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.

[The current concepts on the absorption of monosaccharides, amino acids and peptides in the mammalian small intestine]. / Sovremennye predstavleniia o vsasyvanii monosakharidov, aminokislot i peptidov v tonkoi kishke mlekopitaiushchikh.

The review is mainly devoted to the development of ideas about absorption, or transport, of basic nutrients in the small intestine in humans and higher animal. The absorption processes have been characterized on the example of such substances, vital for organism, as carbohydrates and proteins. The review considers a molecular structure of transporters--protein molecules, which take part in a transfer of the products of lumenal and membrane digestion of carbohydrates (glucose, galactose, fructose) and proteins (amino acids, oligopeptides) across the enterocyte membranes. An information is presented about genetic disturbances of transport of certain amino acids during such diseases as Hartnup disease, cystinuria, and iminoglycineuria.

[Academician Aleksandr Mikhailovich Ugolev. The development of his scientific ideas]. / Akademik Aleksandr Mikhailovich Ugolev. Razvitie nauchnykh idei.

The science suffered a severe loss with the sudden death in November 2, 1991, of and outstanding physiologist Academician Alexander Mikhailovich Ugolev at the age of 66. All life of this man was devoted to the development of the national science. It was the aim and meaning of his life, he has contributed to scientific progress in every possible way. The major scientific interests of A. M. Ugolev were focused on the basic and applied areas of food assimilation, including the physiology of nutrition and digestion. The works of the scientist in this field of knowledge were world-wide recognized. During his scientific activity Dr. Ugolev conducted a number of brilliant studies which remained unforgettable trace in physiology and for ever will be a part of golden fund of science. This was promoted by a unique combination of his illustrious abilities of physiologist and general biologist as well as evolutionist with the gift of experimenter and excellent surgeon. His experimental skill can serve and example of an elegant combination of bold intentions and their masterly realizations. But the most remarkable and world-wide recognized discovery of the scientist was that of membrane digestion--previously unknown mechanism of food assimilation. Just this basic discovery is emphasized in the review devoted to the memory of unforgettable master. The fragmentariness of material depends on a desire to present concisely the major characteristics of membrane digestion.

[Enzymatic adaptation of the pancreas and small intestine to food composition (the example of carbohydrases)]. / Fermentnye adaptatsii podzheludochnoi zhelezy i tonkoi kishki k kompozitsii pishchi (na primere karbogidraz).

In two series of experiments on rats, the effect of high protein as well as fat and carbohydrate diets of carbohydrases activity of the pancreas and the small intestine was studied; in the first series, the animals had the specialized diets for 4, 15 and 30 days, while in the second series--for 7 days. In the 1st series food composition produced an obvious effect on gamma--amylase, maltase and sacrase activities of the intestinal mucosa, whereas in the 2nd series the effect was insignificant which can be accounted for by somewhat different conditions of the experiments. The data suggest that adaptation of pancreatic and small intestine's enzyme systems to food composition should be considered an integrated reaction.

[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.

[Hydrolases of the digestive organs during ontogenesis]. / Gidrolazy pishchevaritel'nykh organov v ontogeneze.

A wide variety of enzymes actualizing membrane hydrolysis of nutrients and their distribution in the gastro-intestinal tract were characterized in one-day old and adult rats and rabbits. The comparison of enzymatic activities of various animal species suggests that some enzymes are responsible for the adaptation to milk diet (lactase), others--to definitive nutrition (invertase, maltase). A number of enzymes (peptidase, alkaline phosphatase) do not depend on the type of nutrition. High activity of some hydrolases in the colon has been demonstrated confirming the A. M. Ugolev hypothesis of its digestive functions in the early ontogenesis.

[Radial and proximo-distal distribution of hydrolytic enzymes in the small intestine at rest and during digestion]. / Radial'noe i proksimo-distal'noe raspredelenie gidroliticheskikh fermentov v tonkoi kishke v pokoe i pri pishchevarenii.

Alpha-amylase was present in considerable amount in the chyme in rats both at rest and during digestion, in the latter case its level being increased. Activity of intrinsic intestinal enzymes dominated in the homogenate of the mucosa. Transition from fasting to feeding and digestion increased the level of alkaline phosphatase and dipeptidase activities in the homogenate of the mucosa. The data obtained suggest the final stages of biopolymers' hydrolysis to be localized in various enterocyte structures. The surface mucus, owing to absorbed pancreatic enzymes, binding proteins and other types of physiologically active molecules, is assumed to play the role of a specific molecular filter and take part in the initial stages of digestion.

[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.

[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.

[The hydrolases of the mucosal, submucosal and muscular layers of the small intestine and their functions]. / Gidrolazy mukoznogo, submukoznogo i myshechnogo sloev tonkoi kishki i ikh funktsii.

Proceeding from the newly obtained evidence on the distribution of digestive enzymes in mucosal, submucosal and muscular layers of the small intestine in rats, a hypothesis has been formulated: in addition to the enterocyte enzyme complex forming the epithelial barrier, there seems to exist a postepithelial barrier formed by the enzymes localized in subepithelial structures. The efficiency of this second barrier was manifested by the study of the peptide hydrolysis and transport using everted intact and de-epithelialized intestinal sacs.

[The functional topography of the digestive enzymes in the gastrointestinal tract and in nondigestive organs (liver, kidney) in the ontogeny of rats and the effect of inducing agents]. / Funktsional'naia topografiia pishchevaritel'nykh fermentov v zheludochno-kishechnom trakte i nepishchevaritel'nykh organakh (pechen', pochka) v ontogeneze u krys i vliianie nekotorykh indutsiruiushchikh agentov.

Activities of the digestive enzymes (lactase, maltase, saccharose, di-, tri- and aminopeptidases, alkaline and acid phosphatase) in the gastrointestinal tract, the kidney and the liver in one-day-old and adult rats as well as in 10-day-old rats after injections of hydrocortisone (H) or thyroxine (T4) were determined. On the basis of the results obtained it is summarized that (1) high levels of the digestive enzyme activities are observed in the nondigestive organs in immature and adult rats; (2) H induces the enzyme activities in the small intestine more than in other organs; T4 does not influence the activities of the most of enzymes studied both in the digestive and in the nondigestive organs; (3) high activities of a number of enzymes in the colon in one-day-old rats implies its involvement in the digestion at early stages of the ontogenesis.