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PubMed1985

Presynaptic calcium diffusion from various arrays of single channels. Implications for transmitter release and synaptic facilitation.

A one-dimensional model of presynaptic calcium diffusion away from the membrane, with cytoplasmic binding, extrusion by a surface pump, and influx during action potentials, can account for the rapid decay of phasic transmitter release and the slower decay of synaptic facilitation following one spike, as well as the very slow decline in total free calcium observed experimentally. However, simulations using this model, and alternative versions in which calcium uptake into organelles and saturable binding are included, fail to preserve phasic transmitter release to spikes in a long tetanus. A three-dimensional diffusion model was developed, in which calcium enters through discrete membrane channels and acts to release transmitter within 50 nm of entry points. Analytic solutions of the equations of this model, in which calcium channels were distributed in active zone patches based on ultrastructural observations, were successful in predicting synaptic facilitation, phasic release to tetanic spikes, and the accumulation of total free calcium. The effects of varying calcium buffering, pump rate, and channel number and distribution were explored. Versions appropriate to squid giant synapses and frog neuromuscular junctions were simulated. Limitations of key assumptions, particularly rapid nonsaturable binding, are discussed.

Action Potentials↗
PubMed1980

The evaluation of initial rates in the presence of background noise.

A three-parameter model was used to test a curvilinear regression for its gamma-intercept. It was applied in the kinetic analysis of a model reaction and of real experiments. The model reaction consisted of a dialysis experiment with increasing amounts of background added at t0. In the presence of this background, initial rates of solute exchange were determined precisely enough to be in agreement with Fick's law of diffusion. The biological experiments were concerned with the saturation kinetics of the acid-soluble radioactivity of cells exposed to 3H-labelled amino acids. Two models of regression were applied, facilitated and physical diffusion, both consistent with the assumption of a substantial amount of background. This background was assumed to be caused by adsorption of 3H-labelled amino acids to cell-surface proteins. Without further consideration it caused a systematic overestimation of the initial rates.

Amino Acids↗
PubMed1988

Epigenetic selection: an alternative mechanism of pattern formation.

It is commonly assumed that patterned development is specified by pre-patterns or programs, actual development being a necessary consequence of previous conditions. However, the variability of normal development and its regenerative capacities are evidence for additional patterning processes. Predictable mature structures could result from continued "epigenetic selection" of the most appropriate developmental events. This selection would occur from an excess of possibilities that are genetically equivalent. The final balanced state would be specified by the genes, and the developmental system could gravitate towards this state without a detailed program. Selection could result from competition between cells and tissues for limiting developmental signals. The success or continuation of events that could start randomly would depend on feedback relationships with complementary developmental events. Specialized processes could be gradually localized if differentiation itself consumed limiting signals and if this consumption increased as differentiation proceeded. Spatial patterns could be formed if the movement of signals was gradually facilitated along the axes where it were initiated by diffusion. For example, induced facilitated transport could be the basis of an advantage of multicellular centers over scattered cells that have specialized in the same way. If epigenetic selection has a developmental role it requires a revision of common views concerning the cellular traits and the gene functions necessary for patterned development. An example of these traits is that cells should be expected to respond to changes in signal availability, not necessarily to signal concentration at any given time.

Animals↗
PubMed2004

Overexpression of the barley aquaporin HvPIP2;1 increases internal CO(2) conductance and CO(2) assimilation in the leaves of transgenic rice plants.

The internal conductance for CO(2) diffusion (g(i)) and CO(2) assimilation rate were measured and the related anatomical characteristics were investigated in transgenic rice leaves that overexpressed barley aquaporin HvPIP2;1. This study was performed to test the hypothesis that aquaporin facilitates CO(2) diffusion within leaves. The g(i) value was estimated for intact leaves by concurrent measurements of gas exchange and carbon isotope ratio. The leaves of the transgenic rice plants that expressed the highest levels of Aq-anti-HvPIP2;1 showed a 40% increase in g(i) as compared to g(i) in the leaves of wild-type rice plants. The increase in g(i) was accompanied by a 14% increase in CO(2) assimilation rate and a 27% increase in stomatal conductance (g(s)). The transgenic plants that had low levels of Aq-anti-HvPIP2;1 showed decreases in g(i) and CO(2) assimilation rate. In the plants with high levels of Aq-anti-HvPIP2;1, mesophyll cell size decreased and the cell walls of the epidermis and mesophyll cells thickened, indicating that the leaves had become xeromorphic. Although such anatomical changes could partially offset the increase in g(i) by the aquaporin, the increase in aquaporin content overcame such adverse effects.

Aquaporins↗
PubMed1977

Mycobacterium smegmatis fatty acid synthetase. Polysaccharide stimulation of the rate-limiting step.

An initial activity burst lasting 5 to 10 s is observed for both de novo synthesis with acetyl-CoA as primer and for elongation of palmitoyl-CoA catalyzed by the multienzyme complex fatty acid synthetase from Mycobacterium smegmatis. After the initial burst, synthetase activity slows at least 6-fold to the steady state rate. The size of the initial burst is proportional to the amount of synthetase protein and corresponds to the synthesis of a small number C three to five) of C24 or C26 acyl chains per mol of enzyme. During the initial burst, C24, C26 acyl enzyme is formed and can be isolated by ammonium sulfate precipitation. On incubation with CoA, enzyme-bound acyl chains undergo transacylation to form the corresponding CoA derivatives. Diffusion of C24-CoA and C26-CoA from the enzyme is slow and rate-limiting for overall fatty acid synthesis. Mycobacterial polysaccharides markedly accelerate this rate-determining step but bovine serum albumin does not. This facilitation of product diffusion accounts for the large stimulation of de novo synthesis and of elongation of mycobacterial polysaccharide. It is also shown that the high apparent Km for acetyl-CoA (approximately 400 micrometer) in the steady state reflects the substrate concentration required to shift the product pattern in favor of shorter chain fatty acids (C16,C18). These conditions circumvent the slow, rate-limiting diffusion of C24-CoA and C26-CoA.

Acetyl Coenzyme A↗
PubMed1998

Intracellular oxygen diffusion: the roles of myoglobin and lipid at cold body temperature.

Cold temperature can constrain the rate of oxygen movement through muscle cells of ectothermic animals because the kinetic energy of the solvent-solute system decreases and the viscosity of the aqueous cytoplasm increases during cooling within the physiological range of body temperatures. These factors affect the movement of both dissolved oxygen and oxymyoglobin, the two predominant routes of intracellular oxygen diffusion in vertebrate oxidative muscles. In addition, reductions in temperature have been shown to increase the affinity of myoglobin for oxygen and to slow the rate of Mb O2-dissociation, compromising the ability of this oxygen-binding protein to facilitate intracellular oxygen diffusion. Experiments with both seasonally cold-bodied fishes and polar fish species suggest that several factors combine to overcome these limitations in delivery of oxygen from the blood to the mitochondria. First, reductions in body temperature induce increases in mitochondrial density of oxidative muscle cells, reducing the mean diffusional pathlength for oxygen between capillaries and mitochondria. Second, cold body temperature in both temperate-zone and polar fishes is frequently correlated with a high content of neutral lipid in oxidative muscles, providing an enhanced diffusional pathway for oxygen through the tissue. Third, recent data indicate that myoglobins from fish species bind and release oxygen more rapidly at cold temperature than do those from mammals. Data from both oxidative skeletal muscle and cardiac muscle of fishes suggest that these factors in various combinations contribute to enhance the aerobically supported mechanical performance of the tissues at cold cellular temperatures.

Animals↗
PubMed2006

Auxin transport: a field in flux.

Polar auxin transport is crucial for plant growth and development. Auxin moves between plant cells through a combination of membrane diffusion and carrier-mediated transport. Several classes of membrane proteins that facilitate auxin uptake and efflux have recently been identified in Arabidopsis. The relative contribution to auxin transport made by the different facilitators and by membrane diffusion is unclear. In this Opinion article, we assess the significance of auxin diffusion versus carrier-mediated transport and then discuss the physiological importance of the transport facilitators within the context of the multiple trans-cellular auxin fluxes recently described in the Arabidopsis root apex.

Arabidopsis↗
PubMed2004

LDL transcytosis by protein membrane diffusion.

Endothelial cell (EC) cultures of different, selected vascular beds and/or organs were screened for receptor-mediated transport of proteins with a semipermeable filter assay. In SVEC4-10 cells, a mouse lymphoid endothelial cell line, orosomucoid, albumin, insulin and LDL were transcytosed from the apical (luminal) to basal (abluminal) side by a receptor-mediated pathway. Specific LDL transcytosis involved transport of intact LDL. A pathway of degradation of LDL and basal release involved vesicles in transport to lysosomes and amino acid merocrine secretion. This newly described transcellular passage of LDL via lysosomes, as well as the standard pathway, were reduced to 70% by PEG(50)-cholesterol (PEG-Chol). Combined results of temperature-dependence analysis and PEG(50)-cholesterol sensitivity show that two pathways contribute to general LDL transcellular passage. We suggest a mechanism of domain hopping by protein membrane diffusion of receptors as the pathway for intact LDL delivery. Based on theoretical considerations we propose that active transport by protein membrane diffusion can be facilitated by an organizational structure of lipid microdomains and polar cellular organization.

Animals↗
PubMedDate not supplied

Investigation of multistate foodborne disease outbreaks.

The U.S. food supply is characterized increasingly by centralized production and wide distribution of products, and more foodborne disease outbreaks are dispersed over broad geographic areas. Such outbreaks may present as a gradual, diffuse, and initially unapparent increase in sporadic cases. Recognition and reporting by clinicians and local public health officials and the ordering of laboratory tests by clinicians continue to be cornerstones of detecting all outbreaks. New methods--such as active laboratory-based surveillance, automated algorithms for detecting increases in infection rates, and molecular subtyping--facilitate detection of diffuse outbreaks. Routines have evolved for the investigation of multistate outbreaks; they are characterized by rapid communication between local, state, and federal public health officials; timely review of epidemiologic data by expert panels; collaboration on tracebacks with food safety regulatory agencies; and communication with the public and media. Rapid, efficient investigation of multistate outbreaks may result in control of acute public health emergencies, identification and correction of hazardous food production and processing practices, and consequent improvement in food safety.

Centers for Disease Control and Prevention, U.S.↗
PubMed1971

The diffusion capsule, a novel device for the addition of a solute at a constant rate to a liquid medium. Its application to metabolic regulation.

The diffusion capsule consists of a cylindrical container that can be completely filled with a solution and sealed with a small semi-permeable membrane at one end. In use, the capsule is immersed in an agitated liquid. Experiments on concentrated solutions in the capsule showed that, contrary to diffusion theory, the rate of diffusion of solute (sugars or amino acids) out of the capsule remained virtually constant until about 65% of the solute had diffused out of the capsule. Thus the device has been used to maintain constant material feed rates for periods exceeding 30h. The capsule is a simple and compact substitute for a pump and is superior to a pump for small feed rates in many applications. The capsule greatly extends the scope of the shake-flask culture technique for micro-organisms in that substrate-limited growth, possibly the aspect of greatest interest, is readily achieved simply by dropping in the flask a capsule containing the substrate. Diffusion feed should facilitate study of the metabolism of toxic substrates: also it is likely to provide an improved means for supplying a pulse of tracer to a culture.

Amino Acids↗
PubMed1992

Transport of salicylic acid through monolayers of a kidney epithelial cell line (LLC-PK1).

LLC-PK1 cells were used as a model of renal proximal epithelium to study the nonionic diffusion of salicylic acid (SAL). The apparent [14C]SAL transcellular permeability (PSal) and intracellular content were estimated at 20-21 degrees C from fluxes measured across cell monolayers grown on filters, in both apical-to-basolateral and basolateral-to-apical directions. The medium pH of the cis-side was varied from 6.0 to 7.4, and the medium pH of the trans-side was kept at 7.4. In the apical-to-basolateral direction, PSal increased linearly with the calculated concentration of nonionized SAL, indicating that SAL permeability was essentially the result of nonionic diffusion. In the basolateral-to-apical direction, PSal was about 2.5-fold higher than in the apical-to-basolateral direction and was not linearly related to the concentration of nonionized SAL molecules (0-4.5 nM), suggesting that besides nonionic diffusion, SAL was transported in its ionized form by a facilitated mechanism still active at 21 degrees C. This was confirmed by measuring basolateral-to-apical fluxes at 37 degrees C and observing that probenecid, an inhibitor of organic anion secretion, and cold SAL decreased PSal. Interestingly, at 37 degrees C, PSal in the apical-to-basolateral direction was also decreased by probenecid and cold SAL, suggesting the existence of a facilitated transport in this direction. These data demonstrated that the secretory transport of SAL is present in LLC-PK1 cells. The facilitated transport observed in the apical-to-basolateral direction suggests that in proximal tubule, SAL reabsorption might occur by facilitated mechanism and nonionic diffusion.

Biological Transport↗
PubMed2002

Effects of HgCl(2) on CO(2) dependence of leaf photosynthesis: evidence indicating involvement of aquaporins in CO(2) diffusion across the plasma membrane.

Experiments were conducted to examine whether mercury-sensitive aquaporins facilitate photosynthetic CO(2) diffusion across the plasma membrane of leaf mesophyll cells. Discs without abaxial epidermes from Vicia faba leaflets were treated with HgCl(2), an inhibitor of aquaporins. Hydraulic conductivity of the plasma membrane of these discs, measured as the weight loss of the discs in the 1 M sorbitol solution, was inhibited by sub-mM concentrations of HgCl(2) by 70 to 80%. Photosynthetic CO(2) fixation was also inhibited by the HgCl(2) treatment in a similar concentration range. When 0.3 mM HgCl(2) solution was fed to the V. faba leaflets with intact epidermes via the transpiration stream, the rate of photosynthesis on leaf area basis (A) measured at photosynthetically active photon flux density of 700 micromol m(-2) s(-1) and at leaf temperature of 25 degrees C, decreased by about 20 to 30% at any CO(2) concentration in the intercellular spaces (C(i)). However, when CO(2) concentration in the chloroplast stroma (C(c)) was calculated from fluorescence and gas exchange data and A was plotted against C(c), A at low C(c) concentrations did not differ before and after the treatment. The conductance for CO(2) diffusion from the intercellular spaces to the chloroplast stroma (g(i)) decreased to 40 and 30% of the control value, when the leaflets were fed with 0.3 mM and 1.2 mM HgCl(2), respectively. Similar results were obtained with leaves of Phaseolus vulgaris. Although effects of HgCl(2) were not specific, the present results showed that HgCl(2) consistently lowered g(i). It is, thus, probable that the photosynthetic CO(2) uptake across the plasma membrane of the mesophyll cells is facilitated by mercury-sensitive aquaporins.

Algorithms↗
PubMed1996

Mechanism of H(+)-coupled formate transport in rabbit renal microvillus membranes.

It has been proposed that a major fraction of Cl- absorption in the mammalian proximal tubule occurs by Cl-/formate exchange across the apical membrane with recycling of formate by nonionic diffusion. The purpose of this study was to characterize the mechanism of formate recycling in rabbit renal microvillus membrane vesicles. Formate uptake was stimulated by an inside-alkaline pH gradient. When external pH (pH alpha) was varied at constant internal pH (pHi), the initial rate of formate uptake was less than predicted for nonionic diffusion of formic acid at constant formic acid permeability. When pHi was varied at constant pHi, the initial rate of formate uptake exhibited cooperative and saturable kinetics with respect to pHi, in contrast to the pHi independence predicted for nonionic diffusion. pH gradient-stimulated [14C]formate uptake was stimulated by internal formate, indicating formate/formate exchange. pH gradient-stimulated formate influx was sensitive to inhibition by 1 mM 4,4'-diisothiocyanostilbene-2, 2'-disulfonic acid but not by furosemide or hydroxycinnamate. We conclude that pH gradient-stimulated formate uptake takes place by a carrier-mediated process of H(+)-formate cotransport, OH-/formate exchange, or facilitated formic acid diffusion, rather than solely by passive nonionic diffusion through the lipid bilayer.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1996

Single-domain/bound calcium hypothesis of transmitter release and facilitation.

1. We describe a model of transmitter release that is based on the finding that release can be gated during the opening of individual Ca2+ channels, suggesting that the release site can be activated by the Ca2+ domain under a single channel. In this model each release site contains four independent Ca2+ binding sites or gates with unbinding kinetics graded from slow to fast and affinities ranging from high to low. All four gates must be bound for release to occur. Thus synaptic dynamics are governed by the kinetics of Ca2+ binding and unbinding from release sites, not Ca2+ diffusion. 2. Fast facilitation occurs when an action potential invades a terminal with one or more ions remaining bound to the release sites. Residual free Ca2+ is not necessary for facilitation with this mechanism, but if present it would enhance facilitation by binding to high-affinity gates between pulses. 3. This model can account for key features of release. These include fourth-power cooperativity with regard to external Ca2+; a release time course that is virtually independent of an increase in quantal content; an inverse relation between external Ca2+ and the degree of facilitation; and a steplike increase in facilitation with increasing stimulus frequency, with each step corresponding to a unitary decline in the Ca2+ cooperativity. 4. Facilitation of single-channel-based secretion is shown to be robust even if channel opening is stochastic. Spontaneous release of transmitter, assumed to be due in part to spontaneous Ca2+ channel openings, is shown to be elevated during and after a train of impulses. 5. An extension of the model to include multiple Ca2+ channels per release site demonstrates that one role of overlapping Ca2+ domains may be to accentuate depolarization-evoked release relative to spontaneous release.

Action Potentials↗
PubMed2005

Modelling diffusive O(2) supply to isolated preparations of mammalian skeletal and cardiac muscle.

The purpose of this study was to use A. V. Hill's equation describing diffusion of O(2) into cylindrical muscles to assess the adequacy of O(2) supply for commonly used isolated preparations of mammalian cardiac and skeletal muscles. The diffusion equation was solved numerically to give the maximum, steady state O(2) diffusion distances (i.e. the distance from the surface of the muscle to the radial location where P(O(2)) is 0) for both resting and contracting muscles and for a range of temperatures. Non-steady state solutions for the rest-to-work transition were also determined to estimate how long contractile activity could be continued before anoxia develops at the muscle centre. The influence on muscle oxygenation of myoglobin-facilitated O(2) diffusion was also assessed. The analysis was performed for typical sized, whole muscles from adult rats and mice, for frog sartorius muscle and for a range of temperatures. Muscle O(2) consumption rates were taken from the literature. The results indicated that (1) diffusive O(2) supply would be adequate to support resting metabolism of soleus and EDL muscles of rat and mouse but may not be adequate to support the transient high resting metabolic rate of papillary muscles shortly after dissection, (2) during steady contractile activity of soleus and EDL muscles, particularly those from the rat, over a reasonable range of duty cycles, adequate O(2) supply could only be ensured if the radii of preparations was substantially smaller than those of whole muscles and (3) for cardiac muscles, diffusive O(2) supply could only support steady-state metabolism at twitch frequencies <1 Hz for whole papillary muscles from rat and <3 Hz for those from mouse. Reducing experimental temperature markedly enhances O(2) supply to skeletal, but not cardiac, muscle. O(2) supply from myoglobin had only minimal effects on oxygenation under typical isolated muscle conditions.

Animals↗
PubMed1977

Movement and Endogenous Levels of Abscisic Acid during Water-Stress-induced Abscission in Cotton Seedlings.

In an effort to investigate possible involvement of abscisic acid (ABA) in foliar abscission processes, its movement and endogenous levels were examined in cotyledons taken from cotton seedlings (Gossypium hirsutum L.) subjected to varying degrees of water deficit, a condition which initiates leaf abscission. Using a pulse-labeling technique to avoid complications of uptake and exit from the tissue, ABA-1-(14)C movement was observed in both basipetal and acropetal directions in cotyledonary petioles taken from well watered, stressed, and rewatered plants. The label distribution patterns obtained after 1 and 3 hours of transport under all situations of water supply were diffusive in nature and did not change when tested under anaerobic conditions. The transport capacity of the petioles ranged from 3.6 to 14.4% ABA-1-(14)C transported per hour at estimated velocities of 0 to 2 millimeters per hour. Comparison of basipetal and acropetal movement indicated a lack of polarity under all conditions tested. These low transport capacities and slow velocities of movement, when compared to the active transport systems associated with auxin movement, as well as the lack of anaerobic effects and polarity, suggest that ABA movement in cotton cotyledonary petiole sections is facilitated by passive diffusion. Increases in free and bound ABA in the lamina with increased water stress did not correlate with patterns of cotyledonary abscission. Thus, no evidence was found to suggest that ABA is directly involved in stress-induced abscission processes.

Journal Article↗
PubMed1988

Purine nucleobase transport in human erythrocytes. Reinvestigation with a novel "inhibitor-stop" assay.

A novel "inhibitor-stop" method for the determination of initial rates of purine nucleobase transport in human erythrocytes has been developed, based on the addition of seven assay volumes of cold 19 mM papaverine to terminate influx. In view of our finding that the initial velocities of adenine, guanine, and hypoxanthine influx into human erythrocytes were linear for only 4-6 s at 37 degrees C, the present method has been used to reexamine the kinetics of purine nucleobase transport in these cells. Initial influx rates of all three purine nucleobases were shown to be the result of concurrent facilitated and nonfacilitated diffusion. The nonfacilitated influx rates could be estimated either from the linear concentration dependence of nucleobase influx at high concentrations of permeant or from residual influx rates which were not inhibited by the presence of co-permeants. Appropriate corrections for nonfacilitated diffusion were made to the influx rates observed at low nucleobase concentrations. Kinetic analyses indicated that adenine (Km = 13 +/- 1 microM, n = 7), guanine (Km = 37 +/- 2 microM, n = 5), and hypoxanthine (Km = 180 +/- 12 microM, n = 6) were mutually competitive substrates for transport. The Ki values obtained with each nucleobase as an inhibitor of the influx of the other nucleobases were similar to their respective Km values for influx. Furthermore, the transport of the purine nucleobases was not inhibited by nucleosides (uridine, inosine) or by inhibitors of nucleoside transport (6-[(4-nitrobenzyl)thio]-9-beta-D-ribofuranosylpurine, dilazep, dipyridamole). It is concluded that all three purine nucleobases share a common facilitated transport system in human erythrocytes which is functionally distinct from the nucleoside transporter.

Adenine↗
PubMed1995

The use of ultrasound as an enhancer for transcutaneous drug delivery: phonophoresis.

Phonophoresis is the use of ultrasound (US) to enhance the delivery of topically applied drugs. The purposes of this article are (1) to review the basic principles of transcutaneous drug delivery, (2) to summarize the functional anatomy of the skin pertinent to phonophoresis, (3) to outline the physiological principles of US as an enhancer of topically applied drugs, (4) to review the literature on the efficacy of phonophoresis, (5) to discuss the relevance of US as an enhancer of topical drugs in the practice of physical therapy, and (6) to outline areas of needed research. Seventy-five percent of the studies reviewed reported positive effects of US on local subcutaneous drug diffusion, but some systemic effects were reported. This research review indicates that to maximize the clinical effectiveness of phonophoresis: (1) the topical drug (both the drug and the carrying agent) should transmit US; (2) the skin should be pretreated with US, heating, moistening, or shaving; (3) the patient needs to be positioned to maximize circulation during treatment; (4) a dressing that seals the area and prevents the escape of moisture should be applied after treatment; (5) an intensity of 1.5 W/cm2 should be used to capture both the thermal and nonthermal effects of the US; and (6) low-intensity US (0.5 W/cm2) should be used when treating open wounds or acute injuries. Research is needed to clarify what parameters of US will most efficiently facilitate topical drug diffusion, how often and for what duration US should be used to maximize local absorption of drugs, and which topical drugs can most effectively be used for phonophoresis.

Adrenal Cortex Hormones↗