RESUMO
The structure of AF-ZrF(4) system (A(+) = Li(+), Na(+), K(+)) compounds in the liquid state is studied using an approach combining EXAFS spectroscopy with molecular dynamics simulations. A very good agreement is observed between the two techniques, which allows us to propose a quantitative description of the liquids. From the Zr(4+) solvation shell point of view, we observe a progressive stabilization of the 7-fold and then of the 6-fold coordinated complexes when passing from Li(+) to Na(+) and K(+) as a "counterion". Particular attention is given to the systems consisting of 35 mol % of ZrF(4). At that particular composition, the ZrF(6)(2-) complex predominates largely whatever the nature of the alkali. The calculated vibrational properties of this complex are in excellent agreement with a previous Raman spectroscopy experiment on molten KF-ZrF(4). The most important differences are observed for the lifetime of these octahedral units, which increases importantly with the size of the monovalent cation. On a larger scale, an intense first sharp diffraction peak is observed for the Zr(4+)-Zr(4+) partial structure factor, which can be attributed to the correlations between the octahedral units formed.
RESUMO
We propose in this paper an original approach to study the structure of the molten LiF-ZrF(4) system up to 50 mol % ZrF(4), combining high-temperature nuclear magnetic resonance (NMR) and extended X-ray absorption fine structure (EXAFS) experiments with molecular dynamics (MD) calculations. (91)Zr high-temperature NMR experiments give an average coordination of 7 for the zirconium ion on all domains of composition. MD simulations, in agreement with EXAFS experiments at the K-edge of Zr, provide evidence for the coexistence of three different Zr-based complexes, [ZrF(6)](2-), [ZrF(7)](3-), and [ZrF(8)](4-), in the melt; the evolution of the concentration of these species upon addition of ZrF(4) is quantified. Smooth variations are observed, apart from a given composition at 35 mol % ZrF(4), for which an anomalous point is observed. Concerning the anion coordination, we observe a predominance of free fluorides at low concentrations in ZrF(4), and an increase of the number of bridging fluoride ions between complexes with addition of ZrF(4).
RESUMO
Sildenafil, a potent type 5 nucleotide-dependent phosphodiesterase (PDE) inhibitor, has been recently proposed as a therapeutic tool to treat or prevent pulmonary artery hypertension (PAHT). We thus studied the effect of sildenafil on both the calcium signaling of isolated pulmonary artery smooth muscle cells (PASMCs) and the reactivity of pulmonary artery (PA) obtained from chronic hypoxia (CH)-induced pulmonary hypertensive rats compared with control (normoxic) rats. CH rats were maintained in an hypobaric chamber (50.5 kPa) for 3 wk leading to full development of PAHT. Intracellular calcium concentration ([Ca2+]i) was measured in PASMCs loaded with the calcium fluorophore indo 1. Unlike in control rats, sildenafil (10-100 nM) decreased the resting [Ca2+]i value in PASMCs obtained from CH rats. In PASMCs from both control and CH rats, sildenafil concentration dependently inhibited the [Ca2+]i response induced by G-coupled membrane receptor agonists such as angiotensin II and phenylephrine but had no effect on the amplitude of the [Ca2+]i response induced by caffeine. Sildenafil (0.1 nM-1 microM) concentration dependently reduced basal PA tone that is present in CH rats and relaxed PA rings precontracted with phenylephrine in both control and CH rats. These data show that sildenafil is a potent pulmonary artery relaxant in CH rats and that it normalizes CH-induced increases in resting [Ca2+]i and basal tone. Consequently, pharmacological inhibition of sildenafil-sensitive PDE5 downregulates the Ca2+ signaling pathway involved in this model of pulmonary hypertension.
