Structure and function of fusion pores in exocytosis and ectoplasmic membrane fusion.

Several proteins involved in exocytosis have been identified recently, but it is still completely unclear which molecules perform the fusion event itself. Although in viral fusion the fusion proteins are known, even there the molecular mechanism remains controversial. Investigation of single fusion events by electrophysiological techniques together with fluorimetric measurements have now provided some insight into the properties of the first aqueous connection, the fusion pore. This pore has an initial size similar to an ion channel and allows movement of lipids only after it has substantially expanded, indicating that it is initially not a purely lipidic structure, but incorporates lipids when it expands. Although neurotransmitter release may occur through narrow transient fusion pores, the fusion pore of synaptic vesicles probably expands vey rapidly, making it unlikely that secretion is performed by rapid exo/endocytosis without full fusion under normal conditions. Recent recordings from small membrane patches have made it possible to resolve fusion events from vesicles as small as synaptic vesicles. Future experiments using excised patches may provide an approach to identify the molecular machinery of exocytotic membrane fusion

A slowly activating voltage-dependent K+ current in rat pituitary nerve terminals.

A novel slowly activating voltage-dependent K+ current was observed in isolated nerve terminals from rat neurohypophysis using the whole-cell configuration of the patch-clamp technique. 2. The activation kinetics of the slow current could be fitted assuming Hodgkin--Huxley-type kinetics, an exponential, n, of 1.3 and activation time constants decreasing from 4 s at -50 mV to 0.7s at +40 mV. 3. A positive shift of reversal potential was observed when [K+] was increased in the bath solution. The current is carried mainly but not exclusively by K+ ions. 4. When intracellular free [Mg2+] was low (approximately 60 microM), average current density was 74 pA pF-1 at membrane potentials around 0 mV. In 83% of nerve terminals current amplitude was > 10 pA pF-1. 5. The slow current was never observed when the pipette contained 4.6 mM free Mg2+. At a physiological level of free Mg2+ (0.5 mM) the average current density was 16 pA pF-1. 6. When nerve terminals were analysed after patch-clamp experiments for vasopressin content by immunodetection, no difference in current amplitude was found between the terminals containing vasopressin and all analysed terminals. 7. The voltage dependence of activation was fitted by a Boltzmann equation giving a half-activation potential of -37 mV and a slope factor of about 9 mV. 8. Tail current deactivation kinetics was biexponential with time constants of 0.12 and 1.5s. Kinetics was dependent on the duration of the activating pulse. 9. Noise analysis of the slow current indicated a single-channel current of 0.33 pA at +6 mV, corresponding to a single-channel conductance of 4.3 pS. 10. This is the first demonstration of a current similar to the slow K+ current, IKs, in a neurone, suggesting that a protein similar to the IKs-inducing channel protein IsK (minK) may be present in peptidergic nerve terminals. 11. The activation properties are consistent with a role of the slow current in inhibition of excitability, at least at the level of the nerve terminal

Air entrainment mechanisms from artificial supercavities: Insight based on numerical simulations

Using multiphase computational simulations based on the Navier-Stokes equations, we examine the internal gaseous flows of artificially ventilated supercavities. These simulations indicate that air shear layers that develop on the cavity-wall (the air-liquid interface surrounding the cavity) are an important mechanism of air entrainment. This corroborates previous theory developed for toroidal cavities, and indicates that similar mechanisms occur in twin-vortex cavities and cavities closing on bodies. The importance of these shear layers on the cavity behavior potentially impacts computational simulations, experiments, and design-level models. Lastly, a more inclusive, semi-empirical air entrainment model is presented that attempts to accommodate the observed processes.http://deepblue.lib.umich.edu/bitstream/2027.42/84310/1/CAV2009-final136.pd

An examination of thermal modeling affects to the numerical prediction of large-scale cavitating fluid flow

The importance of modeling thermal effects in cavitatingfluid is examined in the context of computational fluid dynamics. Simulations of cavitation in water are used to study the effects of thermal versus and pressure variations in the fluid properties, and their impact on predictions. These studies are extended to evaluate energyconserving approaches compared to isothermal ones, to assess the underlying thermal models influence on the predicted cavities occurring in water. Results indicate that the thermal effects remain important, but only for specific applications that need high-frequency phenomena from the numerical simulation. Low-frequency measures, needed for loading analysis, appear to be relatively insensitive to thermal effects. Lastly, various thermally driven cavitation problems requiring energy-equation conservation are presented to display applications requiring such a formulation.http://deepblue.lib.umich.edu/bitstream/2027.42/84311/1/CAV2009-final137.pd

Small-angle neutron scattering (SANS) characterization of 13.5 Cr oxide dispersion strengthened ferritic steel for fusion applications

Small-angle neutron scattering (SANS) has been utilized for micro-structural investigation on laboratory heats of oxide dispersion strengthened (ODS) 13.5 Cr wt % ferritic steel, with 0.3 wt% Y$_{2}$O$_{3}$ and with variable Ti and W contents. The results show that increasing the Ti content from 0.2 to 0.4 wt% a distribution of nano-clusters develops, tentatively identified as Y$_{2}$Ti$_{2}$O$_{7}$, with average radii as small as 6.5 Å and volume fractions increasing from 0.021 to 0.032. The measured SANS cross-sections show also the growth of much larger defects, possibly Cr oxides. Furthermore, the ratio of magnetic to nuclear SANS components shows that the defect composition varies both with their size and with the Ti and the W content. These results are in qualitative agreement with transmission electron microscopy (TEM) observations, showing a striking influence of Ti addition on particle size refinement. However, while TEM is limited in statistics and minimum observable size of the Ti-rich nano-clusters, the defect distributions obtained by these SANS measurements provide complementary information useful for morphological characterization of the micro-structure in the investigated material

Tethering forces of secretory granules measured with optical tweezers.

Fusion of a vesicle with its target membrane is preceded by tethering or docking. However, the physical mechanism of vesicle-tethering is unknown. To study this mechanism, we used eosinophil secretory granules, which undergo stimulated homotypic fusion events inside the cell during degranulation. Using a dual optical trap system, we observed tether formation between isolated eosinophil secretory granules. The results show that secretory granules interact stochastically with a target membrane forming physical tethers linking the vesicle and target membrane, rather than via interactions with the cytoskeleton. The necessary components are membrane-associated, and the addition of cytosolic components is not required. Tether-lifetime measurements as a function of applied mechanical force revealed at least three kinetically distinct tethered states. The tethered-state lifetimes of isolated eosinophil granules match the residence times of chromaffin granules at the plasma membrane in intact cells, suggesting that the tethering mechanisms reported here may represent the physiological mechanisms of vesicle-tethering in the cell