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Simple Diffusion (simple + diffusion)
Selected AbstractsThe ancestral complement system in sea urchinsIMMUNOLOGICAL REVIEWS, Issue 1 2001L. Courtney Smith Summary: The origin of adaptive immunity in the vertebrates can be traced to the appearance of the ancestral RAG genes in the ancestral jawed vertebrate; however, the innate immune system is more ancient. A central subsystem within innate immunity is the complement system, which has been identified throughout and seems to be restricted to the deuterostomes. The evolutionary history of complement can be traced from the sea urchins (members of the echinoderm phylum), which have a simplified system homologous to the alternative pathway, through the agnathans (hagfish and lamprey) and the elasmobranchs (sharks and rays) to the teleosts (bony fish) and tetrapods, with increases in the numbers of complement components and duplications in complement pathways. Increasing complexity in the complement system parallels increasing complexity in the deuterostome animals. This review focuses on the simplest of the complement systems that is present in the sea urchin. Two components have been identified that show significant homology to vertebrate C3 and factor B (Bf), called SpC3 and SpBf, respectively. Sequence analysis from both molecules reveals their ancestral characteristics. Immune challenge of sea urchins indicates that SpC3 is inducible and is present in coelomic fluid (the body fluids) in relatively high concentrations, while SpBf expression is constitutive and is present in much lower concentrations. Opsonization of foreign cells and particles followed by augmented uptake by phagocytic coelomocytes appears to be a central function for this simpler complement system and important for host defense in the sea urchin. These activities are similar to some of the functions of the homologous proteins in the vertebrate complement system. The selective advantage for the ancestral deuterostome may have been the amplification feedback loop that is still of central importance in the alternative pathway of complement in higher vertebrates. Feedback loop functions would quickly coat pathogens with complement leading to phagocytosis and removal of foreign cells, a system that would be significantly more effective than an opsonin that binds upon contact as a result of simple diffusion. An understanding of the immune response of the sea urchin, an animal that is a good estimator of what the ancestral deuterostome immune system was like, will aid us in understanding how adaptive immunity might have been selected for during the early evolution of the vertebrates and how it might have been integrated into the pre-existing innate immune system that was already in place in those animals. The authors are grateful to Drs Sham Nair and Paul Gross for their critique of the manuscript and helpful suggestions. This work was supported by the National Science Foundation (MCB 9603086). [source] Mechanism of light-induced translocation of arrestin and transducin in photoreceptors: Interaction-restricted diffusionIUBMB LIFE, Issue 1 2008Vladlen Z. Slepak Abstract Many signaling proteins change their location within cells in response to external stimuli. In photoreceptors, this phenomenon is remarkably robust. The G protein of rod photoreceptors and rod transducin concentrates in the outer segments (OS) of these neurons in darkness. Within ,30 minutes after illumination, rod transducin redistributes throughout all of the outer and inner compartments of the cell. Visual arrestin concurrently relocalises from the inner compartments to become sequestered primarily within the OS. In the past several years, the question of whether these proteins are actively moved by molecular motors or whether they are redistributed by simple diffusion has been extensively debated. This review focuses on the most essential works in the area and concludes that the basic principle driving this protein movement is diffusion. The directionality and light dependence of this movement is achieved by the interactions of arrestin and transducin with their spatially restricted binding partners. © 2007 IUBMB IUBMB Life, 60(1): 2,9, 2008 [source] Nitric oxide signalling in salivary glandsJOURNAL OF ORAL PATHOLOGY & MEDICINE, Issue 10 2002Dagnia Looms Abstract Nitric oxide (NO) plays multiple roles in both intracellular and extracellular signalling mechanisms with implications for health and disease. This review focuses on the role of NO signalling in salivary secretion. Attention will be paid primarily to endogenous NO production in acinar cells resulting from specific receptor stimulation and to NO-regulated Ca2+ homeostasis. Due to the fact that NO readily crosses membranes by simple diffusion, endogenous NO may play a physiological role in processes as diverse as modifying the secretory output, controlling blood supply to the gland, modulating transmitter output from nerve endings, participating in the host defence barrier, and affecting growth and differentiation of surrounding tissue. Furthermore, the role of NO in the pathogenesis of human oral diseases will be considered. [source] Estimation of plasmalemma conductivity to ascorbic acid in intact leaves exposed to ozonePHYSIOLOGIA PLANTARUM, Issue 4 2000Irina Bichele To establish the capacity of the leaf mesophyll plasmalemma of Phaseolus vulgaris L. to supply ascorbate (ASC) into the cell wall by simple diffusion, a method for calculating plasmalemma diffusional conductivity to ascorbic acid (AA) in intact leaves was evaluated. The core of the approach is that in the presence of a sink for ascorbate in the cell wall, cell wall total ascorbic acid concentration [TAA]cw (=[ASC]cw+[AA]cw) reaches zero at some positive whole-leaf total ascorbic acid concentration [TAA]l. It is shown that [TAA]l at [TAA]cw=0 is proportional to the sink for ASC in the cell wall and the reciprocal of plasmalemma conductivity. The predicted proportional relationship between [TAA]cw and [TAA]l was confirmed by decreasing TAA levels in leaves through predarkening. Furthermore, increasing the sink intensity for ASC in the cell wall by the acute exposure of leaves to 450 nmol ozone mol,1 during re-illumination, [TAA]cw reached zero at 2.7-fold higher [TAA]l than without ozone, and the slope of the relationship increased twofold. Plasmalemma diffusional conductivities to AA of 2.9×10,6 and 1.8×10,6 m s,1, needed to maintain [TAA]cw at the observed level, were calculated from the increase in [TAA]l at [TAA]cw=0 and from the two different estimates of the sink for ASC. A value of 1.3×10,6 m s,1 was calculated on the basis of the oil-water distribution coefficient for TAA. It is concluded that the demand for ASC in the mesophyll cell wall of the investigated leaves could be met by simple diffusion of AA through the plasmalemma. From the measured increase in the slope of the relationship [TAA]cw versus [TAA]l, an increase in the cell wall pH of 0.3 units was estimated under the influence of ozone. [source] Lack of discontinuous gas exchange in a tracheate arthropod, Leiobunum townsendi (Arachnida, Opiliones)PHYSIOLOGICAL ENTOMOLOGY, Issue 2 2002John R. B. Lighton Abstract The discontinuous gas exchange cycle, characterized by stringent spiracular control and periods of near-zero external CO2 emission separated by ,bursts' of CO2 emission, has evolved independently in several taxa of tracheate arthropods. These include the hexapoda, diplopoda, and several arachnid taxa; ticks, pseudoscorpions and solphugids. This paper presents the first data on gas exchange kinetics in a harvestman (Arachnida; Opiliones). The experimental animal, Leiobunum townsendi Weed, from an arid area of the south-western United States, displayed a metabolic rate similar to those of other arthropods at 25 °C (129 ± 22 µW). Their CO2 emission kinetics showed, when the animals were motionless, only minor variations about a mean value of 0.0217 ± 0.0037 mL/h (n = 6, mean body mass 86 mg). Expressed on an intra-recording basis, the coefficient of variation of CO2 emission (= SD/MEAN), which is an index of short-term gas emission fluctuations and thus of spiracular control, had a mean value of only 0.082. In contrast, the coefficient of variation of animals employing a discontinuous gas exchange cycle is >,1.5. Gas exchange in opilionids, unlike the case with most other tracheate arthropods, may therefore be dominated by simple diffusion without a prominent role for wide modulations of spiracular conductance. Contributory to this conservative spiracular control strategy may be the weak degree of tracheation in opilionids, combined with circulating haemocyanin, which acts as both a transport medium and a buffering reservoir for respiratory gas exchange. [source] Renal glutathione transport: Identification of carriers, physiological functions, and controversiesBIOFACTORS, Issue 6 2009Lawrence H. Lash Abstract Glutathione (GSH) is an endogenous tripeptide composed of the amino acids L -glutamate, L -cysteine, and glycine. It is found in virtually all aerobic cells and plays critical roles in maintenance of cellular redox homeostasis and drug metabolism. An important component of its regulation is transport across biological membranes. Because GSH is a charged, hydrophilic molecule, transport occurs via catalysis by specific carrier proteins rather than by simple diffusion. Although it has been clearly understood that efflux of GSH across membranes such as the canalicular and sinusoidal plasma membranes in hepatocytes and the brush-border plasma membrane in renal proximal tubules is a key step in GSH turnover and interorgan metabolism, the existence and physiological functions of uptake of GSH across various epithelial plasma membranes has been subject to some debate. Besides transport across plasma membranes, GSH transport across intracellular membranes, most notably the mitochondrial inner membrane, has received some attention in recent years because of the importance of mitochondrial redox status and the mitochondrial GSH pool in cellular physiology and pathology. This commentary will focus on renal transport processes for GSH and will discuss some of the controversies that have existed and still seem to exist in the literature, specifically regarding uptake of intact GSH by basolateral membranes of renal proximal tubular cells and uptake of intact GSH by the mitochondrial inner membrane. © 2009 International Union of Biochemistry and Molecular Biology, Inc. [source] Role of multidrug resistance protein 2 (MRP2) in glutathione-bimane efflux from Caco-2 and rat renal proximal tubule cellsBRITISH JOURNAL OF PHARMACOLOGY, Issue 5 2001Sylvie A Terlouw The multidrug resistance protein 2 (MRP2) has been shown to play an important role in the transport of glutathione conjugates in the liver. Its importance in renal excretion, however, is still uncertain and other organic anion transporters may be involved. The objective of the present study was to characterize glutathione conjugate efflux from rat kidney proximal tubule cells (PTC), and to determine the contribution of Mrp2. We used isolated PTC in suspension, as well as grown to monolayer density. For comparison, transport characteristics were also determined in the human intestinal epithelial cell line Caco-2, an established model to study MRP2-mediated transport. The cells were loaded with monochlorobimane (MCB) at 10°C. MCB enters the cells by simple diffusion and is conjugated with glutathione to form the fluorescent glutathione-bimane (GS-B). In primary cultures of rat PTC, no indications for a transporter-mediated mechanism were found. The efflux of GS-B from Caco-2 cells and freshly isolated PTC was time- and temperature-dependent. Furthermore, GS-B transport in both models was inhibited by chlorodinitrobenzene (CDNB), with an inhibitory constant of 46.8±0.9 ,M in freshly isolated PTC. In Caco-2 cells, the inhibitory potency of CDNB was approximately 20 fold higher. Finally, efflux of GS-B from freshly isolated PTC from Mrp2-deficient (TR,) rats was studied. As compared to normal rat PTC, transport characteristics were not different. We conclude that in freshly isolated rat PTC glutathione conjugate excretion is mediated by other organic anion transporters rather than by Mrp2. British Journal of Pharmacology (2001) 134, 931,938; doi:10.1038/sj.bjp.0704284 [source] | |||||||||||||