Passive Transport Mechanisms Research Articles

A trail of research on potassium

A complex pump-leak system involving both active and passive transport mechanisms is responsible for the appropriate distribution of potassium (K) between the intra- and extracellular fluid compartments. In addition, the kidneys, and to a lesser extent the colon, safeguard maintenance of the narrow range of low K concentrations in the extracellular fluid. Early renal clearance studies showed that K is normally both reabsorbed and secreted by renal tubules, and that regulated secretion is the major source of K excretion. Studies at the tubule and cell level have localized secretion and reabsorption of K to principal and intercalated cells in the collecting ducts. Measurements of the electrochemical driving forces across individual cell membranes have permitted the characterization of specific ATPases, K channels and K cotransporters and also provided insights into the molecular structure of individual transporters that regulate K excretion.

Large-Eddy Simulation of Flow and Scalar Transport in a Modeled Street Canyon

This study uses large-eddy simulation (LES) to illustrate the flow and turbulence structure and to investigate the mechanism of passive scalar transport in a street canyon. Calculations for a modeled street canyon with building-height-to-street-width ratio of unity at Reynolds number equal to 12 000 are conducted. When the approaching wind is perpendicular to the street axis, the calculation produces a primary vortex in the street canyon, similar to previous studies. An evaluation of the LES results with wind-tunnel measurements reveals good agreement for both mean and turbulence parameters of the flow and scalar fields. The computed primary vortex is confined to the street canyon and is isolated from the free stream flow such that the removal of a scalar emitted at the street level is accomplished by turbulent diffusion at the roof level. It is determined from the calculations that very little scalar is removed from the street canyon, and 97% of the scalar is retained. The scalar mixing at the roof level occurs primarily on the leeward side of the street canyon. In addition to the primary vortex, three secondary vortices are located in the corners of the street canyon at which scalar mixing is enhanced. An examination of additional simulations shows how the location of the scalar source affects the distribution of the scalar.

Indirect evidence that intestinal bile salt absorption in rats and hamsters is under positive feedback control.

Bile salts are reabsorbed from the intestine by active and passive transport mechanisms with great efficacy. Conflicting data do not allow to judge for certainty whether bile salt absorption is under negative or positive feedback control. To address this issue, we analyzed bile salt absorption in vivo along the entire intestinal tract of rats and hamsters that received intraduodenal bile salt infusions for 54 h following interruption of the enterohepatic circulation. Taurocholate absorption in rats was complete, even when unphysiologically high concentrations of taurocholate were given. The combined infusion of taurocholate together with potent inhibitors of bile salt synthesis such as deoxycholate, taurodeoxycholate or taurochenodeoxycholate, failed to inhibit bile salt absorption. In the hamster, taurochenodeoxycholate and taurocholate absorption was complete and could not be inhibited when given in supraphysiological concentrations. Finally, taurocholate absorption was not impaired when deoxycholic acid was infused. These results provide indirect evidence that bile salt absorption is under positive feedback control regulated by luminal bile salt concentrations.

Evaluation of active and passive transport mechanisms in genital tracts of IUD-bearing women with radionuclide hysterosalpingoscintigraphy

The objective of this study was to evaluate the active and passive transport mechanisms in the genital tracts of copper T-200 intrauterine device (IUD)-bearing women. Tc-99mHMPAO-labeled spermatozoa and Tc-99m-labeled albumin macrospheres were placed into the vagina at midcycle. Serial scintigraphic images were obtained over a period of 2 h. Migration of spermatozoa and particles in the genital tract and the direction of transport related to dominant follicle were evaluated. While active sperm migration was greatly inhibited, the passive transport of the particles was not affected in IUD-bearing women. The direction of radiolabeled particles and spermatozoa was toward the dominant follicle side. Passive transport was not affected, whereas active transport of spermatozoa was strongly inhibited in the genital tract by the presence of the IUD. However, the direction of active and passive transport related to dominant follicle side was unchanged in IUD-bearing women and was preferentially toward the tube ipsilateral to the dominant follicle.

Evaluation of active and passive transport mechanisms in genital tracts of IUD bearing women with radionuclide histerosalpingoscintigraphy

Evaluation of active and passive transport mechanisms in genital tracts of IUD bearing women with radionuclide histerosalpingoscintigraphy

Two co-existing mechanisms for nuclear import of MAP kinase: passive diffusion of a monomer and active transport of a dimer.

In response to extracellular stimuli, mitogen-activated protein kinase (MAPK, also known as ERK) translocates from the cytoplasm to the nucleus. MAP kinase kinase (MAPKK, also know as MEK), which possesses a nuclear export signal (NES), acts as a cytoplasmic anchor of MAPK. Here we show evidence that tyrosine (Tyr190 in Xenopus MPK1/ERK2) phosphorylation of MAPK by MAPKK is necessary and sufficient for the dissociation of the MAPKK-MAPK complex, and that the dissociation of the complex is required for the nuclear translocation of MAPK. We then show that nuclear entry of MAPK through a nuclear pore occurs via two distinct mechanisms. Nuclear import of wild-type MAPK (mol. wt 42 kDa) was induced by activation of the MAPK pathway even in the presence of wheat germ agglutinin or dominant-negative Ran, whereas nuclear import of beta-galactosidase (beta-gal)-fused MAPK (mol. wt 160 kDa), which occurred in response to stimuli, was completely blocked by these inhibitors. Moreover, while a dimerization-deficient mutant of MAPK was able to translocate to the nucleus upon stimulation, this mutant MAPK, when fused to beta-gal, became unable to enter the nucleus. These results suggest that monomeric and dimeric forms of MAPK enter the nucleus by passive diffusion and active transport mechanisms, respectively.

Bone mineral content is not reduced despite low vitamin D status in breast milk-fed infants versus cow's milk based formula-fed infants

The effect of low or borderline vitamin D status on bone mineralization of exclusively breast milk-fed infants has not been studied. The low vitamin D status of Korean breast milk-fed infants may theoretically have adverse effects on bone mineralization. Assuming that bone mineral content (BMC) relates in part to vitamin D status, we hypothesized that serum 25-hydroxyvitamin D (25-OHD) concentration and BMC would be low, and serum osteocalcin concentration high, reflecting active bone turnover, in breast milk- versus formula-fed infants born in the winter. Eighteen breast milk- and 17 formula-fed infants were recruited at ages 2 to 5 months. The BMC of the lumbar 1-4 spine region was measured by using dual energy x-ray absorptiometry. The BMC and serum osteocalcin levels were similar for both groups. The serum 25-OHD level was significantly lower in breast milk- than formula-fed infants; 44% of the breast milk group versus 6% of the formula group had serum 25-OHD levels less than 28 nmol/L (11 ng/ml), the lower limit of normal. The BMC did not correlate with the serum 25-OHD level. Thus BMC and serum osteocalcin levels in 2- to 5-month-old infants were not different by type of feeding, despite low vitamin D status in breast milk-fed infants. We speculate that adequate mineral absorption occurs during this period from a predominantly (vitamin D independent) passive transport mechanism. (J Pediatr 1998;132:641-5.)

Drug exsorption from blood into the gastrointestinal tract.

Drugs are exsorbed from the blood across the gastrointestinal membranes by passive or active processes. In the case of a passive transport mechanism, the exsorption of drugs depends on the concentration gradients between the serosal and mucosal sides. The extent of secretion (exsorption) is determined by numerous factors such as extent of binding to serum proteins, distribution volume, lipophilicity, pKa and molecular size of drugs, and the blood flow rate in the gut. Specific transport systems such as P-glycoprotein (P-gp), organic cation and organic anion transporters are found to be involved in active intestinal secretion of drugs. Intestinal secretory transport systems reduce the extent of drug absorption sometimes resulting in low oral bioavailability. It is, therefore, important to know whether poor drug absorption is due to the involvement of specialized secretory transport systems. Modulation of intestinal secretory transport can be a means to enhance absorption of drugs with low oral bioavailability if exsorption of drugs is based on active secretion pathways that are open for control from the "outside".

Flip-Flop of Doxorubicin across Erythrocyte and Lipid Membranes

Doxorubicin, an anticancer drug, is extruded from multidrug resistant (MDR) cells and from the brain by P-glycoprotein located in the plasma membrane and the blood–brain barrier, respectively. MDR-type drugs are hydrophobic and, as such, enter cells by diffusion through the membrane without the requirement for a specific transporter. The apparent contradiction between the presumably free influx of MDR-type drugs into MDR cells and the efficient removal of the drugs by P-glycoprotein, an enzyme with a limited ATPase activity, prompted us to examine the mechanism of passive transport within the membrane. The kinetics of doxorubicin transport demonstrated the presence of two similar sized drug pools located in the two leaflets of the membrane. The transbilayer movement of doxorubicin occurred by a flip-flop mechanism of the drug between the two membrane leaflets. At 37°, the flip-flop exhibited a half-life of 0.7 min, in both erythrocyte membranes and cholesterol-containing lipid membranes. The flip-flop was inhibited by cholesterol and accelerated by high temperatures and the fluidizer benzyl alcohol. The rate of doxorubicin flux across membranes is determined by both the massive binding to the membranes and the slow flip-flop across the membrane. The long residence-time of the drug in the inner leaflet of the plasma membrane allows P-glycoprotein a better opportunity to remove it from the cell.

Fate and distribution of seminal plasma proteins in the genital tract of the female rat after natural mating.

This paper describes the distribution and fate of seminal plasma proteins in the rat female genital tract after insemination, using immunological detection in tissue sections and in fluids collected from different regions. The localization of seminal plasma proteins in the uterus and the vagina correlated with that of spermatozoa, suggesting that passive transport mechanisms operate in these regions. No seminal plasma proteins were detected in the oviduct, indicating that their presence is probably restricted to the uterine environment. Possible mechanisms for eliminating seminal plasma molecules after copulation include leakage from the uterus after relaxation of the cervical muscles and endocytosis by the endometrial cells. Large amounts of both vesicular and coagulating gland proteins were detected in the vagina of females at the time of cervical relaxation, indicating that the first mechanism of leakage from the uterus after cervical relaxation operates. Immunocytochemical procedures were used and seminal vesicle antigens were detected inside uterine epithelial cells, which indicates that endocytosis is also a mechanism for elimination of these molecules after copulation. Western blot results suggest proteolytic cleavage as a third mechanism. However, coagulating gland antigens are neither endocytosed nor cleaved, and their elimination takes place only by backflow to the vagina. The seminal plasma distribution in experimental situations in which sperm transport is altered was also studied. The implications of our findings for mechanisms of sperm transport in the female are discussed.

Uptake and transport of the ACE-inhibitor ceronapril (SQ 29852) by monolayers of human intestinal absorptive (Caco-2) cells in vitro

The angiotensin-converting enzyme (ACE)-inhibitor ceronapril (SQ 29852) is shown to be a substrate of the intestinal dipeptide transporter. Uptake by Caco-2 cells, grown as confluent monolayers, follows a major saturable pathway ( K m, 0.91 ± 0.11 mM; ∼90% at 1 mM) together with a minor passive component ( k J, 32.3 ± 6.6 ng (10 6 cells) −1 (20 min) −1. Uptake was inhibited by competition with dipeptides such as l-AIa- l-Pro (K i, 2.96 mM) and l-Phe-Gly ( K i, 3.84 mM) but not by cephalosporins such as cephalexin. In contrast, transport was non-saturable, flux increased linearly with concentration and data were consistent with a passive transepithelial transport mechanism. Transport profiles showed a biphasic dependence upon time with an initial flux of 0.83 ± 0.02 ng insert −1 min −1 ( k 1) and a terminal value of 1.65 ± 0.08 ng insert −1 min −1 ( (k 2 ) at 100 μM. It is concluded that the basolateral efflux is retarded so that the passive paracellular transport controls the overall transepithelial transport characteristics in the Caco-2 model. Carrier-mediated uptake into intestinal enterocytres, followed by rate-limiting basolateral efflux, may explain the extended t max in vivo following oral administration.

Identification and localization of calcium channel α1 and β subunit isoforms in the kidney

The distal nephron of the kidney plays a key role in the regulation of renal calcium excretion. Although calcium filtered at the glomerulus is reabsorbed along the whole length of the nephron, active, and by inference transcellular, transport has been demonstrated primarily in the micropuncture distal tubule [1, 2], and more specifically the distal convoluted tubule (DCT) [3] and connecting tubule (CNT) [4]. Active transport has also been observed under certain conditions in the cortical thick ascending limb (CTAL) [4, 5], although this is controversial [6]. Furthermore, the action of parathyroid hormone (PTH) to enhance calcium reabsorption in the CNT [3] and CTAL [5] is responsible for its hypocalciuric effect. The mechanism and regulation of calcium reabsorption at the cellular level have recently been reviewed [7]. Figure 1 illustrates a model of transcellular calcium transport in a distal tubule epithelial cell, based on current knowledge. Cytosolic calcium is actively extruded at the basolateral membrane by a calcium pump or sodium-calcium exchanger [8, 9]. Until recently, however, little was known about how calcium entered these cells at the apical surface, except that it must pass down a steep concentration gradient, from millimolar concentrations in the tubular fluid [1] to submicromolar concentrations within the cytosol [10], and also down a transmembrane electrical gradient of between -35 mV and -87 mV [11, 12], suggesting a passive transport mechanism.

Regulation of epithelial shunt conductance by the peptide leucokinin

Isolated Malpighian tubules of the yellow fever mosquito Aedes aegypti spontaneously secrete NaCl, KCl and water across an epithelium of modest transepithelial resistance (40-80 omega cm2) and high transepithelial voltage (30-70 mV, lumen positive). Transepithelial electrochemical potentials indicate that Na and K are secreted by active and Cl by passive transport mechanisms. The addition of synthetic leucokinin-VIII (LK-VIII, insect myotropic peptide) to the peritubular bath significantly increases the rates of transepithelial NaCl, KCl and water secretion. In parallel, LK-VIII depolarizes the transepithelial voltage from 59.3 to 5.7 mV, decreases the transepithelial resistance from 57.7 to 9.9 omega cm2, and renders the basolateral and apical membrane voltages nearly equipotential (approximately -90 mV). Unilateral step changes of the [Cl] in the peritubular bath or tubule lumen elicit small transepithelial Cl diffusion potentials in the absence of LK-VIII but large transepithelial Cl diffusion potentials, up to 85% of Nernst equilibrium potentials, in the presence of LK-VIII. In Malpighian tubules treated with dinitrophenol for estimates of the shunt resistance Rsh, LK-VIII reduces Rsh from 52.5 to 5.8 omega cm2. Bilateral reductions of the Cl concentration in tubule lumen and peritubular bath fully restore Rsh to 55.8 omega cm2 in the presence of LK-VIII. LK-VIII has no effects when presented from the luminal side. These results suggest that LK-VIII increases the Cl conductance of the epithelial shunt via a receptor located at the basolateral side of the epithelium.

The effect of drug charge type and charge density on corneal transport

In order to understand the effect of charge type on corneal transport of peptides, it is useful to first investigate the mechanism of transport of amino acids across corneal tissue. The transport characteristics of l-[ 14C]lysine and l-[ 14C]glutamic acid were based on in vitro flux measurements. Net flux of l-lysine occurs via a sodium-dependent cotransport mechanism (with a coupling ratio of 1:1). The transport of l-lysine involves the Na +-K +-ATPase pump and requires a stereospecific carrier-mediated transport system. The permeability coefficient ( P) of l-lysine was decreased from 10.1 ± 0.80 to 2.34 ± 0.75 × 10 −6 (cm/s by blocking the active pump and carrier-mediated system. However, l-glutamic acid was apparently absorbed by a passive aqueous transport mechanism which was oubain-insensitive, [Na +]-independent, temperature-insensitive and non-stereospecific. Overall, the cornea was 2–3-fold more permeable to a cation ( l-lysine) than an anion ( l-glutamic acid). In addition, the negatively charged drug, salicylic acid, was 2.3 times less permeable than the cationic drug, benzylamine.

Sodium-Dependent Retention of Prostaglandins in Rat Ileum

An intestinal perfusion technique is reported to study the steady-state ileal mucosal retention of labeled prostaglandins E1, E2, and F2α in rats. Ileal homogenates were analyzed for [3H]prostaglandin E1, [14C]prostaglandin E2, and [14C]prostaglandin F2α after intestinal segments were perfused with Krebs improved Ringer buffers containing sodium or depleted from sodium and replaced with choline, and buffers containing 1 mM ouabain, 2 mM amiloride, or 50 nM tetrodotoxin. Prostaglandin ileal mucosal uptake was also studied after iv injection of tetrodotoxin (20 μg/kg body weight). Prostaglandin concentration-dependent transport studies support passive uptake mechanisms for prostaglandin E1 and E2. Physiological concentrations of sodium increased labeled prostaglandins E1, E2, and F2α ileal mucosal uptake and decreased n-octanol–buffer partition coefficients. Unlike sodium, potassium showed no effect on labeled prostaglandin ileal transport. Ouabain, amiloride, and tetrodotoxin in the perfusates did not significantly alter prostaglandin mucosal uptake. However, injecting tetrodotoxin into rats caused a drastic increase of prostaglandin uptake through the ileal mucosa. Therefore, the role of sodium on labeled prostaglandin E1, E2, and F2α ileal mucosal transport can be postulated to be controlled by either one or both of the following mechanisms: a pH-partition passive transport mechanism, and/or a sodium channel-dependent pathway whereby prostaglandin permeation possibly proceeds via a sodium-prostaglandin ion-pair mechanism which is controlled by a gating phenomenon.

Estuarine Recruitment Mechanisms of Larval Atlantic Croakers

Atlantic croakers Micropogonias undulatus undergo a transition from a coastal, pelagic, planktonic stage to an estuarine, demersal stage. Atlantic croaker larvae were collected at five descrete depths (0 m, 1 m, 6 m, and bottom) at stations along three transects north, south, and inside the mouth of the Chesapeake Bay in September 1980. Vertical distribution of the larvae was compared with salinity and temperature at the collection sites. Distribution patterns on the larvae were not uniform with depth but were related to hydrography. In vertically homogeneous water, larvae were found throughout the water column. In stratified water, they were found in the inward-flowing, lower layer, though not necessarily on the bottom. The distribution of Atlantic croaker larvae at the mouth of the Chesapeake can be accounted for by both passive and active transport mechanisms.

Effect of storage time on red blood cell membrane permeability to creatinine and uric acid.

Although there is considerable evidence that blood storage affects red blood cell viability, deformability, and geometry, available data on the effect of blood storage on solute transport across the red blood cell membrane are highly limited. The authors used a stirred ultrafiltration device, with direct cell-free fluid sampling through a semipermeable ultrafiltration membrane, to obtain accurate data on red blood cell membrane permeability to both creatinine and uric acid. Results for influx and efflux experiments can be adequately explained by a passive transport mechanism. The red blood cell membrane permeability to uric acid increased substantially during storage, whereas trends in creatinine permeability were less clear and suggested possible differences between solute efflux and influx experiments. These results provide important insights into the physical and biochemical changes that occur in the red blood cell membrane during storage and also have important implications for analyses of solute removal in hemodialysis.

Factors relevant in bacterial pyrroloquinoline quinone production

Quinoprotein content and levels of external pyrroloquinoline quinone (PQQ) were determined for several bacteria under a variety of growth conditions. From these data and those from the literature, a number of factors can be indicated which are relevant for PQQ production. Synthesis of PQQ is only started if synthesis of a quinoprotein occurs, but quinoprotein synthesis does not depend on PQQ synthesis. The presence of quinoprotein substrates is not necessary for quinoprotein and PQQ syntheses. Although the extent of PQQ production was determined by the type of organism and quinoprotein produced, coordination between quinoprotein and PQQ syntheses is loose, since underproduction and overproduction of PQQ with respect to quinoprotein were observed. The results can be interpreted to indicate that quinoprotein synthesis depends on the growth rate whereas PQQ synthesis does not. In that view, the highest PQQ production can be achieved under limiting growth conditions, as was shown indeed by the much higher levels of PQQ produced in fed-batch cultures compared with those produced in batch experiments. The presence of nucleophiles, especially amino acids, in culture media may cause losses of PQQ due to transformation into biologically inactive compounds. Some organisms continued to synthesize PQQ de novo when this cofactor was administered exogenously. Most probably PQQ cannot be taken up by either passive diffusion or active transport mechanisms and is therefore not able to exert feedback regulation on its biosynthesis in these organisms.

Nasal drug delivery: An in vitro characterization of transepithelial electrical properties and fluxes in the presence or absence of enhancers

The study of the mechanisms of nasal drug absorption and factors affecting nasal drug absorption have been studied in vivo, but to date lack a simple, effective in vitro model system. We describe the use of Ussing chambers to study the transport of substances across, and the effect of chemicals on nasal mucosal tissue. Sheep nasal mucosa mounted in an Ussing chamber maintained viability for up to 8 hours as determined by measuring the electrical properties of the tissue, response to ion transport modifiers and histological assessment. Inulin (0.01 m M), mannitol (0.01 m M). and propranolol (0.008 m M.) were transported from the mucosal (m) to the serosal (s) side of the tissue at rates of 0.079, 0.249 and 1.02 nmol/cm 2/h, respectively. Transport from s-to-m and m-to-s occurred at equivalent rates, consistent with a passive transport mechanism. The rate of transport of propranolol was directly proportional to concentration over a range of 0.008 to 0.8 m M. The bile salt, deoxycholate, added to the mucosal bathing solution at 1 %, caused an increase in transepithelial conductance and irreversible tissue damage (cell necrosis). At 0.1%, deoxycholate reversibly increased transepithelial conductance (absence of cell necrosis) and enhanced the transport of mannitol and inulin 10–20 fold. These results demonstrate that the Ussing chamber provides a useful technique to investigate the nasal transport of drugs and allows evaluation of the effects of absorption enhancers on both absorption and tissue integrity.

A DPP study of ranitidine permeability

The apparent permeation rate constants and amount of ranitidine transferred in 1 h were measured through everted rat intestine using a perfusion apparatus. The effects of pH and 5% or 10% ethanol on the apparent permeation parameters were evaluated. A selective permeation at pH 7.4 in the jejunum was found for ranitidine. Both ethanol concentrations diminished the apparent permeability in this segment significantly. A good correlation was established between the apparent first-order permeation rate constants and the lipophilicity of the drugs as measured by their apparent partition coefficients. However, a significant contribution of the ionized species to the overall permeation process was determined. Under the experimental conditions employed here, a passive diffusion transport mechanism through the small intestine (jejunum) is followed for ranitidine.