Direct Kinetic Effects Research Articles

Short-term thermal acclimation modulates predator functional response.

Phenotypic plastic responses to temperature can modulate the kinetic effects of temperature on biological rates and traits and thus play an important role for species adaptation to climate change. However, there is little information on how these plastic responses to temperature can influence trophic interactions. Here, we conducted an experiment using marbled crayfish and their water louse prey to investigate how short‐term thermal acclimation at two temperatures (16 and 24°C) modulates the predator functional response. We found that both functional response parameters (search rate and handling time) differed between the two experimental temperatures. However, the sign and magnitudes of these differences strongly depended on acclimation time. Acclimation to 16°C increased handling time and search rate whereas acclimation to 24°C leads to the opposite effects with shorter handling time and lower search rate for acclimated predators. Moreover, the strength of these effects increased with acclimation time so that the differences in search rate and handing time between the two temperatures were reversed between the treatment without acclimation and after 24 h of acclimation. Overall, we found that the magnitude of the acclimation effects can be as strong as the direct kinetic effects of temperature. Our study highlights the importance of taking into account short‐term thermal plasticity to improve our understanding of the potential consequences of global warming on species interactions.

Kinetic effect of different ground conditions on the sole of the claws of standing and walking dairy cows

For the first time, we analyzed the direct kinetic effects of concrete and rubber flooring on the soles of live dairy cows' claws while standing and walking. Ten adult dairy cows were equipped with foil-based pressure sensors (HoofSystem, Tekscan Inc., Boston, MA) under their left hind leg using a leather claw shoe. These sensors captured parameters of pressure distribution and vertical ground reaction forces while the cows walked on the 2 tested flooring types. The mean pressure was 15.1 to 21.1% lower on rubber flooring compared with concrete; mean pressure values (± standard deviation) were 36.32 ± 7.77 N/cm2 for static measures and 57.33 ± 11.77 N/cm2 for dynamic measures. We observed an even more obvious relief on rubber flooring in the maximum pressure loads, which were 30.1 to 32.7% lower on rubber flooring compared with concrete; mean pressure values (± standard deviation) were 98.79 ± 14.49 N/cm2 for static measures and 150.45 ± 20.87 N/cm2 for dynamic measures). The force-time curves of the dynamic measures essentially showed biphasic curve progression, with local peaks at 29 and 79% of the stance phase. However, we found considerable differences in curve progression between individuals and between the lateral and medial claws, which may be verified in further investigations with more animals. The study showed a noticeable reduction in mechanical load during standing and walking on rubber flooring compared with concrete.

Feedback of the Kinesin-1 Neck-linker Position on the Catalytic Site

Kinesin-1 motor proteins step along microtubules by a mechanism in which the heads cycle through microtubule-bound and unbound states in an interlaced fashion. An important contribution to head-head coordination arises from the action of the neck-linker that docks onto the core motor domain upon ATP binding. We show here that the docked neck-linker not only guides the microtubule-unbound head to the next microtubule binding site but also signals its position to the head to which it is attached. Cross-linking studies on mutated kinesin constructs reveal that residues at the interface motor core/docked neck-linker, among them most importantly a conserved tyrosine, are involved in this feedback. The primary effect of the docked neck-linker is a reduced microtubule binding affinity in the ADP state.

Saccular hearing in anurans

Acoustic sensing in anurans is mediated by several inner ear organs, including the amphibian papilla, basillar papilla, and lagena. The contribution of the saccule to hearing in adult ranids tends to be constrained to vibration sensitivity and gravity sensing, although there has been some evidence for low-frequency hearing sensitivity as well. The organs that mediate hearing in larval ranids are less well understood. Several studies have postulated a strong role for saccular-mediated hearing in tadpoles, operating by the sensing of particle motion mediated by direct kinetic effects on otolithic organs (the fenestral pathway). Physiological studies show that, in tadpoles, the medial and lateral vestibular nuclei are sensitive to particle motion stimulation, while the dorsal medullary nucleus is more sensitive to pressure stimulation. Anatomical tract tracing with lipophilic cyanocarbanine dyes placed in the saccular branch of the eighth nerve show projections to both auditory and vestibular medullary nuclei, suggesting that input from the saccule may play a role in both particle and pressure sensation.

Temperature controls of microbial respiration in arctic tundra soils above and below freezing

Winter soil respiration can represent a significant fraction of the annual C cycle of arctic tundra, but the temperature response of microbial CO 2 production in frozen soils is poorly understood. We used a short-term laboratory incubation to describe temperature effects on aerobic respiration in four organic tundra soils from Northern Alaska. Respiration conformed closely to simple first-order exponential equations with constant temperature dependence ( r 2=0.81–0.98). Temperature coefficients ( Q 10) increased abruptly with freezing, varying from 4.6 to 9.4 among the thawed soils (+0.5 to +14 °C), and from 63 to 237 among the frozen soils (−10 to −0.5 °C). The single abrupt increase in temperature dependence with freezing suggests a shift in the dominant process controlling respiration below 0 °C. The Q 10s of frozen soils are too large to represent the direct kinetic effect of temperature, and more likely reflect extracellular barriers to diffusion and/or intracellular desiccation. In thawed soils, respiration Q 10s decreased with increasing soil organic matter quality, as indexed by baseline respiration at 0 °C, consistent with the thermodynamic argument that reactions metabolizing structurally complex, aromatic molecules have higher activation energies and temperature dependence than reactions metabolizing structurally simpler molecules. In frozen soils, Q 10s were unrelated to baseline respiration, suggesting that freezing uncouples the direct link between the C chemistry of microbial substrates and the temperature dependence of respiration. Our results underscore the potential for warming to stimulate microbial activity and the turnover of C and nutrients contained in tundra organic soils.

Interaction of Syntaxins with the Amiloride-sensitive Epithelial Sodium Channel

Amiloride-sensitive sodium channels mediate sodium entry across the apical membrane of epithelial cells in variety of tissues. The rate of Na(+) entry is controlled by the regulation of the epithelial sodium channel (ENaC) complex. Insertion/retrieval of the ENaC complex into the apical membrane as well as direct kinetic effects at the single channel level are recognized mechanisms of regulation. Recent data suggest that the syntaxin family of targeting proteins interact with and functionally regulate a number of ion channels and pumps. To evaluate the role of these proteins in regulating ENaC activity, we co-expressed rat ENaC cRNA (alpha, beta, gamma subunits) with syntaxin 1A or 3 cRNAs in Xenopus oocytes. Basal ENaC currents were inhibited by syntaxin 1A and stimulated by syntaxin 3. Both syntaxin 1A and syntaxin 3 could be co-immunoprecipitated with ENaC subunit proteins, suggesting physical interaction. Interestingly, immunofluorescence data suggest that with either syntaxin isoform the ENaC-associated epifluorescence on the oocyte surface is enhanced. These data indicate that (i) both syntaxin isoforms increase the net externalization of the ENaC channel complex, (ii) that the functional regulation is isoform specific, and (iii) suggest that ENaC may be regulated through mechanisms involving protein-protein interactions.

Accessory factors and the regulation of epithelial sodium channel activity.

The contribution in this issue of the JCI by Abriel et al. (1) brings into sharp focus our expanding knowledge of the regulation of epithelial sodium channel (ENaC) activity by specific accessory proteins. It has long been recognized that interactions with cytoskeletal proteins, such as actin, can regulate ENaC in a number of model systems (2). The interactions with defined cytosolic regions of the ENaC subunits were demonstrated by earlier studies of the ubiquitin ligase Nedd4 by Staub et al. (3). The importance of this interaction was emphasized by the finding that the site of interaction was with specific proline-rich regions of the subunits and that these very sites were found to be mutated, or even missing, in patients with Liddle's syndrome (4). This autosomal dominant syndrome is a rare cause of human hypertension that has clearly been shown to result from the failure to properly regulate ENaC expression and activity, with volume-expanded low-renin hypertension as the direct consequence. It appears that Nedd4 negatively modulates ENaC activity; binding of its WW domains to the proline domains of ENaC is followed by ubiquitination of the channel subunits, with subsequent endocytosis and lysosomal degradation. Indeed, ENaC appears to turn over quite rapidly with critical NH2-terminal lysine residues identified as the sites of ubiquitination of the α and γ subunits (5). In the current studies, the Xenopus oocyte expression system was used to demonstrate that overexpression of Nedd4 with ENaC inhibited channel activity. This effect was critically dependent on the proline domains in the ENaC subunits and on intact ubiquitination activity of the Nedd4 protein (1). Of note, a catalytically inactive Nedd4 construct appeared to interact competitively with the wild-type Nedd4 protein and actually protected the expressed ENaC from ubiquitination. A similar dependence of the regulatory effect of Nedd4 on its COOH-terminal ubiquitin ligase domain was recently demonstrated by Goulet et al. (6). These changes in ENaC activity can be rationalized in terms of changes in the surface expression of the channel complex, consistent with the role of the Nedd4 protein in endocytosis and degradation of the assembled ENaC complex at the surface membrane. While these effects are clearly explained by changes in the surface expression the ENaC complexes, there also appear to be direct kinetic effects of the various mutations described in Liddle's syndrome on the apparent open probability of the expressed ENaC (7). Certainly, endocytosis plays an important role in determining channel density or dwell time in the surface membrane, but other factors may also affect the rate of endocytosis (8), and even exocytosis, of the assembled channel complex to the surface membrane (9, 10). Other factors, including K-Ras2A, a small G protein that may in fact be one of the long-sought aldosterone-induced proteins (11), and an ENaC-associated serine protease termed channel activating protein-1(12) can activate ENaC activity independently of changes in its surface expression. In fact, K-Ras2A markedly increases in ENaC activity despite a decrease in surface expression (11), while we find that syntaxin 1A increases ENaC surface expression but decreases functional ENaC activity (9). This ensemble of studies has revealed important themes and details of the short-term regulation of ENaC activity. Although these studies have relied heavily on the Xenopus oocyte expression system, it can be expected that these results will provide novel approaches to the understanding, and even therapy, of human disorders of ENaC regulation. As the panoply of accessory proteins and factors unfolds, each provides a new candidate for the exploration of the functional regulation of ENaC activity, and even potential genetic linkage approaches to defined subsets of human low-renin hypertension.

Reduced Reaction Mechanisms for Ammonia Oxidation in Premixed Laminar Flames

Abstract Reduced reaction mechanisms are essential for the inclusion of direct kinetic effects into turbulent flame computations for practical problems. In the present paper reduced seven step, five step and four step reaction mechanisms for ammonia oxidation have been developed by the study ofpremixed laminar flames. A systematic reduction technique using steady state assumptions for intermediaries and radicals has been used to obtain the reduced schemes from a comprehensive detailed kinetic mechanism. Special emphasis has been given to the formation and destruction of NO and N2O The reduced mechanisms have been tested by comparing calculations performed with the reduced mechanisms and the detailed starting mechanism. Results for an extensive set of flat laminar premixed H2/O2 flames doped with ammonia or ammonia and nitric oxide diluted with argon and NH3/O2 flames are presented. The reduced mechanisms generally provide excellent agreement with the temperature, major species and nitric-oxide profiles fo...

The Calculation of the Structure of Laminar Counterflow Diffusion Flames Using a Global Reaction Mechanism

Abstract —Counterflow diffusion flames burning undiluted and diluted methane-air and propane-air mixtures have been computed using global reaction schemes involving only major species. Agreement for the undiluted methane-air and propane-air flames has been found to be excellent and realistic variations of species profiles with increasing rates of strain are obtained. Computations of diluted methane-air flames at different rates of strain confirm the experimental findings that the critical parameter determining name extinction is the maximum reaction zone temperature, which is found to be in acceptable agreement with measurements. The critical fuel concentration at the limit of low strain for the case of fuel diluted with nitrogen is also in acceptable agreement with measurements. Effects of varying degrees of approximation of transport properties is also investigated, including the common approximation of unity Lewis numbers. Implications of the results as regards to the inclusion of direct kinetic effect...

The influence of Hg II, Cd II and Cl − on the formation and dissociation rates of FeNCS 2+

The influence of Cd II, Hg II and Cl − on the rates of formation and dissociation of FeNCS 2+ has beeb examined in 0·5 M HClO 4, 0·05 M M(ClO 4) 2. 0·05 M Cd 2+ exhibits no direct kinetic effect but inhibits the rate of formation of FeNCS 2+ by sequestering NCS − in an unreactive form. Fe 3+ reacts readily with Hg(SCN) 2 and Hg(SCN) 3 −, water loss from the primary coordination sphere of Fe 3+ comprising the rate determining step. The reaction FeNCS 2++HgSCN + → Fe 3+ + Hg(SCN) 2 exhibits a high second order rate constant which appears to arise from a very favorable pre-equilibrium reaction forming a dinuclear complex, FeNCSHgSCN 3+, followed by a slow rate of FeNCS splitting. Cl acceleration appears to procede via the formation of a ternary complex, Fe(NCS)Cl +. Rate constants are given and their implication discussed.