A revision of current data and views on membrane hydrolysis and transport in the mammalian small intestine based on a comparison of techniques of chronic and acute experiments: experimental re-investigation and critical review

Literature data and the results of our investigations using both generally accepted and original perfusion techniques of the isolated loop of the rat small intestine in in vivo experiments are reviewed. Significant differences in the functioning of the small intestine under conditions of acute and chronic experiments are revealed. It has been established that in chronic experiments as compared to acute ones: (a) the absorption of glucose, galactose, fructose and glycine is 2-5 times higher; (b) Kt as well as Jmax values of the transport of these nutrients differ considerably; (c) Na+-independent mechanism of glucose and glycine transport predominates; (d) higher rates of membrane hydrolysis and more effective interactions between enzyme and transport systems of the enterocyte brush border membranes are observed; (e) functional characteristics of the small intestine affected by various experimental factors are more stable. The conclusion is made that it is necessary to revise current views of the scale and regularities of digestive-transport processes in the small intestine under physiological conditions. The importance of the suggested approaches for general and comparative physiology and biochemistry is discussed.     

The hypothalamo-hypophysial system of the frog Rana temporaria

Summary The median eminence (ME) of the adult frog, Rana temporaria, was studied by means of electron microscopy including quantitative electron-microscopic autoradiography. In frogs captured in May and June numerous peptidergic neurosecretory fibres extending via the internal zone to the pars nervosa display large swellings containing few granules, mitochondria, neurotubules and cisternae of the smooth endoplasmic reticulum. In addition, few secretory globules up to 1.5 m in diameter occur in these varicosities. In animals collected during the autumn period many of these neurosecretory swellings filled with neurosecretory granules and polymorphic inclusions resemble Herring bodies. Three types of granule-containing neurosecretory fibres were observed in the external zone (EZ) of the ME of adult R. temporaria. Peptidergic A₁- and A₂-type fibres are characterized by granules 150–220 nm and 100–160 nm in diameter, respectively. Monoaminergic fibres of type B with granules approximately 100 nm in diameter represent 50% of all neurosecretory elements in the EZ of the frog ME; 12% of the total number of granule-bearing axons in the EZ actively taking up radiolabelled 5-hydroxytryptophan are thought to be serotoninergic terminals. Neurosecretory terminals of all types and glial vascular endfeet establish direct contacts with the perivascular space of the primary portal capillaries. Some neurosecretory terminals are separated from the lumen of the third ventricle by a thin cytoplasmic lamella of tanycytes. The possible physiological significance of this structural pattern is discussed.     

Utilization of energy stored in the form of Na+ and K+ ion gradients by bacterial cells

The hypothesis that Na+ and K+ gradients have an energy storing function [V. P. Skulachev (1978) FEBS Lett. 87, 171-176] has been tested in experiments with Escherichia coli, the marine bacterium Vibrio harveyi, an extremely halophilic Halobacterium halobium and a fresh-water cyanobacterium Phormidium uncinatum from Lake Baikal living at an extremely low salt concentration. The capability of these microorganisms to maintain delta microH was compared using motility as a delta microH-supported function. It was found that in all cases the gradient of monovalent cations is competent to prolong the period of active motility after other energy sources are exhausted. Maximal prolongation was found in H. halobium, which in a Na+ medium was still motile when light was switched off for 9 h under anaerobic conditions. In V. harveyi the motility was maintained for 1 h, in E. coli for about 10 min and in Ph. uncinatum for about 2 min. Thus the delta microH buffer capacity of the monovalent cation gradient is proportional to the content of these cations in the habitat. It was also found that in Ph. uncinatum only delta pK is effective, whereas in E. coli and V. harveyi both delta pK and delta pNa are. In E. coli when the K+ release is completed and the cells become motionless, motility can be temporarily restored by adding NaCl which initiates an H+ efflux. Under conditions of exhaustion of energy sources, the Na+ and K+ gradient was shown to stabilize potential in H. halobium cells, measured with a tetraphenylphosphonium probe. In H. halobium and E. coli, the anaerobic ATP level was found to stabilize when the Na+ and K+ gradients were present. Addition of N,N'-dicyclohexylcarbodiimide destabilized this level, which indicated that Na+ and K+ gradients could support de novo ATP synthesis. It is concluded that the data obtained are in agreement with the concept of the energy storing by the Na+ and K+ gradients. Other functions of these gradients and the mechanisms of their formation are discussed.     

Cell junctions acting as intestinal valves in nematodes

Serial sections of the rectal valve in Aphelenchoides blastophthorus Franklin and the oesophago-intestinal valve in Thornenema wickeni Yeates were examined electron microscopically. Each valve when closed appears as a convoluted path of closely apposed (10 nm) pairs of three-layered cell membranes. Both valves serve to stop intestinal leakage, open briefly and rapidly by forcible dilatation and are closed by pressure from surrounding tissues, helped perhaps by intermolecular forces.     

Effect of tannic acid on the synthesis of protein and nucleic acid by rat liver

1. As early as 1hr. after the intraperitoneal administration of tannic acid to rats, it could be demonstrated in the liver. At 3hr. the nuclear fraction contained the largest amount of tannic acid. 2. Nuclear RNA synthesis was inhibited in vivo 2hr. after the administration of tannic acid. Induction by cortisol of tryptophan pyrrolase was 90% inhibited at 24hr. 3. Incorporation of [1-(14)C]leucine into protein by liver slices from treated rats was decreased by 50% after 24hr. Its incorporation into postmitochondrial supernatant from treated animals was not inhibited. Incorporation into slices and postmitochondrial supernatants were inhibited in vitro by tannic acid. 4. The sequence of events: concentration of tannic acid in nuclei, inhibition of nuclear RNA synthesis, inhibition of protein synthesis and production of necrosis, is discussed.     

Amino Acid Transport in Pseudomonas aeruginosa

Properties of the transport systems for amino acids in Pseudomonas aeruginosa were investigated. Exogenous (14)C-labeled amino acids were shown to equilibrate with the internal native amino acid pool prior to incorporation into protein. When added at low external concentrations, the majority of the amino acids examined entered the protein of the cell unaltered. The rates of amino acid transport, established at low concentrations with 18 commonly occurring amino acids, varied as much as 40-fold. The transport process became saturated at high external amino acid concentrations, was temperature-sensitive, and was inhibited by sodium azide and iodoacetamide. Intracellular to extracellular amino acid ratios of 100- to 300-fold were maintained during exponential growth of the population in a glucose minimal medium. When the medium became depleted of glucose, neither extracellular nor intracellular amino acids could be detected.     

Amino Acid Pool Formation in Pseudomonas aeruginosa

The accumulation and behavior of various amino acids in the pool of Pseudomonas aeruginosa (ATCC 9027) were investigated. Patterns of pool formation and maintenance varied with different amino acids tested and were dependent, to a considerable extent, upon the ability of the organism to catabolize the particular amino acid. The establishment of steady-state amino acid pool levels depended upon the activity of the amino acid permease involved and upon the rate of protein synthesis. The presence of a relatively large specific amino acid pool did not affect the formation of a pool of a structurally different amino acid, and a preformed steady-state pool was not displaced by structurally unrelated amino acids. Steady-state amino acid pools decreased rapidly in the presence of inhibitors of energy metabolism and at 0 C. Steady-state internal amino acid pools were found to be in equilibrium with the corresponding external amino acid, present at low levels. A multiplicity of proline pools was demonstrated.