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A Valcarcel

Publications and source records attributed to A Valcarcel.

6 recordsLinked to original sources
PubMed2005

DFT characterization of adsorbed NH(x) species on Pt(100) and Pt(111) surfaces.

Periodic density functional theory (DFT) calculations using plane waves have been performed to systematically investigate the adsorption and relative stability of ammonia and its dehydrogenated species on Pt(111) and Pt(100) surfaces. Different adsorption geometries and positions have been studied, and in each case, the equilibrium configuration has been determined by relaxation of the system. The vibrational spectra of the various ammonia fragments have been computed, and band assignments have been compared in detail with available experimental data. The adsorption of NH3 (on top) and NH2 (bridge) is more favorable on Pt(100) than on Pt(111), while similar adsorption energies were computed for NH (hollow) and N (hollow) on both surfaces. The remarkably lower adsorption energy of NH2 over Pt(111) as compared with Pt(100) (the difference being approximately 0.7 eV) can be related to different geometric and electronic factors associated with this particular intermediate. Accordingly, the type of platinum surface determines the most stable NH(x) fragment: Pt(100) has more affinity for NH2 species, whereas NH species are preferred over Pt(111).

Journal Article↗
PubMed2002

Pregnancy rates after transfer of frozen bovine embryos: a field trial.

An efficient and practical technique for bovine embryo cryopreservation is a fundamental issue in the widespread use of embryo transfer. The present study shows results obtained in field experiments. In the first experiment, two slow-freezing methods using glycerol and a one-step method using ethylene glycol were compared: glycerol added in two steps (5 and then 10%), glycerol added in one step (10%) and 1.5 M ethylene glycol with direct transfer. The three methods were equally effective; pregnancy rates of 40.4, 39.1 and 45.4%, respectively were achieved. In the second experiment, using 1.5 M ethylene glycol with direct transfer, 20 and 5 min of equilibration of the cryoprotectant were tested. There were no observed significant differences in pregnancy rates. In the third experiment, ethylene glycol and propylene glycol were combined with three sucrose concentrations (0, 0.1 or 0.3 M) in a one-step method. It was observed that ethylene glycol and 0.1 M sucrose yielded the highest pregnancy rate, not differing from fresh controls. Similar pregnancy rates were noted after using multiple-step or one-step methods, but the one-step method is preferable due to its simplicity and applicability to field conditions.

Animals↗
PubMed1997

Actin localization in ram spermatozoa: effect of freezing/thawing, capacitation and calcium ionophore-induced acrosomal exocytosis.

We have analyzed, by immunofluorescence, the localization of actin in ram spermatozoa, its colocalization with the actin-binding protein, gelsolin, and the effect of freeze/thawing, in vitro capacitation, and induced acrosomal exocytosis on its distribution. The monoclonal anti-actin and anti-gelsolin antibodies used recognized single bands at 43,000 and 90,000 kDa, respectively. In all spermatozoa, intense actin staining was observed in the whole length of the flagellum and, depending on the protocol used, in the neck and postacrosomal region of the head. Comparison of three staining methods, together with the use of NBD-phallacidin, allowed us to characterize ram sperm actin as a monomeric, intracellular, membrane-associated protein. Gelsolin was also present in ram spermatozoa and precisely colocalized with actin. Processes involving alterations in membrane structure such as freezing/thawing, in vitro capacitation, and calcium ionophore-induced acrosomal exocytosis provoked changes in the exposure of actin to the antibody. This strongly suggests a physical association of this protein to the plasma membrane, most likely by its intracellular side. The possible role of actin in sperm function is discussed.

Acrosome↗
PubMed1997

In vitro capacitating effect of gamma-aminobutyric acid in ram spermatozoa.

We have evaluated the capacitating effect of gamma-aminobutyric acid (GABA) in ram spermatozoa in vitro, in a chemically defined medium, by means of the chlortetracycline (CTC) binding assay. Semen from adult Australian Merino rams was collected in an artificial vagina; spermatozoa were washed once in modified Biggers, Whitten, and Wittingham medium (m-BWW), without BSA or serum, and incubated in m-BWW alone or in m-BWW containing GABA, GABA agonists, or antagonists for 2 h at 38.5 degrees C under 5% CO2 in air. Samples were taken for assessment of CTC binding pattern or were further incubated for 15 min in the presence of 5 microM calcium ionophore A23187. Acrosomal exocytosis was evaluated by Pisum sativum agglutinin binding. Addition of GABA to the incubation medium resulted in a concentration-dependent increase in the percentage of CTC forms II and III, corresponding to mid-capacitated and capacitated spermatozoa, respectively. The effect was marginally significant at 1 microM and maximal at 20 microM. The action of 20 microM GABA was mimicked by the GABAB-receptor agonist, muscimol, but not by the GABAA-receptor agonist, baclofen, and completely blocked by the GABAA-receptor antagonists, bicuculline and picrotoxin, which lacked effect per se. In a separate set of experiments, incubation of spermatozoa with GABA at a concentration of 1 microM, which was insufficient to stimulate sperm capacitation, together with the neuroactive steroid allopregnanolone (1 microM) provoked a capacitating effect similar to that achieved by 20 microM GABA alone. These results show that GABA has a capacitating action on ram spermatozoa through a GABAA receptor-mediated mechanism.

Acrosome↗
PubMed1996

Changes in sperm-bound amidase activity suggest subtle damage to ram sperm acrosomes by freezing/thawing, not detected by light microscopy.

We have measured sperm-bound amidase activity in fresh, cooled and frozen/thawed ram spermatozoa, in order to study if freezing and thawing led to some degree of acrosome damage of motile/viable spermatozoa not detected by optical methods. This assay was based on the fact that membrane damage would result in an increased access of the enzyme substrate to the sperm acrosome. Semen was collected from adult Australian Merino rams, and spermatozoa were washed by centrifugation through a Ficoll solution. Sperm-bound amidase activity was measured in whole spermatozoa using the protease substrate benzoyl-arginyl-p-nitroanilide (BAPNA). Acrosomal status was also assessed by light microscopy after Giemsa staining. Most amidase activity was shown to be sperm-bound, as only a minor fraction of the enzyme activity was release into the medium after induced damage. Simultaneous assessment of sperm-bound amidase activity and the percentage of spermatozoa with microscopically evident acrosomal damage, after mild sonication for different times, showed a high correlation between both parameters (r = 0.97, p < 0.001). In separate experiments, fresh, cooled and frozen/thawed semen samples were filtered through Sephadex G-10 to obtain a subpopulation of motile, mostly acrosome-intact spermatozoa. As controls, spermatozoa from the same samples to which extensive acrosome damage was induced were evaluated. Slow cooling to 4 degrees C had no effect on amidase activity or percent acrosomal damage with respect to fresh samples. Freezing and thawing resulted in a sperm population that, after filtration through Sephadex, had a low percentage of acrosome damage (9.4%, vs. 2.1% for fresh filtered controls), which was 11% of that obtained after extensive acrosome damage (83%). However, amidase activity in these samples was markedly increased, showing values of activity that were 56% of those obtained in extensively damaged spermatozoa. This effect was not due to an alteration in the enzyme kinetics. We conclude that sperm-bound amidase activity is useful to detect subtle changes, provoked by a standard freezing/thawing procedure, in the permeability of acrosomes from ram spermatozoa which are not detected by direct observation of the acrosomes after Giemsa staining.

Acrosome↗