accumulation of starch. The mRNA levels of sucrose synthesis enzymes, cytosolic FBPase (EC 3. 1. 3. 11) and sucrose phosphate synthase (EC 2. 4. 1. 14), and the sucrose transporter (OsSUTI) increased at the time of heading, which was largely coincident with a decrease in the mRNA levels of starch synthesis enzymes. In the flag leaf sheaths, changes in mRNA levels of starch synthesis enzymes were not pronounced, however mRNA levels of sucrose synthesis enzymes and the sucrose transporter showed a clear increase throughout the heading period. The different characteristics observed between the two leaf sheaths will be discussed in relation to the sink to source transition. 相似文献
There exists a great variability among plant species regarding their sensitivity and resistance to high salinity in soil, and most often this variability is related with the ability of a particular plant species to regulate ion homeostasis and transport. In this study, we have investigated the effects of NaCl on growth rate, water status, and ion distribution in different cells and tissues of two succulent plants, Aloe vera and Salicornia europaea. Our results showed that the growth of A. vera seedlings was significantly decreased in response to salinity. However, the growth of S. europaea seedlings was greatly stimulated by high concentrations of NaCl. Under saline conditions, S. europaea seedlings maintained K+ and Ca2+ uptake in roots and showed a higher root‐to‐shoot flux of Na+ and Cl– as compared to A. vera. Despite great accumulation of Na+ and Cl– in photosynthetically active shoot cells in S. europaea, its growth was enhanced, indicating S. europaea is capable of compartmentalizing salt ions in the vacuoles of shoot cells. Aloe vera seedlings, however, showed a low transport rate of Na+ and Cl– to leaves and suppressed uptake of K+ and Ca2+ in roots during NaCl treatment. Our results also implicate that A. vera may be able to accumulate Na+ and Cl– in the metabolically inactive aqueous cells in leaves and, as a result, the plant can survive and can maintain growth under saline conditions. 相似文献