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At least 829 records · Page 46Linked to original sources
PubMed1975

Acidification in the rat proximal jejunum.

1. Production of hydrogen ions by the rat proximal jejunum was investigated using the everted sac technique. 2. Acidification occurred in the absence of glucose, increasing on addition of glucose to reach a maximal value. An apparent Km of 1-78 mM was derived for the glucose-dependent process. 3. Acidification in the presence of glucose was inhibited by 10 mM-2:4-dinitrophenol, 10 mM phlorrhizin, 10 mM aminophylline and anaerobiosis. 4. Histamine, ethylenediamine tetraacetic acid (EDTA), ouabain and acetazolamide, compounds known to alter acid production in gastric mucosa had no effect on jejunal acidification. 5. Galactose and 3-O-methylglucose failed to increase acidification; in contrast, mannose and fructose did lead to increases, indicating metabolic origin of the hydrogen ions. 6. Serosal and mucosal lactate production were measured and the calculated percentage of hydrogen ions possibly derived from this source was shown to account for only a small proportion of acidification. 7. The greatest increase in acidification with minimal simultaneous production of lactate occurred with ATP which was shown not to enter intestinal tissues. 8. A hypothesis for acidification, that of the break-down at the mucosal surface of ATP from intracellular metabolic sources, is proposed and its relevance to the postulated microclimate is discussed.

Acetazolamide↗
PubMed1975

Uptake of 5-methyltetrahydrofolic acid by the rat jejunum.

1. Everted sacs of rat jejunum were used to study the transport of 5-methyltetrahydrofolic acid at various concentrations and at different pHs. 2. The transport of 5-methyltetrahydrofolic acid appeared to be linear with increasing incubation time at a 5-methyltetrahydrofolic acid concentration of 10(-5) M in the incubating medium. Tissue uptake was much higher than serosal uptake. 3. At 'zero' concentration gradient of 5-methyltetrahydrofolic acid, the transport from mucosal to serosal sides was negligible but tissue uptake was appreciable. 4. Reducing the incubation temperature from 37 to 27 degrees C gave a Q10 value of 1-8. 5. The characteristics of 5-methyltetrahydrofolic acid uptake suggested a non-saturable transport mechanism. 6. Studies were also carried out with isolated mucosal epithelial cells at different incubation times and at different pH values. 7. The over-all results suggest that 5-methyltetrahydrofolic acid transport is most likely a passive diffusion of its zwitterion and also a solvent drag with water flow. 8. The 5-methyltetrahydrofolic acid may be converted to its zwitterion in an acid microclimate at the surface of the intestinal absorbing cell.

Animals↗
PubMed1976

Aspects of intestinal folate transport in the rat.

1. In vitro preparations of rat jejunum were used to study the metabolic dependence, the structural specificity, and the pH sensitivity of the folic acid absorption process. 2. The presence of 2:4 dinitrophenol, and the absence of oxygen, in the muscosal bathing medium both led to a decrease in the tissue accumulation and serosal transfer of folic acid by everted sacs. 3. 10-formylfolic acid, present in the mucosal medium at a molar ratio of 10:1 with labelled folic acid, appears to compete for tissue accumulation sites but has no significant effect on serosal transfer. 4. The efflux of folic acid from pre-loaded jejunal sacs is stimulated by the presence in the mucosal medium of the folic acid and related compounds. Pteroyl-L-glutamic acid, 10-formylfolic acid and methotrexate elicit a significant increase in the efflux rate; pteroyl-D-glutamic acid was significantly less effective and pteroic acid had no effect. 5. The uptake of folic acid by isolated jejunal cells prepared by enzymic disaggregation of the mucosa, was found to be influenced by the pH of the bathing medium, uptake being enhanced at a pH of between 5 and 6. 6. It is concluded that the effect of metabolic inhibitors and acid pH conditions on the uptake of folic acid in vitro is consistent with a passive absorption mechanism, influenced in intact preparations and in vivo by the jejunal pH microclimate. However the occurrence of competitive inhibition and stimulated efflux may indicate the existence of a structurally specific accumulation process at some site, or sites, within the mucosa.

Animals↗
PubMed2003

Involvement of human organic anion transporting polypeptide OATP-B (SLC21A9) in pH-dependent transport across intestinal apical membrane.

Some organic anions are absorbed from the gastrointestinal tract through carrier-mediated transport mechanism(s), which may include proton-coupled transport, anion exchange transport, and others. However, the molecular identity of the organic anion transporters localized at the apical membrane of human intestinal epithelial cells has not been clearly demonstrated. In the present study, we focused on human organic anion transporting polypeptide OATP-B and examined its subcellular localization and functionality in the small intestine. Localization of OATP-B was determined by immunohistochemical analysis. Transport properties of estrone-3-sulfate and the 3-hydroxy-3-methylglutaryl-CoA reductase inhibitor pravastatin by OATP-B-transfected human embryonic kidney 293 cells were measured. OATP-B was immunohistochemically localized at the apical membrane of intestinal epithelial cells in humans. Uptake of [3H]estrone-3-sulfate and [14C]pravastatin by OATP-B at pH 5.5 was higher than that at pH 7.4. [3H]Estrone-3-sulfate transport was decreased by pravastatin, aromatic anion compounds, and the anion exchange inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, but not by small anionic compounds, such as lactic acid and acetic acid. The inhibitory effect of pravastatin on the uptake of [3H]estrone-3-sulfate was concentration-dependent, and the IC50 value was 5.5 mM. The results suggested that OATP-B mediates absorption of anionic compounds and its activity may be optimum at the acidic surface microclimate pH of the small intestine. Accordingly, OATP-B plays a role in the absorption of anionic compounds across the apical membrane of human intestinal epithelial cells, although it cannot be decisively concluded that pH-dependent absorption of pravastatin is determined by OATP-B alone.

Biological Transport↗
PubMed1973

Ecological genetics and natural selection in molluscs.

Polymorphic snails of the genus Cepaea have been widely used for research in ecological genetics. Natural selection by selective predation is important in controlling morph frequencies in some populations of C. nemoralis in England. The importance of environmental selection in affecting other patterns of local genetic differentiation of population structure (area effects) is a matter of controversy. Some authors emphasize divergent evolution of whole gene pools between area effects, while others feel that climatic selection acting on individual loci is important. Analysis of 500,000 C. nemoralis snails from throughout Europe shows that there is a strong positive association between gene frequencies at the shell color locus and mean summer temperature, but that no climatic correlations are obvious at other loci. Another species, C. vindobonensis, which has a much simpler system of polymorphism than does C. nemoralis, was investigated in Yugoslavia, in a region where there is known to be intense microclimatic differentiation because of the accumulation of cold air in frost hollows. There was a striking tendency for snails with lightly pigmented shell bands to be found in places with a warm microclimate. Physiological and behavioral experiments demonstrate that this is due primarily to differential energy absorption from sunshine by the different shell phenotypes. As in C. nemoralis, other C. vindobonensis phenotypes show no detectable association with the environment. It is possible that genes whose frequencies cannot be related to environmental selection may have evolved strong linkage interactions with other genes in the population's gene pool. Selection by the ecological environment and the genetic environment may therefore both be important in controlling the genetic structure of snail populations.

Animals↗
PubMed1996

Assessment of indoor climate in an apartment by use of a fungal index.

Indoor climate was assessed in an apartment in Isehara City, Kanagawa Prefecture, Japan, by use of a fungal index. The index represents the environmental (climate) capacity to allow fungal growth; it is determined by measuring the growth rate of a biosensor fungus, Eurotium herbariorum J-183. Differences in climate among various parts of the apartment (microclimate) and its changes could be clarified by using the index. The index in the entire apartment was high in summer, low in winter, and intermediate in spring and autumn. According to the part of the apartment, the index was high in water-associated areas and cool areas. This high fungal index in cool areas was caused by the air at the same absolute humidity showing an increase in the relative humidity with a decrease in temperature. Fungal contamination rapidly progressed in areas with a high fungal index in this apartment. A correlation was observed between the fungal index and fungal contamination. Therefore, areas susceptible to fungal contamination can be estimated by use of the fungal index.

Air Microbiology↗
PubMed1983

Determination of acid surface pH in vivo in rat proximal jejunum.

Surface pH of rat intestine was measured in vivo in exteriorised loops using pH-electrodes. Surface pH was approximately 6.1 in Krebs-phosphate buffer in proximal jejunum and was significantly more acid (p less than 0.01) than the bulk medium pH of 7.2. Values for the midgut were approximately 6.5, yet the distal ileum gave values of 7.3, which was marginally more alkaline than the buffer. These results agree with the known acidification phenomenon in the jejunum and the alkalinisation process in the ileum. No difference in surface pH was detected when bicarbonate buffer was used, nor when glucose was included in the buffer. The acid surface pH was almost completely inhibited when jejunal loops were made anoxic by completely occluding the blood supply, showing that the low surface pH is not itself a consequence of preparative anoxia. If glucose was included in the buffer, allowing glucose access from the luminal surface, surface pH was not significantly altered after occlusion. This indicates that previously reported in vitro results in the presence of glucose are similar to the present in vivo findings. The present experiments confirm the existence in vivo of the 'acid-microclimate' proposed to alter the absorption profiles of some dissociable drugs.

Animals↗
PubMed1980

pH dependence of micellar diffusion and dissociation.

A simplified model system was used to examine the effect of pH on the diffusion and dissociation of mixed micelles across an interphase. Diffusion of radioactively labeled taurocholate and oleic acid from a micellar solution into a phosphate buffer of varying pH (5.0-8.0) was measured. The proportion of bile salt and fatty acid as monomers and aggregates in both solutions was determined by ultrafiltration. Results indicate that all of the oleic acid existed as aggregates in the micellar solution, whereas taurocholate was in equilibrium between aggregate and monomer forms. Between pH 5.0 and 7.0, oleic acid and taurocholate diffusion was inversely related to pH and due to the differences in aggregate but not monomer diffusion. Fatty acid diffusion across the interface was always less than taurocholate diffusion, because taurocholate monomers could also diffuse across the interphase. Calculation of the proportion of taurocholate diffusion from aggregate and monomers based on micellar composition was similar to measured values at pH greater than or equal to 6.5, indicating a lack of micelle dissociation. However, at pH less than or equal to 6 measured values for bile salt monomers exceeded calculations, indicating that a low pH microclimate favors dissociation of micelles. A hypothesis is advanced to explain these events occurring in a disequilibrium area adjacent to the epithelial cell membrane.

Colloids↗
PubMed1981

Transport and metabolism of water-soluble vitamins in intestine.

An overview of intestinal absorption of water-soluble vitamins is presented. Evidence relating to absorption of ascorbic acid by the "sodium-gradient" hypothesis and folic acid by the "acidic microclimate" hypothesis is discussed. Over the last decade, vitamin absorption by the intestine has been attributed in large part to simple diffusion. Recently, however, this view has been changing as information continues to be gathered which suggests the participation of membrane-bound "carriers" in transport and the involvement of cellular enzymes in metabolism of the vitamins during absorption. The circumstances that make vitamin absorption difficult to investigate are described. A brief look toward the future stresses the types of information that will be helpful to acquire, identifies some technological advances that will promote progress, and discusses potential application of the results to health care. Some reasons are given for confusion that has prevailed in descriptions of vitamin absorption during the last decade; suggestions are made that might help investigators to correct this situation.

Animals↗
PubMed1987

Folate transport by human intestinal brush-border membrane vesicles.

Transport of folic acid (Pte-Glu) across the brush-border membrane of human intestine was studied using brush-border membrane vesicle (BBMV) technique. The transport of Pte-Glu was higher in BBMV prepared from the jejunum than those prepared from the ileum (0.70 +/- 0.05 and 0.14 +/- 0.02 pmol X mg protein-1 X 10 s-1, respectively). The transport of Pte-Glu appeared to be carrier mediated and was pH dependent and increased with decreasing incubation buffer pH; saturable (Kt = 1.69 microM, Vmax = 4.72 pmol X mg protein-1 X 10 s-1); inhibited in a competitive manner by the structural analogues 5-methyltetrahydrofolate, methotrexate, and 5-formyltetrahydrofolate (Ki = 2.2, 1.4 and 1.4 microM, respectively); not affected by inducing a relatively positive or negative intravesicular compartment; independent of Na+ gradient; and inhibited by 4,4'-diisothiocyanatostlibene-2,2'-disulfonic acid (DIDS), an anion exchange inhibitor. The increase in Pte-Glu transport on decreasing incubation buffer pH appeared to be in part mediated through a direct effect of acidic pH on the transport carrier and in part through the pH gradient imposed by activating Pte-Glu-:OH- exchange and/or Pte-Glu-:H+ co-transport mechanisms. The important role of an acidic extravesicular environment in Pte-Glu transport is consistent with a role for the intestinal surface acid microclimate in folate transport. These results demonstrate that Pte-Glu transport in human BBMV occurs by a carrier-mediated system that is similar to that described for rat and rabbit intestinal BBMV.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1990

Acidic mucin layer facilitates micelle dissociation and fatty acid diffusion.

The presence of a mucin layer on the surface of the intestinal epithelium has been suggested as an important factor in maintaining an acidic microclimate. The presence of such a low-pH compartment has been shown to facilitate fatty acid uptake. The mechanisms leading to the enhancement of fatty acid uptake were investigated in a purified acidic mucin layer. Our results indicate that the presence of a low-pH compartment indeed facilitates the dissociation of mixed micelles made of taurocholate and oleic acid. The released fatty acid formed an emulsion at the mucin layer, and this event could be visualized by the naked eye. When the size of the particles in the micelle solution was examined by photon correlation spectroscopy, it was found that acidification alone can lead to the formation of particles with size substantially greater than that of micelles. With the use of labeled fatty acid, the change in optical density can be correlated to the amount of fatty acid appearing in the mucin layer in an asymptotic fashion, suggesting that using the turbidity as an indicator might underestimate fatty acid diffusion. Despite this limitation, the rate of fatty acid diffusion in the mucin layer was estimated to be 400% of that in the buffer solution.

Animals↗
PubMed1979

Modes of thermal protection in polar bear cubs--at birth and on emergence from the den.

At birth in late December the polar bear is small (700 g), uninsulated, and helpless. It probably has a modest capacity for metabolic heat production and depends on the female and a snow den in which it is born for thermal protection. The microclimate of an artificial polar bear den was investigated at Point Barrow, AK, and the temperature therein found to stay around 0 degrees C provided a heat source (200 W) equivalent to an adult polar bear was introduced. When the bears desert the den in early April the cub has grown to about 10 kg and has a well-developed fur insulation, but almost no subcutaneous fat. The cub has a high resting metabolic rate (4.6 W.kg-1), which is supported by the fat polar bear milk. Its lower critical temperature is about -30 degrees C, and an ambient temperature of -45 degrees C results in only a 33% increase in metabolism. The cub can tolerate a wind chill of 2.3 kW.m2 without apparent stress of drop in rectal temperature. If the cub is immersed in ice water rectal temperature drops 11 degrees C in 30 min. It is concluded that the cub can tolerate extremely low temperatures in air due to fur insulation and high metabolic heat production, but is unable to cope with the chill of ice water for any prolonged period of time.

Adipose Tissue↗
PubMed1987

Cooling different body surfaces during upper and lower body exercise.

The effect of varying the body surface area being cooled by a liquid microclimate system was evaluated during exercise heat-stress conditions. Six male subjects performed a total of six exercise (O2 uptake = 1.2 l/min) tests in a hot environment (ambient temperature = 38 degrees C, relative humidity = 30%) while dressed in clothing having low moisture permeability and high insulation. Each subject completed two upper body exercise (U; arm crank) tests: 1) with only the torso surface (T) cooled; and 2) with the surfaces of both the torso and upper arms (TA) cooled [coolant temperature at the inlet (Ti) was 20 degrees C for all upper body tests]. Each subject also completed four lower body exercise (L; walking) tests: 1) with only the T cooled (Ti = 20 degrees C); 2) with only the T cooled (Ti = 26 degrees C); 3) with torso, upper arm, and thigh surface (TAT) cooled (Ti = 20 degrees C); and 4) with TAT cooled (Ti = 26 degrees C). During U exercise, TA cooling had no effects compared with cooling only T. During L exercise, sweat rates, heart rates, and rectal temperature (Tre) changes were less with TAT cooling compared with cooling only the T. Altering Ti had no effect on Tre changes, but higher heart rates were observed with 26 than with 20 degrees C. These data indicate that cooling arms during upper body exercise provides no thermoregulatory advantage, although cooling the thigh surfaces during lower body exercise does provide an advantage.

Adult↗
PubMedDate not supplied

Physiological changes in skin barrier function in relation to occlusion level, exposure time and climatic conditions.

Skin occlusion is a condition encountered with various articles as part of everyday life and resulting changes in skin barrier physiology often remain unnoticed. In the present study we aimed to understand the impact of absorbent feminine hygiene articles, one vapor-permeable and one vapor-impermeable, on skin hydration in response to exposure time and different environmental climatic conditions. Using a forearm model, volunteers were patched with moistened vapor-permeable and -impermeable articles in parallel for 1, 3, and 6 h and under different climatic conditions (i.e. 20 degrees C/30% relative humidity, 25 degrees C/50% relative humidity, 30 degrees C/ 75% relative humidity). The physiological changes in the skin barrier function were measured via skin hydration, evaporation of superficial water (skin surface water loss, SSWL) and relative humidity in the microclimate between skin and occlusive article (RH(mc)). The results show that skin hydration, SSWL, and RH(mc) under a vapor-permeable article are reduced versus the vapor-impermeable article for all exposure times at 25 degrees C and 50% relative humidity. SSWL and RH(mc) decrease from their 1-hour peak values with increasing exposure time, while skin hydration decreases only after 3 h of exposure. Lower environmental temperature (20 degrees C) and lower relative humidity (30%) have little impact on the reduction of SSWL and RH(mc,) but more so on the reduction of skin hydration. Higher temperature (30 degrees C) and higher relative humidity (75%) increase RH(mc) and skin hydration under both vapor-permeable and -impermeable articles while SSWL is reduced under the vapor-impermeable article under these conditions. In conclusion, vapor-permeability is the key factor for physiological changes in the barrier function of the skin under occlusion, exposure time and climatic conditions being modulating factors. These findings have been integrated into a model of skin hydration under occlusion in the context of absorbent hygiene articles. While current vapor-impermeable articles are effective in reducing the excessive moisture on the skin due to bodily discharge, vapor permeability adds a further measurable benefit in reducing skin overhydration.

Adult↗
PubMed1985

The transmembrane pH gradient drives uphill folate transport in rabbit jejunum. Direct evidence for folate/hydroxyl exchange in brush border membrane vesicles.

In rabbit jejunal, but not ileal brush border membrane vesicles, an outwardly directed OH- gradient (pH 7.7 inside, pH 5.5 outside) markedly stimulated the initial velocity of folate (0.1 microM) uptake compared with uptake in the absence of a pH gradient. Under pH gradient conditions, folate was transiently accumulated at a concentration four times that found at equilibrium (over-shoot), implying uphill transport of the vitamin. Equilibrium folate uptake was inversely proportional to medium osmolality, suggesting uptake into an osmotically sensitive space. pH gradient-stimulated folate uptake was markedly reduced by inhibitors of anion exchange (4,4'-diisothiocyano-2,2'-disulfonic acid stilbene; 4-acetamido-4-isothiocyanostilbene-2,2'-disulfonic acid; furosemide), and was saturable (folate Km = 0.19 +/- 0.02 microM; Vmax = 12.8 +/- 0.4 pmol X mg protein-1 X min-1). Imposition of an inside-positive electrical potential did not stimulate folate uptake, suggesting that stimulation by a pH gradient was not due to an induced electrical potential. In contrast, an inwardly directed Na+ or K+ gradient did not stimulate folate uptake. These findings provide evidence for a carrier on the jejunal brush border membrane that mediates folate/OH- exchange (or H+/folate co-transport), and are consonant with the known presence of an outwardly directed OH- gradient in vivo (brush border acid microclimate), an acidic pH optimum for intestinal folate uptake, and the primary role of the jejunum in folate absorption.

Animals↗
PubMed1999

La Crosse and other forms of California encephalitis.

The California serogroup viruses are mosquito viruses that cause human infections on five continents. They are maintained and amplified in nature by a wide variety of mosquito vectors and mammalian hosts; they thrive in a remarkably wide variety of microclimates (eg, tropical, coastal temperate marshland, lowland river valleys, alpine valleys and highlands, high boreal deserts, and arctic steppes). In 1993, California serogroup viruses caused 71% of all cases of arboviral illness in the United States, principally La Crosse encephalitis. The 30 to 180 annual cases of La Crosse encephalitis represent 8% to 30% of all cases of encephalitis, rendering this illness the most common and important endemic mosquito-borne illness in the USA. Subclinical or mild infections are much more common. Methods and results acquired from intense study of California serogroup viruses have been applied, with benefit, to the study of the ecology and pathogenesis of many more serious human arboviral illnesses. The evolutionary potential of viruses, with particular reference to the development of more virulent strains, has been studied more closely in the California serogroup viruses than in almost any other agent of human disease.

Animals↗
PubMed2004

The dynamics of social-ecological systems in urban landscapes: Stockholm and the National Urban Park, Sweden.

This study addresses social-ecological dynamics in the greater metropolitan area of Stockholm County, Sweden, with special focus on the National Urban Park (NUP). It is part of the Millennium Ecosystem Assessment (MA) and has the following specific objectives: (1) to provide scientific information on biodiversity patterns, ecosystem dynamics, and ecosystem services generated; (2) to map interplay between actors and institutions involved in management of ecosystem services; and (3) to identify strategies for strengthening social-ecological resilience. The green areas in Stockholm County deliver numerous ecosystem services, for example, air filtration, regulation of microclimate, noise reduction, surface water drainage, recreational and cultural values, nutrient retention, and pollination and seed dispersal. Recreation is among the most important services and NUP, for example, has more than 15 million visitors per year. More than 65 organizations representing 175,000 members are involved in management of ecosystem services. However, because of population increase and urban growth during the last three decades, the region displays a quite dramatic loss of green areas and biodiversity. An important future focus is how management may reduce increasing isolation of urban green areas and enhance connectivity. Comanagement should be considered where locally managed green space may function as buffer zones and for management of weak links that connect larger green areas; for example, there are three such areas around NUP identified. Preliminary results indicate that areas of informal management represent centers on which to base adaptive comanagement, with the potential to strengthen biodiversity management and resilience in the landscape.

City Planning↗
PubMed2006

Water vapour absorption in the penicillate millipede Polyxenus lagurus (Diplopoda: Penicillata: Polyxenida): microcalorimetric analysis of uptake kinetics.

The aberrant millipedes of the order Polyxenida are minute animals that inhabit xeric microclimates of bark and rock faces. The lichens and algae that provide their main food substrates tolerate extensive dehydration, effectively eliminating a liquid water source during periods of drought. In this study, we used microcalorimetry to test whether Polyxenus lagurus (L.) exploits active water vapour absorption (WVA) for water replenishment. Individual animals were pre-desiccated to 10-20% mass-loss and heat fluxes then monitored using a TAM 2277 microcalorimeter. The calorimetric cell was exposed to an air stream increasing progressively in humidity from 84% to 96%. WVA was distinguishable as large exothermic fluxes seen in > or = 86% RH. Owing to very small and opposing heat fluxes from metabolism and passive water loss, the measured flux provided a good measure of water uptake. WVA showed an uptake threshold of 85% RH and linear sorption kinetics until >94% RH, when uptake became asymptotic. Uptake was rapid, and would allow recovery from 20% dehydration (by mass) in little over 5 h. The uptake flux scales proportional, variant mass (0.61), suggesting an area-limited mechanism. Polyxenus possesses a cryptonephric system, analogous to that of tenebrionid beetle larvae. Measurements of water absorption and desorption from faecal pellets voided in different humidities gave an estimated rectal humidity of 85.5%. The close congruence between this value and the WVA threshold provides evidence for a cryptonephric uptake mechanism derived independently from that of tenebrionids. Polyxenus represents the first documented example of WVA in the myriapod classes.

Absorption↗