A study of Na(x)Pt3O4 as an O2 electrode bifunctional electrocatalyst

The present study suggests that polytetrafluoroethylene (PTFE) bonded Na(X)Pt3O4 gas porous diffusion electrodes may be a viable candidate for bifunctional O2 reduction and evolution activity. The electrodes exhibited Tafel slopes of about 0.06 V/decade for both O2 reduction an evolution. For O2 reduction, the 0.06 slope doubled to 0.12 V/decade at larger current densities. Preliminary stability testing at 24 C suggest that the Na(x)Pt3O4 electrodes were relatively stable at reducing and oxidizing potentials typically encountered at the O2 electrodes in a regenerative fuel cell

Generation of tuneable 589nm radiation as a Na guide star source using an optical parametric amplifier

We describe a 5.5W 589nm source based on a passively modelocked Nd:YVO4 laser and a multi-stage Lithium Triborate optical parametric amplifier seeded by a tuneable semiconductor laser. We show this system can produce rapidly tuneable, transform-limited pulses in near diffraction-limited beams at 589nm, useful for Na guide star applications. The attraction of this scheme is that it can be assembled from commercially available hardware and is readily scalable to high average powers

Na+-H+ exchange activity in brush-border membrane vesicles isolated from chick small intestine

This study was undertaken to investigate the presence of a Na+{single bond}H+ antiporter in brush-border membrane vesicles (BBMV) isolated from chick small intestine. An outwardly directed proton gradient (pH 5.5 inside, 7.5 outside) stimulated Na+ uptake into BBMV and resulted in a transient accumulation. No accumulation was observed in the absence of a proton gradient. Voltage clamping the membrane with K+ and valinomycin decreased the Na+ overshoot. Amiloride inhibited pH gradient-driven Na+ uptake in a dose-dependent manner with an IC50 of 44 μM. The relationship between pH gradient-driven Na+ uptake and external Na+ concentration followed simple, saturating Michaelis-Menten kinetics. Eadie-Hofstee analysis of the pH gradient-driven Na+ uptake indicated a single transport system with a Vmax of 33 nmol/mg protein per 15 s and a Km for Na+ of 12 mM. The initial rate of pH-driven Na+ uptake increased as the intravesicular pH decreased, with a Hill coefficient close to 1. These findings indicate that BBMV isolated from chicken small intestine posses a Na+{single bond}H+ exchanger. This exchanger does not appear to be the one involved in cell pH regulation.Dirección General de Investigaciones Científicas y Técnicas PB89-061

Calcium/sodium exchange in purified secretory vesicles from bovine neurohypophyses

Purified secretory vesicles isolated from bovine neurohypophyses take up Na+ under the same circumstances where an efflux of Ca2+ takes place, suggesting a Na+/Ca2+ exchange. Potassium cannot substitute for Na+ in this process. Also, a Ca2+/Ca2+ exchange can occur. Inhibiting the latter process by Mg2+ allowed to estimate an apparent KM of 0.7 μM free Ca2+ and a maximal uptake of 1.5 nmol × mg protein−1 × min−1 Ca2+ in exchange for Na+. The vesicles did not contain plasma membrane marker (Na+/K+ ATPase) as shown by distribution analyses on the density gradients on which they were purified. Similarly, distribution studies also showed that no other ATPase activity could be detected in the purified vesicle fraction. It is concluded that a Na+/Ca2+ exchange is operating across the secretory vesicle membrane and that it is not directly dependent on ATP hydrolysis

The Ca2+Ca^{2+}-activated Cl−Cl^- current ensures robust and reliable signal amplification in vertebrate olfactory receptor neurons

Activation of most primary sensory neurons results in transduction currents that are carried by cations. One notable exception is the vertebrate olfactory receptor neuron (ORN), where the transduction current is carried largely by the anion $Cl^-$. However, it remains unclear why ORNs use an anionic current for signal amplification. We have sought to provide clarification on this topic by studying the so far neglected dynamics of $Na^+$, $Ca^{2+}$, $K^+$ and $Cl^-$ in the small space of olfactory cilia during an odorant response. Using computational modeling and simulations we compared the outcomes of signal amplification based on either $Cl^-$ or $Na^+$ currents. We found that amplification produced by $Na^+$ influx instead of a $Cl^-$ efflux is problematic due to several reasons: First, the $Na^+$ current amplitude varies greatly depending on mucosal ion concentration changes. Second, a $Na^+$ current leads to a large increase in the ciliary $Na^+$ concentration during an odorant response. This increase inhibits and even reverses $Ca^{2+}$ clearance by $Na^+/Ca^{2+}/K^+$ exchange, which is essential for response termination. Finally, a $Na^+$ current increases the ciliary osmotic pressure, which could cause swelling to damage the cilia. By contrast, a transduction pathway based on $Cl^-$ efflux circumvents these problems and renders the odorant response robust and reliable.Comment: 31 pages, 10 figures (including SI