An analysis of the D-glucose influx kinetics of in vitro hamster jejunum, based on considerations of the mass-transfer coefficient

Carrier-Mediated Transport of Glycerol in the Perfused Rat Small Intestine

Studies using the closed loop and everted sacs of the rat small intestine recently prompted us to suggest that carrier-mediated transport is involved in the intestinal absorption of glycerol. Although it could be mediated by a novel carrier system, little information is available. The aim of the present study was to kinetically characterize carrier-mediated glycerol transport in the perfused rat small intestine to help in identifying the carrier involved and to explore the possibility that the carrier might be used as a pathway for oral drug delivery and a target for drug development. In situ single-pass perfusion was conducted using a 10-cm midgut segment of the male Wistar rat, and the absorption of [3H]glycerol was evaluated by its disappearance from the intestinal lumen. The absorption of glycerol was saturable and significantly reduced by removing Na+ from the perfusion solution, suggesting the involvement of a Na+-dependent carrier-mediated transport system. The concentration-dependent absorption profile was successfully analyzed by assuming Michaelis-Menten type carrier-mediated transport and simultaneous passive (diffusive) transport. The maximum transport rate (J(max)) was 77.0 pmol/s/cm2 and the Michaelis constant (K(m)) was 1.04 mM, giving a J(max)/K(m) of 7.39 x 10(-5) cm/s. The membrane permeability coefficient for passive transport (P(m,d)) was 6.89 x 10(-5) cm/s, slightly smaller than J(max)/K(m). Therefore, it could be the major mechanism of intestinal glycerol absorption in the low concentration range where carrier-mediated transport conforms to linear kinetics represented by J(max)/K(m). Furthermore, carrier-mediated glycerol transport was found to be inhibited by glycerol 3-phosphate, monoacetin and diglycerol, indicating that the carrier system may be shared by these structural analogues. Thus, the present study has successfully demonstrated and characterized carrier-mediated glycerol transport in the perfused rat small intestine which is a physiologically relevant model.

The effect of auxiliary conditions on intestinal unstirred layer diffusion modelled by numerical simulation

Estimation of intestinal unstirred layer thickness usually involves inducing transmural potential difference changes by altering the content of the solution used to perfuse the small intestine. Osmotically active solutes, such as mannitol, when added to the luminal solution diffuse across the unstirred water layer (UWL) and induce osmotically dependent changes in potential difference. As an alternative procedure, the sodium ion in the luminal fluid can be replaced by another ion. As the sodium ion diffuses out of the UWL, the change in concentration next to the intestinal membrane alters the transmural potential difference. In both cases, UWL thickness is calculated from the time course of the potential difference changes, using a solution to the diffusion equation. The diffusion equation solution which allows the calculation of intestinal unstirred layer thickness was examined by simulation, using the method of numerical solutions. This process readily allows examination of the time course of diffusion under various imposed circumstances. The existing model for diffusion across the unstirred layer is based on auxiliary conditions which are unlikely to be fulfilled in the same intestine. The present simulation additionally incorporated the effects of membrane permeability, fluid absorption and less than instantaneous bulk phase concentration change. Simulation indicated that changes within the physiologically relevant range in the chosen auxiliary conditions (with the real unstirred layer length kept constant) can alter estimates of the apparent half-time. Consequently, changes in parameters unassociated with the unstirred layer would be misconstrued as alterations in unstirred layer thickness.

Two-sided culture of human placental trophoblast. Morphology, immunocytochemistry and permeability properties

We describe the culture of human term placental trophoblast cells on cell-free amniotic membrane, with medium on both sides. Over the course of 2 days, the isolated cells, initially simple, mononucleated and probably cytotrophoblast, form a confluent layer of multinucleated syncytial cells with morphological and immunocytochemical properties of syncytiotrophoblast. This layer becomes polarized with respect to morphology, alkaline phosphatase distribution and hCG secretion. Contamination with amnion cells, and with other cell types that are present in placental tissue, was less than 1 per cent. A preliminary investigation of the permeability properties of the preparation showed that the trophoblast cell layer, rather than the amniotic membrane, was rate-limiting to transtrophoblast transfer, but that possible effects of the supporting membrane should be considered. The transtrophoblast transfer of D-glucose and the non-metabolisable analogue, 3-O-methyl-D-glucose (3OMG), had saturable and non-saturable/leak components in both directions, indicating that carrier-mediated processes were involved. The non-metabolisable amino acid 2-aminoisobutyrate (AIB) was both accumulated within the trophoblast cells, and transferred by saturable and non-saturable processes from the microvillous side, but no saturable accumulation or transfer was observed from the basal side, at the concentrations tried. The results suggest that this model may prove suitable for studies of transtrophoblast transfer.

Changes in intestinal glucose transport over the lifespan of the rat

Age-related changes in intestinal glucose absorption were studied using everted intestinal sacs and brush border membrane vesicles prepared from male F344 rats. Glucose uptake by everted intestinal sacs was greatest in young (2-3-month-old) as compared with adult (12-14-month-old) and old (24-month-old) rats. The greatest decrease in glucose uptake occurred between 2 and 12 months. The addition of phloridzin reduced glucose uptake to similar levels in all age groups, suggesting that the age-related change was in the carrier-mediated component of glucose transport. In order to localize the site of decreased carrier-mediated glucose transport, experiments were performed using brush border membrane vesicles. Vesicular glucose uptake in the presence of Na was significantly greater in vesicles prepared from 2-month-old rats (133 +/- 18 pmol/mg/s), compared with those prepared from 12-month-old rats (82 +/- 13 pmol/mg/s). Kinetic studies performed under non-equilibrium conditions demonstrated that the major effect of age was on the Na-dependent component of the brush border transport system. There was a reduction in the Vmax from 335 +/- 37 pmol/mg/s in the young to 217 +/- 22 pmol/mg/s in the adult, but there was no change in the Km. Isotope exchange studies performed under equilibrium conditions confirmed a decrease in the activity of the glucose transporter with age. No age-related changes in Na uptake by brush border membrane vesicles were observed. These findings suggest that a decrease in the number and/or activity of Na-linked glucose carriers may account for the decrease in intestinal glucose transport with age.

Determination of kinetic parameters of a carrier-mediated transport in the perfused intestine by two-dimensional laminar flow model: effects of the unstirred water layer

The kinetic parameters of a carrier-mediated transport for D-glucose and for taurocholate were determined from rat in situ intestinal single perfusion experiments. The true parameters were obtained by the two-dimensional laminar flow model, in which the solute concentration at the aqueous-intestinal membrane interface can be calculated numerically without assuming the aqueous diffusion layer, discriminating the effects of the unstirred water layer. The true Michaelis constant was 4.5 mM for D-glucose and 1.5 mM for taurocholate. The true maximal transport velocity was 3.4 nmol/s per cm2 for D-glucose and 0.29 nmol/s per cm2 for taurocholate. The apparent Michaelis constant was raised by the factor of 6.6 for D-glucose and 3.6 for taurocholate due to the effects of the unstirred water layer. The maximal transport velocity was relatively unaffected by the unstirred water layer in both compounds. The values of the effective (operational) thickness of the unstirred water layer were compatible with those reported previously by employing various experimental methods. The kinetic parameters obtained in vitro everted sacs, for comparison, almost coincided with the true ones in situ. Therefore, the two-dimensional laminar flow model is shown to be valid not only for determining the kinetic parameters of a carrier-mediated transport in situ but also for predicting the absorption rate in situ from the uptake rate in vitro.

Placental amino acid uptake. VI. Regulation by intracellular substrate

Amino acid uptake by human placental tissue is regulated by intracellular amino acids. alpha-Aminoisobutyric acid (AIB) uptake was reduced at intracellular AIB concentrations of 0.8 mM. The magnitude of reduction increased sharply between 1 and 3 mM and reached a maximum of 45% at 5 mM. Suppression was specific to the "A" system. It occurred only when both the amino acid used for preloading and that used as an uptake substrate were active with that system. In the "L" system, facilitation apparently occurs, and in the "ASC" system there is no apparent effect. The system specificity as well as other evidence indicated that suppression is caused by substrate present intracellularly rather than by dilution of extracellular substrate. Suppression was independent of inhibitors of protein synthesis and was not seen in membrane vesicles prepared from preloaded tissue, indicating that intracellular substrate interacts directly with the carrier (transinhibition) rather than altering its synthesis or degradation. The A system transinhibition has the potential to regulate syncytial uptake in vivo and limit variation due to changes in maternal plasma amino acid concentration.

Effects of unstirred layers on the kinetics of carrier-mediated solute transport by two systems

Equations describing the effect of unstirred layers on the simultaneous transport of a solute via two mediated systems have been derived. It has been previously established that unstirred layers cause convex curvature of Eadie-Hofstee transformations of kinetic data for single mediated transport systems. On the other hand, multiple transport systems produce concave curvature in this plot. A numerical example is presented which illustrates the fact that under some circumstances the opposing effects can nearly cancel out, producing an erroneous apparently linear relation. Furthermore, statistical analysis of these curves suggests that the normal error encountered in physiological transport measurements can readily obscure evidence of unstirred layers. It is suggested that kinetic analysis of transport, particularly in epithelial tissues, should whenever possible, include independent measurements of unstirred layers, employ a broad range of substrate concentrations, and vary the stirring rate of the medium. Evaluation of the 'true' kinetic constants can then be made with the relations derived here. In addition, several limiting cases have been discussed which reduce to simpler kinetic descriptions published earlier.

Influence of viscous incubation media on the resistance to diffusion of the intestinal unstirred water layer in vitro

Everted sacs of rat intestine were incubated in media made viscous with Na-carboxymethylcellulose (CMC). This increased the resistance to diffusion of the mucosal unstirred water layer, as measured by the half-time for the establishment of osmotic streaming potentials. The increase ws not however linearly related to viscosity, but reached a maximum, equivalent to an apparent thickness of 500 mu, at a viscosity of about 100 cP (shear rate = 50 s-1). The inhibition of maltose hydrolysis by CMC conformed to theoretical expectations, and was characterised by an increased apparent Km, with no change in Vmax. This was not true however of galactose transport by everted sacs which showed a decline in maximum transport rate, with no apparent increase in Km. It appears that increased unstirred layer resistance may induce partial anoxia in the everted sac preparation and thus reduce the energy available for active transport. We conclude that the effect of enhanced viscosity is to reduce turbulent flow, and hence stirring, in the intervillous compartment. Such an effect may be brought about by viscous components of the diet and may influence rates of absorption and digestion in the intact intestine in vivo.

Comparison of kinetic constants of hexose uptake in four animal species and man

1. The rate of in vitro uptake of glucose (gl), galactose (gal), 3-O-methyl glucose (MG), and fructose (fr) was determined using jejunal discs of hamster (ham), rat, rabbit (rab) and guinea pig (gp). 2. The maximal transport rates (Jdm) of each hexose were ham greater than rab greater than gp = rat; 3. The Michaelis constants were different for the four sugars; for gl the values were gp greater than rat greater than ham greater than rab. 4. The passive permeability coefficients (Pd) were similar for each sugar, and were greatest in ham and rat, and lowest in rat and gp. 5. The effective resistance of the intestinal unstirred water layer was lowest in ham and highest in gp and rat. 6. There were differences in the kinetic constants for gl estimated using discs and biopsies of jejunum, and the greater uptake into human biopsies than in the other species was due to the higher Jdm and Pd.

Intestinal folic acid absorption and the acid microclimate. The effects of compounds relevant to folate malabsorption

The effect s of various substances and circumstances on folic acid absorption and surface pH were investigated in vitro in rat proximal jejunum. Prior consumption of alcohol and libitum, phenytoin and in vitro methotrexate had no effect on folate absorption. In contrast, ethanol in vitro, low sodium ion containing buffers in vitro and oral methotrexate given prior to experiment reduced absorption. Folate absorption did not correlate with water movement since methotrexate decreased folate but enhanced fluid absorption, ruling out direct coupling. Surface pH was elevated by the low sodium ion containing buffers, ethanol in vitro and oral methotrexate, correlating inversely with folate absorption. The parallelisms between absorption and changes in surface pH support the microclimate hypothesis of folic acid absorption where the rate of transport is largely determined by the surface pH of the proximal jejunum.

Virtual elimination of the interference of unstirred water layers on intestinal sugar transport kinetics by use of the tissue accumulation method at appropriate shaking rates

The intestinal transport of sugars and amino acids seems to follow Michaelis-Menten kinetics, but the presence of unstirred water layers at the outer face of the brush border membrane may distort kinetic measurements. According to current theory, the capacity parameter, Jmc max would not be affected, but the Kt would be increased to a higher value, Kt ', in proportion to the thickness of the unstirred water layer, d. We reasoned that by increasing the shaking rate in the tissue accumulation method, d might drop to such small values that Kt ' would fall to a constant level practically equal to the "true" Kt. We measured D-galactose influx into rings of everted hamster intestine as a function of both the substrate concentration and the shaking rate. Our results show that as the circular stirring rate increases from 0.38--6.2 Hz, J mc max remains constant, as expected, but Kt ' first drops, then levels off to reach a plateau between 2 and 6.2 Hz. We conclude that the average Kt values in this frequency range (Kt = 7.4 mM) represent the true transport Kt. Furthermore, all previous kinetic work performed in our laboratory has been carried out under identical conditions, including shaking rates of 4 Hz. The validity of our preceding results is thus upheld.

Kinetic constants for intestinal transport of four monosaccharides determined under conditions of variable effective resistance of the unstirred water layer

Theoretical considerations have suggested that variations in the resistance of the unstirred water layer (UWL) have a profound effect on the kinetic constants of intestinal transport. In this study, a previously validated in vitro technique was employed to determine the unidirectional flux rate of glucose, galactose, 3-O-methyl glucose and fructose into the rabbit jejunum under carefully-defined conditions of stirring of the bulk phase known to yield different values for the effective resistance of the UWL. For each monosaccharide, uptake is much greater when the resistance of the UWL is low than when high. The maximal transport rate, Jd m, of glucose was half as large as the Jd m of galactose and 3-O-methyl glucose (3-O-MG), and was twice as great as the Jd m of fructose. The apparent affinity constant, Km * of glucose is less than that of fructose, which was lower than the Km * of galactose and 3-O-MG. The use of the Lineweaver-Burk double reciprocal plot is associated with an overestimation of both Jd m and Km *. This discrepancy between the true and apparent values of the kinetic constants is much greater for lower than for higher values of Jd m and Km *; variations in the resistance of the unstirred layer influences the magnitude and direction of the discrepancy. The apparent passive permeability coefficient is similar for each sugar, but because of the different values of Jd m, passive permeation contributes relatively more to the uptake of glucose and fructose than of galactose or 3-O-MG. Under conditions of high unstirred layer resistance, differences in uptake rates of the sugars are due to differences in their Jd m rather than their Km *. Kinetic analysis is compatible with the suggestion that the glucose carriers are predominantly near the tip of the villus, whereas those for galactose and 3-O-MG are located along the entire villus and the Km * of their carriers at the tip is lower than their Km * towards the base of the villus. It is proposed that there are multiple or heterogeneous intestinal carriers for glucose, galactose and 3-O-methyl glucose in the jejunum of the rabbit.

Rat jejunum perfused in situ: effect of perfusion rate and intraluminal radius on absorption rate and effective unstirred layer thickness

In anaesthetized rats a jejunal segment was perfused in situ varying the perfusion rate (0.1, 0.2, 0.5 ml/min) in a randomized order. The intraluminal radius of the segments was small (1.7 mm) or enlarged (3.1 mm) by increasing the intraluminal pressure. The appearance rate of butanol, antipyrine, salicylic acid, D- and L-phenylalanine but not of urea in the venous blood of the jejunal segments was increased up to 35%, when the intraluminal perfusion rate was raised from 0.1 to 0.5 ml/min. Two factors contribute to this effect: the flattening of the concentration gradient down the segment and the reduction of the effective unstirred layer thickness. The length and the intraluminal radius of the perfused segments was not altered, when the perfusion rate was varied. Therefore, a change of the absorbing area did not contribute to the increase of the absorption rate induced by the increase of the perfusion rate. In the series with small intraluminal radius the experimental data corresponded to the theoretical predictions obtained for a laminar intraluminal flow. In the segments with enlarged intraluminal radius the increase of the absorption rate by raising the perfusion rate was less than expected for a laminar flow indicating that the flow might have been turbulent. The enlargement of the intraluminal radius from 1.7 to 3.1 mm increased the absorption rate up to 100%.

Transport kinetics of D-glucose in human small intestinal mucosa: rate constants in histologically normal and abnormal mucosal biopsies

Using D-glucose as the probe molecule, we analysed conditions which must be fulfilled in mucosal biopsy studies before the kinetic nature of the transport process can be established. Mucosal biopsies were obtained from the region of the ligament of Treitz from four healthy volunteers and from 47 patients: 29 of the 47 had histologically normal mucosa; seven had mild abnormalities; and 11 had moderate or severe ("flat") abnormalities in villus architecture. The rate of uptake of 40 mM glucose was constant only between 4 and 10 min, extrapolating through zero uptake at zero time with a constant adherent mucosal fluid volume. Incubation for shorter or longer periods was associated with over- or underestimation of the rate of uptake. Failure to use a nonabsorbable marker was also associated with overestimation of the rate of uptake. When biopsies were incubated for 6 min, a curvilinear relationship was observed between uptake rate and concentration. In biopsies with moderate and severe abnormalities, there was a marked reduction in the magnitude of the maximal transport rate and the apparent passive permeability coefficient, with little change in the magnitude of the apparent affinity constant. When human mucosal biopsies are used to study nutrient absorption, certain criteria must be fulfilled to establish valid rate constants and to make comparisons between normal and abnormal mucosa.

Limitations of the Eadie-Hofstee plot to estimate kinetic parameters of intestinal transport in the presence of an unstirred water layer

In the presence of an intestinal unstirred water layer, the relationship between substrate concentration (C1) and unidirectional flux (Jd) is not described by the equation for a rectangular hyperbole. Accordingly, transformations of the Michaelis-Menten equation may not necessarily be linear and may lead to serious errors in the estimation of the affinity constant (Km) and maximal transport rate (Jdm) of carrier-mediated processes. An equation has previously been derived which described Jd under conditions of varying effective thickness or surface area of the unstirred water layer, the free diffusion coefficient of the probe molecule, and the distribution of transport sites along the villus. These theoretical curves have been analyzed by using the Eadie-Hofstee transformation (Jd vs. Jd/C1) of the Michaelis-Menten equation. Use of this plot leads to serious discrepancies between the true and apparent affinity constants and between true and apparent maximal transport rates. These differences are further magnified by failure to correct for the contribution of passive permeation. The Eadie-Hofstee plot is of use, however, to infer certain qualitative characteristics of active transport processes, such as the variation in affinity constants and overlying resistance of the unstirred water layer at different sites along the villus and to predict the adequacy of the correction for the contribution of passive permeation.

The influence of gestational age and size on the absorption of D-xylose and D-glucose from the small intestine of the human neonate

D-Xylose absorption (using the one-hour blood levels) and active D-glucose absorption (calculated from changes in transmural electrical potential difference) have been measured after intraduodenal infusion of the appropriate sugar, in groups of neonates who were full-term or preterm, and who were in addition either appropriately sized for gestational age, or growth-retarded in utero. The one-hour D-xylose blood levels were significantly lower in preterm infants, and were even lower in the growth-retarded group. In follow-up studies D-xylose absorption rose in all groups of infants over the first three weeks of postnatal life but there was no sign of 'catch-up' in the results of the growth-retarded infants. In the studies of D-glucose absorption the theoretical kinetic parameters 'apparent P.D.max' (equivalent to Vmax) and 'apparent Km' were calculated. Normal full-term infants had values for both parameters that were approximately two-thirds of published adult values. Malnourished neonates had significantly lower P.D.max and apparent Km, and preterm infants had a lower P.D.max, than the values obtained in those infants who were full-term and appropriately grown.

Role of unstirred layer in intestinal absorption of phenylalanine in vivo

The appearance rate of L- and D-phenylalanine in the venous blood of rat jejunal loops in vivo is increased up to 60% if the intraluminal solution is mixed more efficiently by the simultaneous perfusion of air. The effect decreases as the luminal concentration is increased to 100 mmol/l. Thus, the apparent Michaelis constants are by 50% lower in the case of the reduced unstirred layer (26 to 17 for L- and 9 to 6 mmol/l for D-phenylalanine). The enhancement of the absorption and the reduction of the Michaelis constants can be attributed to the reduction of the effective unstirred layer thickness by about 400--500 micrometer.

[Kinetic properties of the fructose influx across the brush border of the rat jejunum. Effects of a diet rich in fructose]

The unidirectional influx (i.e. initial rate of uptake) of D-fructose across the brush border of rat jejunum is a saturable function of concentration, with a Kt of 125 mM, which implicates a carrier mechanism. This mechanism appears to be very specific for fructose in view of the lack of influx inhibition observed in the presence of large concentrations of the sugars or polyols, D-glucose, D-galactose, D-mannose, D-xylose, L-sorbose, D-tagatose, sorbitol or mannitol. D-Fructose uptake is inhibited by incubation, preceded by a 30-min preincubation in the same inhibitory conditions, in the absence of Na, or in the presence of metabolic poisons, NaF, 2,4-dinitrophenol, monoiodoacetate. Phloridzin (10-3 M), with or without preincubation, has no effect on uptake. D-Fructose influx is stimulated by fructose feeding, mainly because the augmentation of the number of active sites of transfer: Jmax is increased two-fold, Kt is more weakly affected.

Dependence of intestinal absorption in vivo on the unstirred layer

The appearance rate of butanol, antipyrine, salicylic acid, and urea in the venous blood of rat jejunal loops perfused in vivo is increased up to 64%, if the intraluminal solution is mixed more efficiently by the simultaneous perfusion of air. The enhancement of the absorption can be attributed partly to the enlarged absorbing area but mainly to the reduction of the effective unstirred layer thickness by about 500 micrometers. The unstirred layer reduces the phenylalanine absorption at 0.1 mmol l(-1) but not at 100 mmol l(-1), since at high concentrations a full saturation of the transport system can be achieved in spite of the unstirred layer resistance. The interference of the unstirred layer increases with increasing absorbability of the substances.

Discrimination between different entry mechanisms for neutral amino acids in rabbit ileal mucosa

1. Influx of neutral and basic amino acids into the mucosal epithelium of the rabbit ileum was measured in the presence of high and low concentrations of Na. Uptake measured at an amino acid concentration of 1 mM varied from 10 to 16 nmole . cm-2 . min-1. Removal of Na inhibited the uptake of short-chain amino acids more than long-chain amino acids. 2. Inhibition of uptake of glycine, alanine, leucine and phenylalanine by a selection of non-radioactive amino acids was found to follow a particular pattern. Long-chain amino acids inhibited the uptake of all test amino acids; short-chain amino acids inhibited preferentially the uptake of glycine. 3. The maximum inhibition serine could cause to the uptake of other amino acids was found to vary. Serine inhibited completely the uptake of glycine but a portion of uptake of long-chain amino acids was found to persist, even in the presence of high concentrations of serine. This is taken as evidence for the presence of an amino acid uptake mechanism having no affinity for serine. 4. The apparent affinities of neutral amino acids for the serino-inhibitable system (system 1) varied from about 0.5 mM (for long-chain amino acids) to about 3 mM (for short-chain amino acids). The total uptake of individual amino acids by system 1 was essentially similar when compared at an amino acid concentration of 1 mM. 5. The serine-resistant uptake of neutral amino acids (system 2) constituted up to two fifths of total uptake for long-chain amino acids, measured at amino acid concentrations of 1mM. The affinities of long-chain amino acids for system 2 is thought to be less than for system 1. Serine appears not to interact with system 2. 6. A second component to serine uptake was found to be related linearly to the concentration of serine in the medium. A similar component may contribute to the total uptake of phenylalanine. The possibility that such a component could arise as a space marker artifact is discussed. 7. An independent kinetic analysis of phenylalanine uptake by rabbit ileal mucosa showed that it could not be accounted for on the basis of a single entry system. However uptake could be described kinetically, assuming two systems of mediated entry to be present. The possible presence of a third non-saturable component to uptake does not affect these conclusions. 8. It is concluded that least two systems exist for the mediated entry of neutral amino acids into rabbit ileal mucosa. This fact should be taken into account in any future mechanistic interpretation of carrier-mediated amino acid transport in the small intestine.

Sugar transport by brush border membrane vesicles isolated from human small intestine

Uptake of D- and L-glucose, and fructose by purified brush border membrane vesicles isolated from human small intestine was studied using a rapid filtration technique. The uptake of D-glucose by the vesicles was osmotically sensitive and represented transport into an intravesicular space and not binding to the membranes. Transport of both, D- and L-glucose was inhibited by phlorizin. Uptake of D-fructose into the brush border vesicles was not stimulated by sodium. In the presence of a sodium gradient D-glucose was taken up 5 times faster than L-glucose. The amount of D-glucose transported into the vesicles in the presence of a sodium gradient was transiently higher than the amount of D-glucose taken up at equilibrium (over-shoot). D-Glucose transport was stimulated only by a sodium gradient; other monovalent cations had no effect. In the presence of a sodium gradient D-glucose transport was increased by the nearly impermeable anion sulfate as compared with uptake of D-glucose in the presence of a sodium chloride gradient. This indicates an influence of the electrical membrane potential on the sodium coupled non-electrolyte transport.

Unstirred layer and kinetics of electrogenic glucose absorption in the human jejunum in situ

Using an electrical technique we estimated the thickness of the unstirred layer in the human jejunum during kinetic studies of electrogenic glucose absorption. The unstirred layer in seven healthy volunteers (632 +/- 24 mum: mean +/- SEM) was significantly thicker than in 10 patients with active coeliac disease (442 +/- 23 mum) but not significantly different in seven patients who had responded to treatment by gluten withdrawal (585 +/- 49 mum). There were similar differences in the values of ;Apparent Km' for electrogenic glucose absorption between healthy control subjects (36 +/- 6 mM) active coeliac patients (11 +/- 1 mM) and treated coeliac patients (31 +/- 5 mM). The changes in PDmax however, showed a different pattern. The PDmax in the active coeliac group (6.8 +/- 0.7 mV) was lower than in controls (7.6 +/- 0.6 mV) but not significantly so, while the PDmax in the treated coeliac group (10.6 +/- 0.9 mV) was significantly higher than in both the active coeliac and control groups. It should be noted that both operational kinetic parameters obtained in the present study are much lower than those obtained previously (Read et al., 1976b) because of the use of siphonage. Analysis of the results using a computer simulation indicates that the reduction in Apparent Km in active coeliac disease can be caused by the interaction of the decreased maximal absorption rate for glucose (Jmax) with the attenuated unstirred layer. In these circumstances it is not necessary to postulate any change in the affinity of the transport mechanism for glucose (;Real Km'). It is remarkable that the disease process produces an Apparent Km which is much closer to the Real Km than that found in health.

Correction of the apparent Michaelis constant, biased by an unstirred layer, if a passive transport component is present

An unstirred layer shifts the permeation curve due to a carrier-mediated transport system (non-passive component) to the right, so that a higher Km value is determined (apparent Michaelis constant biased by an unstirred layer). If a substance is transported simultaneously by a non-passive and a passive mechanism, and if the non-passive component can be inhibited, the permeation curve due to the non-passive component can be obtained by subtraction of the curve due to the passive component alone from the curve obtained with intact passive and non-passive component. But in the presence of an unstirred layer the difference curve lies always below the curve which would be obtained in the presence of the non-passive component alone. This error increases with increasing unstirred layer thickness and increasing magnitude of the passive transport component. By means of an appropriate equation the apparent Michaelis constant biased by an unstirred layer and determined from the difference curve can be corrected, if the unstirred layer thickness and area are known.

Myocardial glucose uptake and breakdown during adenosine-induced vasodilation

In isolated K+ (16.2 mM)-arrested cat hearts perfused at constant pressure adenosine infusions (0.8 mumoles - min-1 - 100 g-1 for 10 min) caused an increase in myocardial 14C-glucose uptake and release of 14CO2 + H14CO3- AND 14C-lactate simultaneously with a rise in coronary flow. The ratio of the release of 14CO2 + H14CO3- to that of 14C-lactate and the specific activity of lactate in the effuate were not altered. In K+ -arrested hearts perfused with constant volume neither glucose uptake nor glucose breakdown were influenced by 0.8 or 100 mumoles - min-1 - 100 g-1 adenosine with 0.1 - 5 mM glucose in the perfusion medium. It is concluded that adenosine does not affect directly the myocardial glucose carrier system, aerobic or anaerobic glucose breakdown or glycogenolysis, but enhances glucose uptake secondarily by increasing coronary flow. This interpretation is substantiated by the finding that mechanically produced increases in perfusion volume caused similar increases in myocardial glucose uptake as were observed with comparable adenosine-induced coronary flow increments.

Electrogenic glucose absorption in untreated and treated coeliac disease

Using a method for measuring changes in transmural potential difference across the human jejunum in vivo, the operational kinetic parameters of 'Apparent Km' and PD max for the active electrogenic component of glucose absorption were estimated in a group of healthy volunteers and in patients with coeliac disease. Both the 'Apparent Km' (17+/2mM; mean +/SEM) and the PD max (8.6+/0.7 mV) in nine patients with untreated coeliac disease were significantly lower (p less than 0.005) than in the control group ('Apparent Km' = 74+/5mM; PD max 12.8+/0.9mV, n=20). Treatment of five coeliac patients by gluten withdrawal for less than three months increased significantly the values of both the "Apparent Km (35+/6mM) and the TPD max (11.4+/1.2mV). Treatment of five patients for more than six months caused a further increase in the values of both kinetic parameters ('Apparent Km' = 108+/13mM; PD max =15.6+/2.7mV) to levels which exceeded those in healthy subjects. The possible interpretations of the differences in the kinetic characteristics of electrogenic glucose transport between coeliac patients and healthy subjects are discussed.