Structural and functional analysis of glucose absorption mechanisms in the rat small intestine in vivo.

The absorption of glucose (free, and released from membrane hydrolysis of maltose) and water in the isolated loop of the rat small intestine was studied in chronic experiments. Even at maximum glucose (75 mmol/l) or maltose (37.5 mmol/l) concentrations the rate of glucose transfer by solvent drag and by diffusion did not exceed 13% and 25%, respectively, of the total rate of glucose absorption. Electron microscopic and immunocytochemical analysis revealed a significant widening of intercellular gaps in the basal epithelium region and an increase of actin density in the vicinity of the tight junctions and between the apical root filaments in enterocytes after glucose or glycine load in chronic and acute experiments. However, very rarely (in 1% of all cases), and only in chronic experiments, structural changes in the tight junctions such as "blisters" and dilatations were also recognised. It is concluded that under normal physiological conditions the absorption of glucose (free, and released from maltose hydrolysis) mainly uses active transport across the apical membrane of the enterocytes.

Membrane digestion and transport under physiological conditions: a review of available data.

A technique to study membrane digestion and transport in the small intestine under physiological conditions has been developed. The technique is based on a continuous perfusion of a chronically isolated loop of the rat small intestine. Membrane hydrolysis and transport of some nutrients in the rat small intestine in chronic, as well as in acute (in situ) experiments was investigated. The absorption of hexoses and amino acids has been found to be 2.5-4 times higher under physiological conditions than in acute in situ experiments. Both the active transport of glucose released from maltose hydrolysis and the hydrolysis of the latter is increased under physiological conditions. A coupling between the final stages of hydrolysis and the initial stages of transport in chronic experiments was shown to be highly efficient; practically all or nearly all glucose released is being transported without entering the luminal phase. The hydrolysis rate of starch during the perfusion of a small intestinal segment in chronic experiments is many times higher than that in acute experiments or under anaesthesia. The enzymatic and transport activities revealed using a widely accepted technique in situ, the more so, in vitro account for only a small fraction of those which are typical of undisturbed processes under conditions close to the physiological. The levels of functioning of the digestive-transport systems of the small intestine considered as natural levels developed in the process of evolution, actually reflect only residual processes and, in most cases, they account for 1/3 to 1/10 of the true level of an actual physiological process.

[Canalicular system of enterocytes at rest and its changes during lipid absorption]. / Kanalikuliarnaia sistema énterotsitov v "pokoe" i ee izmeneniia pri vsasyvanii lipidov.

A study was made of the structural changes in different organoids of enterocytes of the rat small intestine at "rest" and during lipid absorption using ultrathin sections, impregnation with Ur-Pb-Cu, and continuous impregnation with OsO4. With the latter technique a specific canalicular system was found in the enterocytes, which we failed to observe on ultrathin sections. During lipid absorption the canalicular system underwent fragmentation. The vesicules on the trans-surface of the Golgi apparatus were enlarged, and the number of flattened cisterns was significantly lower. The changes in cellular organoids and their interrelations observed in the present study support the earlier hypothesis of the resynthesis of triglycerides within the enterocytes, the formation of chylomicrons and their transfer into the intercellular space.

[Isolated enterocytes as an object of functional morphology]. / Izolirovannye énterotsity kak ob''ekt funktsional'noi morfologii.

Morphological examination of isolated enterocytes obtained-from the rat jejunum initial region by mechanical method after the previous treatment with ethylendiamintetracetic acid disodium salt included staining with hematoxylin, carmin and janus green. Histochemical reaction to alcaline phosphatase, succinate dehydrogenase, lactate dehydrogenase, ATP-ase and glycosaminoglycans were performed. Significant resemblance between the main cytological and cytochemical characteristics of the isolated enterocytes and those of the intestinal epithelium was demonstrated. Morphophysiological examination of isolated enterocytes, incubated in media, containing different substrates (glucose, maltose, glycine, trioleine) and under effect of oxygenated and not-oxygenated media demonstrated that mitochondria intracellular topography and succinate dehydrogenase activity significantly depend on their functional state.

[Membrane digestion and some general problems in physiology]. / Membrannoe pishchevarenie i nekotorye obshchie problemy fiziologii.

A number of general problems arising from investigations of various aspects of membrane digestion has been considered in the paper. The development of this field of knowledge allowed to characterize from a new point of view the role of glycocalix, molecular physiology of intestinal enzymes, the role of recirculation of membrane proteins and intestinal barrier as well as the scale of membrane hydrolysis and transport in conditions approximated to physiological ones. The level of membrane hydrolysis and transport in vivo is several times that of in vitro experiments and of acute experiments in situ. From the data obtained it is evident that some present concepts of the level efficiency of in vivo physiological processes in the small intestine and mechanisms of their regulation ought to be reconsidered.

[Structural and functional integration of the processes of membrane hydrolysis and transport ("permeome" hypothesis)]. / Strukturnaia i funktsional'naia integratsiia protsessov membrannogo gidroliza i transporta (gipoteza "permeoma")

It was suggested and experimentally proved that the transport system's input is a part of an enzyme-transport ensemble and an allosteric regulator of enzyme activity. In special experiments, a typical for cooperation sygmoidal kinetics of saccharose and maltose hydrolysis was shown, as well as of their hydrolysis products transport, if the enzyme is involved in the actively transporting intestine mucosa. On the contrary, it is the hyperbolic curve which is specific for isolated digestive enzymes and for glucose accumulation. Replacement of oxygen with nitrogen and specific blockage of the glucose transporting agent with floridzine leads to disappearance or sharp reducing of the sygmoidality. The term "ergome" is suggested for designation of the minimal ensemble of elements able to carry out an elementary physiological process, the term "permeome"--for the transporting ergome.

[Induced autolysis as an important mechanism of the initial stages of ordinary digestion]. / Indutsirovannyi autoliz kak vazhnyi mekhanizm nachal'nykh stadii pishchevareniia v estestvennykh usloviiakh.

The critical review of generally accepted ideas on initial stages of digestion in higher animals is presented. A brief account is given of the hypothesis of the induced autolysis as an important mechanism of food assimilation. According to this hypothesis digestive juices (in addition to their enzyme activities) contain factors capable to induce the autolysis of food structures. An attempt was made to estimate the value of gastric secretions and induced autolysis in the splitting of complex biological food structures. The splitting of native structures of food object was found to be more effective and complete than the splitting of the same structures subjected to heat denaturation. The results obtained suggest that the initial stages of digestion should be considered as a complex process provided both with the enzymes of gastric juice and with the enzymes of food itself, with lysosomal enzymes in particular.

[The transport system that pumps monosaccharides from enterocytes across the basolateral membrane]. / Transportnaia sistema, otkachivaiushchaia monosakharidy iz énterotsitov cherez bazolateral'nuiu membranu.

Using a new, in vitro, experimental approach the transport of glucose, galactose, fructose and glycine in seroso-mucosal direction in the small intestine, was studied under different modes of oxygenation. In case of monosaccharides under the oxygenation from serosal surface, the substrate concentration is lower in the small intestine tissue than under anoxia. The effect of the oxygenation from serosal surface on the galactose transport was shown to be sensitive to absence of sodium ions, ouabain, phloretin, and not sensitive to phlorizin. The data obtained corroborate the hypothesis of the existence of a mechanism pumping off monosaccharides enterocytes through the basolateral membrane.

[Role of basolateral and apical respiration in the active transport of glucose and maltose in different segments of the small intestine under different functional conditions]. / Rol' bazo-lateral'nogo i apikal'nogo dykhaniia v aktivnom transporte gliukozy i mal'tozy v razlichnykh segmentakh tonkoi kishki pri raznykh funktsional'nykh sostoianiiakh poslednei.

The active transport of free glucose and glucose released in membrane hydrolysis involves oxygenation of mucosal and seroseal surfaces as well as bilateral one. The glucose accumulation against the electrochemical gradient increases in transition from fasting to satiety. The data obtained suggest a hypothesis stating that enterocytes and, probably, other functionally polarized cells have not a single (basolateral) but double types of the cellular respiration: the apical as well as the basolateral ones. In addition to diffusion exchange of O2 and CO2 between the blood and the basolateral surface of cells, the gas exchange is supposed to occur at the apical surface as well. The O2 transport to this particular surface of cells can be based on the Na+ microcirculation. The interrelationship between the apical and basolateral types of respiration was shown to depend on the functional condition of the organism (satiety or hunger).

[New data on the mechanisms of glucose transport in the small intestine based on an analysis of the role of serous-mucous flows]. / Novye dannye o mekhanizmakh transporta gliukozy v tonkoi kishke, osnovannye na analize roli serozno-mukoznykh potokov.

The transport of glucose, other monosaccharides and disaccharides in the rat small intestine has been studied under the condition of a predominance of serosal-to-mucosal (SM) flows over mucosal-to-serosal (MS) ones. A dependence has been found of the efflux of glucose into the serosal solution on the duration of incubation, starting glucose concentration in the serosal solution, intestinal segment under study (proximal-distal gradient), action of glucose transport inhibitors (phlorizin, absence of Na+ in the incubation medium, anoxia) and temperature. In this paper, the hypothesis on entero-hematic circulation has been proposed, according to which recycling of glucose is one of the important manifestations of processes of microcirculation of substances between blood and the intestine.

[The enzymatic barrier of the small intestine]. / Enzimaticheskii bar'er tonkoi kishki.

The small intestine's barrier functions are reviewed. The data on mechanical (passive) and active protective systems of the organism against various antigens, toxic substances and proteins, is presented. An important role of these protective systems as an enzyme apparatus of epithelial and postepithelial layers of the small intestine's mucose, is shown.

[Trans-sorption as an important mechanism of molecular transport in biological systems]. / Transsorbtsiia kak vazhnyi mekhanizm molekuliarnogo transporta v biologicheskikh sistemakh.

The mechanism of trans-sorption was shown to be based on mobile adsorption of various substances at specific and unspecific sites localised in spatially organised systems integrated within premembrane structures. Some specifics and possible physiological significance of trans-sorption are discussed using mathematical models of nutrients transfer across autonomous premembrane layer in the small intestine.

[New method of studying membrane hydrolysis and transport as well as metabolic processes in the small intestine in vitro (everted small intestinal sac with bilateral oxygenation)]. / Novyi metod issledovaniia membrannogo gidroliza i transporta, a takzhe metabolicheskikh protsessov v tonkoi kishke in vitro (vyvernutyi meshok tonkoi kishki s dvustoronnei oksigenatsii).

The effect of bilateral oxygenation (from mucosal and serosal surfaces) of the small intestinal preparation on glucose accumulation has been investigated. It has been demonstrated that such a type of oxygenation significantly (two fold) increases the absorption of glucose as compared to that under oxygenation only from musocal surface. This data sugested that oxygenation from serosal site of the small intestine had an important role in the transport of free glucose.

[The effect of stress on the enzyme systems of the enterocyte microvilli in rat pups and the possible mechanism of the transmission of stress effects from dam to progeny]. / Vliianie stressa na fermentnye sistemy mikrovorsinok énterotsitov u krysiat i vozmozhnyi mekhanizm peredachi stressornykh éffektov ot materi k potomstvu.

In suckling rats whose mothers had had an immobilisation stress or been injected HC and T4 at the last day of their pregnancy, the sucrase induction decreased in the small intestine, if the HC was injected on the 10th day of life. The hormone injection to 1-day suckling rats did not affect the induction. These effects seem to be exerted not by the direct permeation of hormone from mother to litter but to be mediated rather by a special factor "stressin". The latter is transferred from mother to litter and decreases the sensitivity of intestinal enzymes to exogenous HC, thus exerting an antistressor effect.

[The detailed characteristics of the enzyme spectrum of the small intestine in rats in the early postnatal period]. / Detal'naia kharakteristika fermentnogo spektra tonkoi kishki krys v rannem postnatal'nom periode.

The activity of the small intestine's peptide hydrolases is higher in 1-day old rats than in adult rats, whereas levels of activity of alkaline phosphatase and diglycyl glycine peptidase do not differ significantly in these two groups of the rats. Our own data on carbohydrases corroborate other authors' evidence and reveals that activities of lactase, sucrase and maltase are either absent or very low in the first days of life and sharply increase by the third week of postnatal development. Adaptive changes of regulatory properties of lactase and alkaline phosphatase are revealed.

[The characteristics of the absorption in the small intestine of 2 casein hydrolysates and of an equivalent amino acid mixture]. / Nekotorye osobennosti vsasyvaniia v tonkoi kishke dvukh gidrolizatov kazeina i ékvivalentnoi smesi aminokislot.

Using casein hydrolysates and the amino-acid mixture applied to mucosal or serosal surfaces, their effect on the transport of free glycine, glycine formed during glycyl glycine hydrolysis, and galactose, was studied in the small intestine. The data obtained suggest a mechanism of peptide transport in the enterocytes as well as a possibility of a regulatory effect of the peptides contained in casein hydrolysates on the transport of some basic nutrients.

[Interrelationships between the enzymatic functions of the pancreas and small intestine during adaptive processes]. / Vzaimootnosheniia fermentativnykh funktsii podzheludochnoi zhelezy i tonkoi kishki pri adaptivnykh protsessakh.

In rats fed for 4, 15, and 30 days with increased amount of proteins, lipids, and carbohydrates, considerable shifts occurred in activity of enzymes of the pancreas (amylase, protease, and lipase) and small intestine (gamma--amylase, maltase group, invertase, peptidhydrolase, monoglyceriflipase). Mathematical analysis suggested a close connection between the adaptive shifts in the enzyme systems maintaining the lumen and the membrane types of digestion. The protein diet augments the proteolytic enzyme chain the lipid diet--the lipolytic chain, and the carbohydrate diet--the carbohydrate chain. The shifts should be regarded as an integrative adaptive response of the enzyme spectrum of the pancreas and small intestine to alterations in the food composition.

[Glycerinized models of enterocytes and several of their properties]. / Glitserinizirovannye modeli énterotsitov i nekotorye ikh svoistva.

The method for the preparation of glycerol-treated models of the enterocytes with preserved membrane has been described. Using such a simplified model of the enterocyte suspension allows membrane digestion and enzyme properties to be studied. Moreover, preserving the important properties of the native isolated cells the glycerol-treated enterocytes have a number of essential advantages as compared with other known preparations of the small intestinal mucosa. Some characteristics of the glycerol-treated models of the enterocytes have been described in the paper.

[The digestive enzymes in the gastrointestinal tract, kidney, liver and spleen in different functional states]. / Pishchevaritel'nye fermenty v zheludochno-kishechnom trakte, pochke, pecheni i selezenke pri razlichnykh funktsional'nykh sostoianiiakh.

A wide range of digestive enzymes with different cellular location (membrane, intracellular, Lysosomal) was determined in gastrointestinal organs (stomach, duodenum, jejunum, ileum, colon) as well as in undigestive organs (liver, kidney, spleen) in normal conditions and in altered functional states (fasting, refeeding). High levels of peptidase activity was noted in undigestive organs and the colon as compared to that in the small intestine. Adaptive responses were (revealed not only for a number of membrane enzymes but also for intracellular ones including those in undigestive organs.

[The enzyme-transport characteristics of the small intestine of rats during aging]. / Fermentativno-transportnye kharakteristiki tonkoi kishki krys pri starenii.

Ageing was shown to be accompanied by changes of the activity levels of the small intestine's carbohydrases, dipeptidases, and alkaline phosphatases. The changes occurred both under normal conditions and after operations on the small intestine. In ageing, the small intestine's capacity for the homeomorphosis decreases irrespective of either increase or decrease in the functional loading. The age involution of the gastrointestinal tract is followed by disturbances not only in hydrolytic processes but in transport those as well: particularly deceleration of glucose accumulation in preparations of the rat intestine mucose. Regulatory properties of some enzymes were changed, too, in old rats.