Augmentation is a potentiation of the exocytotic process. (1/1433)

Short-term synaptic enhancement is caused by an increase in the probability with which synaptic terminals release transmitter in response to presynaptic action potentials. Since exocytosed vesicles are drawn from a readily releasable pool of packaged transmitter, enhancement must result either from an increase in the size of the pool or an elevation in the fraction of releasable vesicles that undergoes exocytosis with each action potential. We show here that at least one major component of enhancement, augmentation, is not caused by an increase in the size of the readily releasable pool but is instead associated with an increase in the efficiency with which action potentials induce the exocytosis of readily releasable vesicles.  (+info)

Osmosensing by bacteria: signals and membrane-based sensors. (2/1433)

Bacteria can survive dramatic osmotic shifts. Osmoregulatory responses mitigate the passive adjustments in cell structure and the growth inhibition that may ensue. The levels of certain cytoplasmic solutes rise and fall in response to increases and decreases, respectively, in extracellular osmolality. Certain organic compounds are favored over ions as osmoregulatory solutes, although K+ fluxes are intrinsic to the osmoregulatory response for at least some organisms. Osmosensors must undergo transitions between "off" and "on" conformations in response to changes in extracellular water activity (direct osmosensing) or resulting changes in cell structure (indirect osmosensing). Those located in the cytoplasmic membranes and nucleoids of bacteria are positioned for indirect osmosensing. Cytoplasmic membrane-based osmosensors may detect changes in the periplasmic and/or cytoplasmic solvent by experiencing changes in preferential interactions with particular solvent constituents, cosolvent-induced hydration changes, and/or macromolecular crowding. Alternatively, the membrane may act as an antenna and osmosensors may detect changes in membrane structure. Cosolvents may modulate intrinsic biomembrane strain and/or topologically closed membrane systems may experience changes in mechanical strain in response to imposed osmotic shifts. The osmosensory mechanisms controlling membrane-based K+ transporters, transcriptional regulators, osmoprotectant transporters, and mechanosensitive channels intrinsic to the cytoplasmic membrane of Escherichia coli are under intensive investigation. The osmoprotectant transporter ProP and channel MscL act as osmosensors after purification and reconstitution in proteoliposomes. Evidence that sensor kinase KdpD receives multiple sensory inputs is consistent with the effects of K+ fluxes on nucleoid structure, cellular energetics, cytoplasmic ionic strength, and ion composition as well as on cytoplasmic osmolality. Thus, osmoregulatory responses accommodate and exploit the effects of individual cosolvents on cell structure and function as well as the collective contribution of cosolvents to intracellular osmolality.  (+info)

Osmotic response element is required for the induction of aldose reductase by tumor necrosis factor-alpha. (3/1433)

Induction of aldose reductase (AR) was observed in human cells treated with tumor necrosis factor-alpha (TNF-alpha). AR protein expression increased severalfold in human liver cells after 1 day of exposure to 100 units/ml TNF-alpha. An increase in AR transcripts was also observed in human liver cells after 3 h of TNF-alpha treatment, reaching a maximum level of 11-fold at 48 h. Among the three inflammatory cytokines: TNF-alpha, interleukin-1, and interferon-gamma, TNF-alpha (100 units/ml) gave the most induction of AR. Differences in the pattern of AR induction were observed in human liver, lens, and retinal pigment epithelial cells with increasing concentrations of TNF-alpha. A similar pattern of AR promoter response was observed between TNF-alpha and osmotically stressed human liver cells. The deletion of the osmotic response element (ORE) abolished the induction by TNF-alpha and osmotic stress. A point mutation that converts ORE to a nuclear factor-kappaB (NF-kappaB) sequence abolished the osmotic response but maintained the TNF-alpha response. Electrophoretic gel mobility shift assays showed two NF-kappaB proteins, p50 and p52, capable of binding ORE sequence, and gel shift Western assay detected NF-kappaB proteins p50 and p65 in the ORE complex. Inhibitors of NF-kappaB signaling, lactacystin, and MG132 abolished the AR promoter response to TNF-alpha.  (+info)

A second mechanism of respiratory control. (4/1433)

According to the chemosmotic hypothesis, ATP is synthesized in mitochondria, bacteria and chloroplasts via the proton motive force delta p, the energy-rich intermediate of electron transport and photosynthetic phosphorylation. The general applicability of the chemosmotic hypothesis, however, was disputed until present. In particular the relationship between the rate of respiration and delta p in mitochondria was found variable, depending on the experimental conditions. Recently, a new mechanism of respiratory control was found, based on binding of ATP or ADP to subunit IV of cytochrome c oxidase, which is independent of delta p and could explain many previous results contradicting the chemosmotic hypothesis.  (+info)

Tissue sources and blood flow limitations of osmotic water transport across the peritoneum. (5/1433)

Despite the daily use of hypertonic solutions to remove fluid from patients throughout the world who are undergoing peritoneal dialysis, the tissue sources of this water flow are unknown. To study this phenomenon in specific tissues, small plastic chambers were affixed to parietal and visceral surfaces of the peritoneum and were filled with either an isotonic or hypertonic solution. The volume changes over 60 to 90 min were determined and divided by the chamber area to yield the volume flux. The hypertonic solution produced a positive flux into the chamber of 0.6 to 1.1 microl/min per cm2 in all tissues tested. In contrast, the isotonic solution resulted in a net loss or an insignificant change in the chamber volume. Additional experiments tested the influence of blood flow on the hypertonic water flux during periods of control, reduced (50 to 80%), or postmortem (no) blood flow, as determined by laser Doppler flowmetry. With the exception of the liver, small but insignificant changes in the flux into the chamber were observed during the period of reduced flow; all water fluxes were markedly depressed during the postmortem period. It is concluded that both parietal and visceral tissues are sources of osmotically induced water flow into the cavity. Except for the liver, marked blood flow reductions have small but insignificant effects on osmotic water transport.  (+info)

Expression of the vasotocin gene in the hypothalamus of intact and osmotically stimulated bullfrogs during metamorphosis. (6/1433)

To study the ontogeny of the vasotocin (VT) system and its contribution to anuran metamorphosis, VT mRNA levels were determined by Northern blot analysis in metamorphosing bullfrog tadpoles. Effects of osmotic stimulation on VT mRNA levels were also analyzed in order to follow the development of osmotic responsiveness of VT neurons. The intensity of hybridization signals for VT mRNA gradually increased during prometamorphic development. The increase became marked thereafter until metamorphic climax. Plasma osmolality and hematocrit remained unchanged before metamorphosis, and increased after metamorphic climax, indicating that climactic tadpoles in a semi-terrestrial environment were in a dehydrated condition. These increases correlated well with the increase in VT mRNA level. Immersion of tadpoles in 30% seawater (approximately 350 mOsmol) for 3 days increased plasma osmolality at all stages. No significant changes were observed in the VT mRNA level in response to this treatment during premetamorphic stages. The VT mRNA levels were significantly higher in the treated tadpoles after preclimax stages. Hyperosmotic treatment also increased hematocrit until early metamorphic climax, but did not alter it in tadpoles at late metamorphic climax. These results suggest that the responsiveness of VT-producing neurons to hyperosmotic or hypovolemic stimulation, or both, is established by the time of the metamorphic climax in bullfrog. The marked increase in VT mRNA levels at metamorphic climax stages of intact individuals is probably induced by dehydration. VT-stimulated water absorption and reabsorption in the target organs probably prevented the increase in hematocrit at late metamorphic climax. Thus VT may contribute importantly to osmoregulatory mechanisms in relation to adaptation to a semi-terrestrial habitat through the metamorphosis.  (+info)

The relationship between the uptake of glucose and 3-O-methylglucose and soluble carbohydrate and polysaccharide in the fungus Dendryphiella salina. (7/1433)

When mycelium of Dendryphiella salina, pre-incubated in D-[I-14C]mannitol such that this is the only major labelled soluble carbohydrate present, absorbs glucose or the non-metabolized sugar 3-O-methylglucose, there is a specific stimulation of incorporation of 14C into alkali-insoluble, trichloroacetic-acid-soluble (1 leads to 4)-alpha-glucan, probably glycogen. There is also a net increase in the amount of glucan caused by stimulation of synthesis and inhibition of breakdown. Addition of 3-O-methylglucose results in the loss of mannitol into the medium. This loss and the reduced rate of replenishment from the glucan is important in the osmotic regulation of the hyphae. If osmotic adjustment does not occur, the hyphae do not show the specific incorporation of 14C into the glucan.  (+info)

Bioenergetic aspects of halophilism. (8/1433)

Examination of microbial diversity in environments of increasing salt concentrations indicates that certain types of dissimilatory metabolism do not occur at the highest salinities. Examples are methanogenesis for H2 + CO2 or from acetate, dissimilatory sulfate reduction with oxidation of acetate, and autotrophic nitrification. Occurrence of the different metabolic types is correlated with the free-energy change associated with the dissimilatory reactions. Life at high salt concentrations is energetically expensive. Most bacteria and also the methanogenic Archaea produce high intracellular concentrations of organic osmotic solutes at a high energetic cost. All halophilic microorganisms expend large amounts of energy to maintain steep gradients of NA+ and K+ concentrations across their cytoplasmic membrane. The energetic cost of salt adaptation probably dictates what types of metabolism can support life at the highest salt concentrations. Use of KCl as an intracellular solute, while requiring far-reaching adaptations of the intracellular machinery, is energetically more favorable than production of organic-compatible solutes. This may explain why the anaerobic halophilic fermentative bacteria (order Haloanaerobiales) use this strategy and also why halophilic homoacetogenic bacteria that produce acetate from H2 + CO2 exist whereas methanogens that use the same substrates in a reaction with a similar free-energy yield do not.  (+info)