The influence of skeletal muscle cell volume on carbohydrate metabolism in contracting skeletal muscle

This study investigated the regulation of carbohydrate metabolism through changes in skeletal muscle cell volume immediately post contraction and during recovery. Using an established in vitro isolated muscle strip model, soleus (SOL) and extensor digitorum longus (EDL) were dissected from male rats and incubated in an organ bath (perfused with 95% O2; 5% CO2, pH 7.4, temperature 25°C) containing medium- 199 altered to a target osmotic condition (iso-, hypo- or hyper-osmotic; 290, 1 80, 400 mmol/kg). Muscles were stimulated for 10 minutes (40 Hz SOL; 30 Hz EDL) and then either immediately flash frozen or allowed to recover for 20 minutes before subsequent metabolite and enzyme analysis. Results demonstrated a relative water decrease in HYPER vs. HYPOosmotic condition (n=8/group; p<0.05) regardless of muscle type. Specifically, the SOL HYPER condition had elevated metabolite concentrations after 10 minutes of stimulation in comparison to both HYPO and ISO (p<0.05), while EDL muscle did not show any significant difTerences between the HYPER or HYPO conditions. After 20 minutes of recovery, metabolic changes occurred in both SOL and EDL with the SOL HYPER condition showing greater relative changes in metabolite concentrations versus HYPO. The results of the current study have demonstrated that osmotic imbalance induces metabolic change within the skeletal muscle cell and muscle type may influence the mechanisms utilized for cell volume regulation.

Measurement of forces between and within bilayers of neutral phospholipids

Phospholipids in water form lamellar phases made up of alternating layers of water and bimolecular lipid leaflets. Three complementary methods, osmotic, mechanical, and vapour pressures, were used to measure the work of removing water from lamellar phases composed of frozen dipalmitoylphosphatidylcholine ( DPPC ), melted DPPC, egg phosphatidylethanolamine or equimolar mixtures of DPPC and cholesterol ( DPPC/CHOL ), Concurrently the structural changes that resulted from this water removal were measured using X-ray diffraction. The work was divided into that which forces the bilayers together ( F ) and that which compresses the molecules together within the bilayers ( F )# A large repulsive force exists between bilayers composed of each of the lipids studied and this force increases exponentially as bilayer separation is decreased. F is affected by the nature of the head groups, conformation of the acyl chains and heterogeneity of these chains. In general all of the melted phosphatidylcholines ( melted DPPC, egg lecithin and DPPC/CHOL ) have large equilibrium separations in excess water resulting from large repulsive hydration forces between these bilayers. By comparison, egg PE has an increased attractive force, and frozen DPPC has a decreased hydration force; each results in smaller separations in water for these two lipids. The chemical potentials of the water between the bilayers for all these lipids lie on a continuum, indicating that interbilayer water cannot be characterized by two discrete states, usually referred to as "bound" or "non**bound". For all lipids studied a maximum of 25 % of the total work done on the system goes into deforming the bilayers. The method used here viii to separate repulsion from deformation, developed for us by v. A. Parsegian, provides a unique method for the measurement of lateral pressure of a bilayer and its modulus of deformability ( Y ). Lateral pressure is affected by the nature of the head group, conformation and heterogeneity of the acyl chains. For small changes in molecular surface area ( A ) near equilibrium, both melted and frozen DPPC have similar values for the deformability modulus. Thus in this regime it requires about the same force to change the angle of tilt of frozen chains as it does to compress the fluid bilayer. The introduction of cholesterol into bilayers of DPPC reduces dramatically the lateral pressure of the bilayers over a large range of molecular surface areas ( A ). The variation in the magnitude of bilayer repulsion with different phospholipids provides a basis for the mechanism of lipid segregation in mixed lipid systems and suggests that interacting heterogeneous membranes may influence or modulate the composition of the opposing membrane. The measurements of deformabilities of bilayers provides a direct comparison of them with the properties of monolayers.

Sedimentology and micromorphology of drumlin sediments at Chimney Bluffs State Park, New York, United States of America

The drumlin sediments at Chimney Bluffs, New York appear to represent a block-inmatrix style glacial melange. This melange comprises sand stringers, lenses and intraclasts juxtaposed in an apparently massive diamicton. Thin section examination of these glacigenic deposits has revealed microstructures indicative of autokinetic subglacial defonnation which are consistent with a deformable bed origin for the diamicton. These features include banding and. necking of matrix grains, oriented plasma fabrics and the formation of pressure shadows at the long axis ends of elongate clasts. Preservation of primary stratification within the sand intraclasts appears to suggest that these features were pre-existing up-ice deposits that were frozen, entrained, then deposited as part of a defonning till layer beneath an advancing ice sheet. Multi-directional micro-shearing within the sand blocks is thought to reflect the frozen nature of the sand units in such a high strain environment. It is also contended that dewatering of the sediment pile leading to the eventual immobilisation of the defonning till layer was responsible for opening sub-horizontal fissures within the diamicton. These features were subsequently infilled with mass flow poorly sorted sands and silts which were subjected to ductile defonnation during the waning stages of an actively deforming till layer. Microstructures indicative of the dewatering processes in the sand units include patches of fine-grained particles within a coarser-grained matrix and the presence of concentrated zones of translocated clays. However, these units were probably confined within an impermeable diamicton casing that prevented massive pore water influxes from the deforming till layer~ Hence, these microstructures probably reflect localised dewatering of the sand intraclasts. A layered subglacial shear zone model is proposed for the various features exhibited by the drumlin sediments. The complexity of these structures is explained in terms of ii superposing deformation styles in response to changing pore water pressures. Constructional glaciotectonics, as implied by the occurrence of sub-horizontal fissuring, is suggested as the mechanism for the stacking of the sand intraclast units within the diamicton. The usefulness of micromorphology in complimenting the traditional sedimentology of glacigenic deposits is emphasised by the current study. An otherwise massive diamicton was shown to contain microstructures indicative of the very high strain rates expected in a complexly deforming till layer. . It is quite obvious from this investigation that the classification of diamictons needs to be re-examined for evidence of microstructures that could lead to the re-interpretation of diamicton forming processes. RESUME Le pacquet de sediments drumlinaire de Chimney Bluffs, New York, represent un "bloc-en-matrice" genre de melange glaciale. Des structures microscopique comprennent l'evidence pour la defonnation intrinseque attribuee a l'origine lit non resistant du drumlin. PreselVation des structures primaires au coeur des blocs arenaces suggere que ceux sont des depots preexistant qui furent geles, entraines et par la suite sedimentes au milieu d'une couche de debris sous-glaciaires en voie de deformation. Des failles microscopiques a l'interieur des blocs arenaces appuient aussi l'idee d'un bloc cohesif (c'est-a-dire gele) au centre d'un till non resistant. Des implications significatives s'emergent de cette etude pour les conditions sous-glaciaire et les processus de la formation des drumlin.

Study of new yeast strains as novel starter cultures for Riesling icewine production

Icewine is a sweet dessert wine fermented from the juice of grapes naturally frozen on the vine. The production of Icewine faces many challenges such as sluggish fermentation, which often yields wines with low ethanol, and an accumulation of high concentration of volatile acidity, mainly in the form of acetic acid. This project investigated three new yeast strains as novel starter cultures for Icewine fermentation with particular emphasis on reducing acetic acid production: a naturally occurring strain of S. bayanus/S. pastorianus isolated from Icewine grapes, and two hybrids between S. cerevisiae and S. bayanus, AWRI 1571 and AWRI 1572. These strains were evaluated for sugar consumption patterns and metabolic production of ethanol, glycerol and acetic acid, and were compared to the performance of a standard commercial wine yeast KI-VI116. The ITS rONA region of the two A WRI crosses was also analyzed during fermentations to assess their genomic stability. Icewine fermentations were performed in sterile filtered juice, in the absence of indigenous microflora, and also in unfiltered juice in order to mirror commercial wine making practices. The hybrid A WRI 1572 was found to be a promising candidate as a novel starter culture for Icewine production. I t produced 10.3 % v/v of ethanol in sterile Riesling Icewine fermentations and 11.2 % v/v in the unfiltered ones within a reasonable fermentation time (39 days). Its acetic acid production per gram sugar consumed was approximately 30% lower in comparison with commercial wine yeast K I -V 1116 under both sterile filtered and unfiltered fermentations. The natural isolate S. bayanus/S. pastorianus and AWRI 1571 did not appear to be suitable for commercial Icewine production. They reached the target ethanol concentration of approximately 10 % v/v in 39 day fermentations and also produced less acetic acid as a function of both time and sugar consumed in sterile fermentations compared to KI-V1116. However, in unfiltered fermentations, both of them failed to produce the target concentration of ethanol and accumulated high concentration of acetic acid. Both A WRI crosses displayed higher loss of or reduced copies in ITS rDNA region from the S. bayanus parent compared to the S. cerevisiae parent; however, these genomic losses could not be related to the metabolic profile.

Icewine fermentation by Saccharomyces cervisiae : fundamental stress responses and comparative fermentation dynamics

Please consult the paper edition of this thesis to read. It is available on the 5th Floor of the Library at Call Number: Z 9999.5 B63 P54 2007

Inniskillin fonds, 1970-2014, n.d.

Inniskillin Wines was founded by Karl Kaiser and Donald Ziraldo in 1975 in Niagara-on-the-Lake, Ontario. They had met the previous year, when Karl Kaiser, a winemaker and chemist, purchased some grapes from Donald Ziraldo, who owned and operated Ziraldo Nurseries. The two shared a vision of producing better quality Canadian wines and formed a partnership, with Kaiser making the wine and Ziraldo serving as company President. In 1975, they were granted a winery license by the LCBO, the first one granted since 1929. The company name Inniskillin was derived from the Inniskilling Fusilliers, an Irish regiment whose Colonel once owned the land that Ziraldo Nurseries occupied. This was the original site of the winery, although in 1978 the winery moved to the Brae Burn Estate, their current location. In 1982 the winery expanded by 50 acres with the addition of the Montague Vineyard, and another 50 acres was acquired in 1991. The Niagara-on-the-Lake vineyard produces single vineyards bottlings of Chardonnay, Pinot Noir, Merlot and Pinot Grigio/Pinot Gris. In 1984, Karl Kaiser began producing icewine from Vidal grapes frozen naturally on the vine. Inniskillin garnered international acclaim for the quality of their icewines, receiving the prestigious Grand Prix d’Honneur at VinExpo in 1991, for their 1989 Vidal icewine. This established Inniskillin as a producer of world class wines, while also raising the profile of Canadian wines in general. The company branched out their operations, first acquiring vineyards in the Napa Valley in 1989 to form Inniskillin Napa (producing wines under the Terra label), and in 1994 establishing Inniskillin Okanagan in the Okanagan Valley in British Columbia. The Napa valley venture ceased in the mid 90’s, while Inniskillin Okanagan continues to operate. In 2006, Karl Kaiser and Donald Ziraldo left Inniskillin. Kaiser retired, while Ziraldo became chair of the Vineland Research and Innovation Center (2006-2011), and remains involved in the wine industry. In 2007, Bruce Nicholson joined Inniskillin as winemaker. Nicholson continues to produce award-winning wines under the Inniskillin label, receiving the top award, the Premio Speciale Gran Award, at Vinitaly 2009 for his 2006 Gold Vidal and his 2006 Sparkling Vidal Icewine. In 2012, he received several awards for the 2008 Riesling Icewine, including gold at the International Wine and Spirits Competition in London, UK, the San Francisco International Wine Championships, and Selections Mondials des Vins Canada.

Donald Ziraldo Fonds, 1968-2012, n.d.

Donald J. P. Ziraldo, C.M., BSc., LLD was born in St. Catharines, Ontario on October 13, 1948 to Fredrick and Irma (Schiratti) Ziraldo. He graduated Denis Morris High School in St. Catharines in 1967, and received his B.Sc. in Agriculture at the University of Guelph in 1971. In 1974, Ziraldo was running Ziraldo Nurseries when he met Austrian born schoolteacher, chemist and winemaker Karl J. Kaiser. They realized that there was a gap in the premium varietal wine market and decided to plant a premium traditional European variety of grape vine species, the Vitis vinifera. This was an innovation in the Niagara region because the current wine producers were not using premium European grapes at the time. Ziraldo and Kaiser founded and then formally incorporated Inniskillin Wines Inc. in Niagara-on-the-Lake, Ontario on July 31, 1975. Ziraldo successfully lobbied General George Kitching, CEO of the LCBO, for a winery license. In 1975, Kitching granted him a winery license, the first in Ontario since Prohibition ended. From the beginning, there was a division of labour where Kaiser focused on the winemaking and Ziraldo focused on the marketing and promotion of the wines. Ziraldo also became president of the company. Ziraldo and Kaiser worked on improving their winemaking techniques and promoting their products and company. Ziraldo has been called ‘one of the founding fathers of the Canadian wine industry’, and it is widely acknowledged that both men played a large role in the success and growth of the Canadian wine industry. Together they pioneered the estate winery movement in Canada. A major turning point Inniskillin came in 1984 when Karl Kaiser successfully harvested the first Icewine crop from frozen grapes on the vine and bottled Eiswein Vidal (Icewine). In 1990, Inniskillin received worldwide recognition for this Icewine when their 1989 Vidal Icewine won the most prestigious award in the wine world, the Grand Prix d’Honneur, given at Vinexpo in France. This victory has been called ‘the award heard round the world’ and it launched Inniskillin into the international wine arena. At the same time, this helped lift the profile of Canadian wines in general. Inniskillin not only became Canada’s leading producer of Icewine, but it also became known for producing ‘one of the world’s great wines’. After the 1990 award, Ziraldo began a major public relations campaign to promote Inniskillin and build Icewine into a worldwide brand. He travelled broadly every year to promote the brand and products and networked extensively with politicians, celebrities, chefs, sommeliers, etc. To ensure worldwide and long-term success, Ziraldo introduced Icewine to Asia and the United States which were new markets. He developed a new Icewine glass with George Riedel. Tony Aspler has called Ziraldo ‘Canada’s Wine Ambassador’. Ziraldo was President of Inniskillin Wines Inc. (Niagara) from 1975 to 2006. In 1992, Inniskillin merged with Cartier Wines, and in 1993 Cartier Inniskillin Vintners Inc. merged with T.G. Bright & Co. Limited, forming the new company Vincor International Inc. Inniskillin wines was now a subsidiary of Vincor. Ziraldo became a Director at Vincor International Inc. from 1993 to 2004. From 1989 to the mid 1990s, Ziraldo also became President of Inniskillin Napa, in Napa Valley, California. Inniskillin purchased Napa Valley vineyards and produced wines under the Terra label. In 1994, Ziraldo set up a subsidiary estate winery of Inniskillin in Oliver, British Columbia which was called Inniskillin Okanagan Vineyards Inc. He became President of the winery. This started as a partnership between Inniskillin and the local Inkameep Indian Band in the Okanagan. In 2006, Ziraldo left Inniskillin and since that time he has been involved in other Icewine related ventures such as running Ziraldo Estate Winery and producing Ziraldo Riesling Icewine 2007. He also is in partnership with the Niagara based Equifera Estate Winery to produce Equifera Icewine. His most recent projects include planting Picolit grapes in his parent’s hometown, in a project called Picolit Di Fagagna and becoming Managing Director of the Senhora Do Convento Port Winery in Portugal. Donald Ziraldo was instrumental in the creation of the Vintners Quality Alliance (VQA) in Ontario and was its founding Chair from 1988-1995. The VQA was established as a regulatory and appellation system which secured the quality and origin of Canadian wines made under this system. The VQA designation and bottle label gave the consumer confidence that the wines they were purchasing were 100% local products. The VQA system was set up first in Ontario and then in British Columbia.

Winter at base of American Falls

The description for the image reads "Niagara in winter, huge bank of frozen water below the American Falls. The beginning of the ice-bridge".

Force potentiation as a modulator of contractile performance: Implications for control of skeletal muscle force and energetics of work

This thesis investigated the modulation of dynamic contractile function and energetics of work by posttetanic potentiation (PTP). Mechanical experiments were conducted in vitro using software-controlled protocols to stimulate/determine contractile function during ramp shortening, and muscles were frozen during parallel incubations for biochemical analysis. The central feature of this research was the comparison of fast hindlimb muscles from wildtype and skeletal myosin light chain kinase knockout (skMLCK-/-) mice that does not express the primary mechanism for PTP: myosin regulatory light chain (RLC) phosphorylation. In contrast to smooth/cardiac muscles where RLC phosphorylation is indispensable, its precise physiological role in skeletal muscle is unclear. It was initially determined that tetanic potentiation was shortening speed dependent, and this sensitivity of the PTP mechanism to muscle shortening extended the stimulation frequency domain over which PTP was manifest. Thus, the physiological utility of RLC phosphorylation to augment contractile function in vivo may be more extensive than previously considered. Subsequent experiments studied the contraction-type dependence for PTP and demonstrated that the enhancement of contractile function was dependent on force level. Surprisingly, in the absence of RLC phosphorylation, skMLCK-/- muscles exhibited significant concentric PTP; consequently, up to ~50% of the dynamic PTP response in wildtype muscle may be attributed to an alternate mechanism. When the interaction of PTP and the catchlike property (CLP) was examined, we determined that unlike the acute augmentation of peak force by the CLP, RLC phosphorylation produced a longer-lasting enhancement of force and work in the potentiated state. Nevertheless, despite the apparent interference between these mechanisms, both offer physiological utility and may be complementary in achieving optimal contractile function in vivo. Finally, when the energetic implications of PTP were explored, we determined that during a brief period of repetitive concentric activation, total work performed was ~60% greater in wildtype vs. skMLCK-/- muscles but there was no genotype difference in High-Energy Phosphate Consumption or Economy (i.e. HEPC: work). In summary, this thesis provides novel insight into the modulatory effects of PTP and RLC phosphorylation, and through the observation of alternative mechanisms for PTP we further develop our understanding of the history-dependence of fast skeletal muscle function.

Canada Starch Company (CASCO) Fonds, 1980-1987, n.d.

CASCO (Canada Starch Company) began operations in 1982 and was officially opened in May of 1983. Premier William Davis was in attendance. CASCO is a company whose roots can be traced back to 1858 when it was founded by W.T. Benson in Cardinal, Ontario. The company grew as corn uses were developed. Corn derived products now include: corn oil, liquid sweetener and feed for dairy and cattle. Starch is used as a finish for fine papers, a component in dry cell batteries, pharmaceuticals, wallpaper, film, tires, surgical dressings, plastics and plywood. Corn syrup is used in beverages, canned fruit, frozen seafood, licorice, ice cream and baking products. Corn solubles are used in animal feed, rubber substitutes, soap, paint and varnish. There are more than 250 industrial and food uses for corn

Standardization of cytokine flow cytometry assays

Affiliation: Faculté de médecine, Université de Montréal & CANVAC

Federal Constitutionalism and Aboriginal Difference

The author outlines a new legal approach, which he labels federal constitutionalism, to the question of aboriginal difference in Canada. This approach has the potential to open up more fruitful avenues for the resolution of aboriginal law issues than either the “frozen rights” approach currently adopted by the Supreme Court of Canada or the treaty federalism approach, which posits that treaties should be used to resolve all differences between aboriginals and non-aboriginals. The author outlines the difficulties inherent in both the frozen rights and treaty federalism approaches. Federal constitutionalism, in contrast, draws its vitality from an organic understanding of Canada’s constitutional experience. It would allow aboriginal peoples to be seen as federal actors who have historically shaped the Canadian federation. Federal constitutionalism is a multi-faceted approach that would permit aboriginal questions to be addressed using the federal principle, thereby allowing the legal focus to move away from section 35 of the Constitution Act, 1982. Aboriginal peoples would be able to exercise the rights of sovereignty over their own internal affairs, while individual aboriginals could participate directly in federal and provincial governments without having to proceed through the intermediary of aboriginal representatives. Federal constitutionalism would allow aboriginal peoples a guaranteed sphere of autonomy, while permitting recognition of their historical interdependence with non-aboriginal peoples.

Modèles de dysfonction érectile chez le rat

Des modèles de dysfonction érectile (DE) ont été développés et étudiés chez le rat. Cependant, peu de choses sont connues dans certains modèles présentant des facteurs de risque de la DE, en l’occurrence le rat soumis à une diète riche en glucose, le rat avec une restriction de croissance intra-utérine (RCIU) et le rat avec l’infusion continue de l’angiotensine (Ang) II en ce qui concerne l’évaluation de la fonction érectile in vivo. C’est ainsi que nous nous sommes proposés cette étude. La fonction érectile a été mesurée par la stimulation électrique du nerf caverneux à des cohortes de rats mâles Sprague-Dawley de différents âges comme suit: 1) des rats jeunes de 12 semaines d’âge, adultes reproducteurs à la retraite d’environ 30 semaines d’âge et des rats de 27 et 29 semaines soumis à une diète riche en glucose par l’eau de breuvage (10%) à court (1 semaine), moyen (6 semaines) et long (12 ou 13 semaines) termes; 2) des rats jeunes âgés de 12 semaines recevant une infusion soit de l’Ang II (200ng/kg/min) soit de la saline par mini-pompes osmotiques sous-cutanées pendant 1 et 2 semaines; et 3) des rats adultes âgés de 36 semaines avec une RCIU. La fonction érectile a été représentée par des courbes de variations de pression intracaverneuse (PIC) et d’aire sous la courbe (ASC) normalisées avec la pression artérielle moyenne (PAM) en réponse à la stimulation nerveuse à différents volts. À la fin des expériences, les animaux ont été sacrifiés, le sang recueilli pour le dosage de la glycémie et de l’insulinémie; le pénis, l’aorte thoracique et le cœur prélevés à des fins d’analyses protéique et histologique. La diète riche en glucose à court terme a eu un effet néfaste sur la fonction érectile chez le jeune rat mais bénéfique chez le rat adulte reproducteur; de même que pour les études à long terme, la diète riche en glucose a amélioré ou renversé la DE associée au vieillissement. Les rats avec RCIU ont exhibé une baisse très significative de la fonction érectile. Ces détériorations ou améliorations de la fonction érectile avec le glucose ou dans la RCIU ont été principalement associées à des modifications de l’expression de l’Akt, en plus d’une augmentation de l’insuline sérique dans les groupes avec le glucose. L’Ang II a entraîné une baisse de la fonction érectile, statistiquement très significative après 1 semaine et maintenue après 2 semaines de traitement, sans causer des changements morphologiques au cœur et à l’aorte thoracique. En conclusion : l’atteinte du système érectile se fait précocement dans les complications cardiovasculaires liées à l’activation du SRA, l’implication de l’Akt dans la modulation de la fonction érectile dépend de type de vieillissement et de la diète, le RCIU est un facteur de risque pour la DE.

Anisotropie de la photoluminescence dans des nanostructures organiques chirales autoassemblées

Nous investiguons dans ce travail la dynamique des excitons dans une couche mince d’agrégats H autoassemblés hélicoïdaux de molécules de sexithiophène. Le couplage intermoléculaire (J=100 meV) place ce matériau dans la catégorie des semi-conducteurs à couplage de type intermédiaire. Le désordre énergétique et la forte interaction électronsphonons causent une forte localisation des excitons. Les espèces initiales se ramifient en deux états distincts : un état d’excitons autopiégés (rendement de 95 %) et un état à transfert de charge (rendement de 5%). À température de la pièce (293K), les processus de sauts intermoléculaires sont activés et l’anisotropie de la fluorescence décroît rapidement à zéro en 5 ns. À basse température (14K), les processus de sauts sont gelés. Pour caractériser la dynamique de diffusion des espèces, une expérience d’anisotropie de fluorescence a été effectuée. Celle-ci consiste à mesurer la différence entre la photoluminescence polarisée parallèlement au laser excitateur et celle polarisée perpendiculairement, en fonction du temps. Cette mesure nous donne de l’information sur la dépolarisation des excitons, qui est directement reliée à leur diffusion dans la structure supramoléculaire. On mesure une anisotropie de 0,1 après 20 ns qui perdure jusqu’à 50ns. Les états à transfert de charge causent une remontée de l’anisotropie vers une valeur de 0,15 sur une plage temporelle allant de 50 ns jusqu’à 210 ns (période entre les impulsions laser). Ces résultats démontrent que la localisation des porteurs est très grande à 14K, et qu’elle est supérieure pour les espèces à transfert de charge. Un modèle numérique simple d’équations différentielles à temps de vie radiatif et de dépolarisation constants permet de reproduire les données expérimentales. Ce modèle a toutefois ses limitations, notamment en ce qui a trait aux mécanismes de dépolarisation des excitons.

Quotients d'une variété algébrique par un groupe algébrique linéairement réductif et ses sous-groupes maximaux unipotents

La construction d'un quotient, en topologie, est relativement simple; si $G$ est un groupe topologique agissant sur un espace topologique $X$, on peut considérer l'application naturelle de $X$ dans $X/G$, l'espace d'orbites muni de la topologie quotient. En géométrie algébrique, malheureusement, il n'est généralement pas possible de munir l'espace d'orbites d'une structure de variété. Dans le cas de l'action d'un groupe linéairement réductif $G$ sur une variété projective $X$, la théorie géométrique des invariants nous permet toutefois de construire un morphisme de variété d'un ouvert $U$ de $X$ vers une variété projective $X//U$, se rapprochant autant que possible d'une application quotient, au sens topologique du terme. Considérons par exemple $X\subseteq P^{n}$, une $k$-variété projective sur laquelle agit un groupe linéairement réductif $G$ et supposons que cette action soit induite par une action linéaire de $G$ sur $A^{n+1}$. Soit $\widehat{X}\subseteq A^{n+1}$, le cône affine au dessus de $\X$. Par un théorème de la théorie classique des invariants, il existe alors des invariants homogènes $f_{1},...,f_{r}\in C[\widehat{X}]^{G}$ tels que $$C[\widehat{X}]^{G}= C[f_{1},...,f_{r}].$$ On appellera le nilcone, que l'on notera $N$, la sous-variété de $\X$ définie par le locus des invariants $f_{1},...,f_{r}$. Soit $Proj(C[\widehat{X}]^{G})$, le spectre projectif de l'anneau des invariants. L'application rationnelle $$\pi:X\dashrightarrow Proj(C[f_{1},...,f_{r}])$$ induite par l'inclusion de $C[\widehat{X}]^{G}$ dans $C[\widehat{X}]$ est alors surjective, constante sur les orbites et sépare les orbites autant qu'il est possible de le faire; plus précisément, chaque fibre contient exactement une orbite fermée. Pour obtenir une application régulière satisfaisant les mêmes propriétés, il est nécessaire de jeter les points du nilcone. On obtient alors l'application quotient $$\pi:X\backslash N\rightarrow Proj(C[f_{1},...,f_{r}]).$$ Le critère de Hilbert-Mumford, dû à Hilbert et repris par Mumford près d'un demi-siècle plus tard, permet de décrire $N$ sans connaître les $f_{1},...,f_{r}$. Ce critère est d'autant plus utile que les générateurs de l'anneau des invariants ne sont connus que dans certains cas particuliers. Malgré les applications concrètes de ce théorème en géométrie algébrique classique, les démonstrations que l'on en trouve dans la littérature sont généralement données dans le cadre peu accessible des schémas. L'objectif de ce mémoire sera, entre autres, de donner une démonstration de ce critère en utilisant autant que possible les outils de la géométrie algébrique classique et de l'algèbre commutative. La version que nous démontrerons est un peu plus générale que la version originale de Hilbert \cite{hilbert} et se retrouve, par exemple, dans \cite{kempf}. Notre preuve est valide sur $C$ mais pourrait être généralisée à un corps $k$ de caractéristique nulle, pas nécessairement algébriquement clos. Dans la seconde partie de ce mémoire, nous étudierons la relation entre la construction précédente et celle obtenue en incluant les covariants en plus des invariants. Nous démontrerons dans ce cas un critère analogue au critère de Hilbert-Mumford (Théorème 6.3.2). C'est un théorème de Brion pour lequel nous donnerons une version un peu plus générale. Cette version, de même qu'une preuve simplifiée d'un théorème de Grosshans (Théorème 6.1.7), sont les éléments de ce mémoire que l'on ne retrouve pas dans la littérature.