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While the effect of long-chain fatty acids on adipose tissue (AT) lipogenic activities has been described in non-lactating ruminants (Vernon, 1977), little is known about their effects on the mammary gland and the AT in lactating animals. However, in cows in mid lactation, duodenal rapeseed oil infusion decreased the rate of fatty acid (FA) synthesis in AT and increased milk yield of long-chain FA (18[ratio ]1, 18[ratio ]2 and 18[ratio ]3) and decreased medium-chain FA (14[ratio ]0 and 16[ratio ]0), suggesting a depressive effect of fat feeding on mammary lipid synthesis de novo (Chilliard et al. 1991). On the other hand, in goat species, the addition of vegetable lipids to the diet led to an increase in the milk fat content and yield (Chilliard et al. 2003) suggesting that the possible negative effect of long-chain FA on FA synthesis in the lactating mammary gland could be more than compensated by increasing the supply of FA brought to the mammary gland for milk synthesis. Elsewhere, AT from various anatomical sites are characterized by different FA composition in goat (Bas et al. 1987) together with different patterns of lipogenic gene expression in sheep (Barber et al. 2000). These results suggest that each AT site is characterized by a specific metabolism. However, in lactating ruminants, few data are available on the extent of expression and regulation of genes coding for lipogenic enzymes in AT. Therefore, the current study was performed in three lipogenic tissues of lactating goats, namely the mammary gland, an internal AT site (perirenal AT) and an external AT site (subcutaneous AT).

The objectives of this study were to investigate different feeding strategies of high-yielding dairy cows during dry-off. With a 12- to 13-mo calving interval and increasing milk yield, metabolic and health problems during the dry-off period will increase. Twenty-two dairy primiparous and multiparous cows were randomly assigned to 2 feeding treatments. One group was fed straw ad libitum (straw), and the other group was fed silage (4 kg/d of dry matter) daily and straw ad libitum (silage). At the dry-off point (d 0), the cows had an average milk yield of 17.1 +/- 0.8 kg/d. All cows were milked in the morning on d 3 and 5 during the dry-off period. Rumen fluid was analyzed for volatile fatty acids (VFA), pH, NH3-N, and protozoa were counted from samples collected at d -3, 4, and 17. Total VFA concentration decreased at dry-off in both treatments and the drop was most pronounced among cows fed straw. Rumen pH increased significantly in both groups, and cows fed straw had significantly higher pH during dry-off. Ammonia N in rumen decreased significantly at dry-off and there was a tendency to lowered NH(3)-N in cows fed straw at dry-off. The plasma concentration of nonesterified fatty acids was markedly elevated during the dry-off period among cows in the straw treatment group, but was less pronounced among the cows fed silage with straw. The glucose level in plasma was not significantly affected during the dry-off period, and the insulin concentration was markedly reduced in both treatment groups. Plasma leptin concentration was lower in the lactating state than in the dry period. Both the beta-hydroxybutyrate and urea concentrations in plasma were significantly reduced during dry-off. Our results indicate that dry-off markedly affected the metabolism in the blood and in the rumen of the cows, and that the cows offered only straw during the dry-off were most affected.

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In order to study the effect of underfeeding and overfeeding on the fat deposition in the dromedary camel, 14 camels were divided into three groups: an overfed-underfed group (OV-UN), an underfed-overfed group (UN-OV) and a control group (CTRL). After a 4-wk adaptation, a cross-over design was applied for 16 weeks including two periods of 8 weeks each. The three energy levels in the diet corresponded to 17% (UN), 68% (CTRL) and 134% (OV) of the theoretical maintenance energy requirements. Body weight and barymetric parameters were measured weekly. Hump fat samples were collected monthly for determination of adipocyte size and lipid content. Overfeeding had or tended to have a significant effect on body weight, hump size, hump lipid content and adipocyte volume. Thus, the increase of the hump weight was 71% in the OV-UN group and 19% in the UN-OV group. Hump fat content increased from 52.9 to 63.6% in the OV-UN group and from 54.2 to 64.7% in the UN-OV group. Similarly, trends were observed for the adipocyte volume with an increase from 138 to 253 pL and from 275 to 346 pL in the OV-UN and UN-OV groups, respectively. Underfeeding had reverse effects: the hump weight decreased by 41% in the OV-UN group and 4% in the UN-OV group. Similarly, the hump lipid content decreased significantly in the OV-UN group from 63.6 to 53.0% and not significantly from 58.0 to 54.2% in the UN-OV group. Underfeeding decreased the adipocyte volume from 253 to 167 pL (OV-UN group) and from 292 to 275 pL (UN-OV group). A high significant positive correlation was observed between the hump lipid content and adipocyte volume. Low speed changes in the hump size, volume and lipid content could be linked to the camel adaptation to underfeeding conditions in dry areas., Effet du niveau d'alimentation sur le poids vif, la taille, la teneur en lipides et le volume des adipocytes de la bosse chez le dromadaire. Les effets de la suralimentation et de la sous-alimentation sur les réserves lipidiques chez le dromadaire ont été mesurés chez 14 chamelles divisées en trois lots : un lot suralimenté - sous-alimenté (OV-UN), un lot sous-alimenté - suralimenté (UN-OV) et un lot témoin (CTRL). Après 4 semaines d'adaptation un schéma expérimental croisé a été appliqué pendant 16 semaines comprenant 2 périodes de 8 semaines chacune. L'apport énergétique des 3 niveaux d'alimentation représentait 17, 68 et 134 % des besoins énergétiques théoriques d'entretien respectivement pour le régime sous-alimentation, témoin et suralimentation. Le poids vif et les paramètres barymétriques ont été mesurés chaque semaine. Les biopsies de la bosse pour la détermination de la taille des adipocytes et la teneur en lipides ont été réalisées mensuellement. La suralimentation a un effet significatif sur le poids vif, la taille de la bosse et sa teneur en lipides ainsi que sur le volume des adipocytes. Le poids de la bosse augmente de 71 % pour le lot OV-UN et de 19 % pour le lot UN-OV. La teneur en matière grasse augmente de 52,9 à 63,6 % pour le lot OV-UN et de 54,2 à 64,7 % pour le lot UN-OV. Des évolutions similaires sont observées pour le volume des adipocytes qui augmente de 138 à 253 pL et de 275 à 346 pL pour les lots OV-UN et UN-OV, respectivement. La sous-alimentation a un effet inverse avec une diminution du poids de la bosse de 41 % pour le lot OV-UN et de 4 % pour le lot UN-OV. La teneur en lipides de la bosse diminue également significativement de 63,6 à 53,0 % pour le lot OV-UN et de manière non significative de 58,0 à 54,2 % pour le lot UN-OV. De même, la sous-alimentation diminue significativement le volume des adipocytes de 253 à 167 et de 292 à 275 pL pour les lots OV-UN et UN-OV, respectivement. Une corrélation hautement significative est observée entre le volume des adipocytes de la bosse et sa teneur en lipides. La faible vitesse de variation de la taille de la bosse, de sa teneur en lipides et de la taille de ses adipocytes sont probablement des particularités liées aux mécanismes d'adaptation du dromadaire à la sous alimentation dans les zones arides.

International audience; Plasma leptin concentrations depend on energy intake and fat stores and are modified by hormones, such as glucocorticoids. We have measured plasma leptin concentrations in pre-term calves (born on day 277 of gestation) during the first week of life, in full-term calves (290 days of gestation), fed similar amounts of nutrients with colostrum or a milk-derived formula, combined with or without dexamethasone treatment (to simulate a high glucocorticoid status), during the first five days of life, and in calves fed with an automatic feeder or by bucket or suckling on dams up to day 28. Leptin concentrations increased (P < 0.05) from birth to day 7 in pre-term calves. In full-term calves leptin concentrations were stable from day 1 to day 4 in colostrum-fed animals, but decreased (P < 0.05) and were lower (P < 0.05) if fed a formula with similar amounts of energy and macronutrients as colostrum. Concentrations increased (P < 0.05) from day 1 to day 2 in calves suckling on dams and then remained elevated, but did not change and were lower in calves fed with the automatic feeder or by bucket than in suckling calves. Dexamethasone only slightly elevated leptin concentrations. There was no episodic secretion pattern, and there were no consistent associations of leptin with various metabolites and hormones. In conclusion, plasma leptin in young calves with respect to effects of nutrition (low energy intake) and hormones (glucocorticoids) and in association with metabolic changes behaved differently from what is known in mature cattle.

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Metabolic adaptations to variations in food supply are incompletely understood in ruminant animal adipose tissue (AT) and muscle. To explore this, we studied lipid metabolism and glucose transport potential in one internal and one external AT, as well as in one oxidative and one glycolytic muscle from control, 7 d underfed and 21 d refed adult cows. Refeeding increased (+79 to +307 %) the activities of enzymes involved inde novolipogenesis (fatty acid synthase, malic enzyme, glucose-6-phosphate dehydrogenase) in perirenal and subcutaneous AT; underfeeding did not modify these variables. Underfeeding decreased the activities of lipoprotein lipase (LPL) in perirenal AT (−70 %) and cardiac muscle (−67 %), but did not modify the activities in subcutaneous AT andlongissimus thoracis. Refeeding increased LPL activities in all tissues (+40 to +553 %) to levels comparable with (cardiac muscle) or greater than (AT,longissimus thoracis) those observed in control cows. Such variations in perirenal and cardiac muscle LPL activities did not result from variations in LPL mRNA levels, but suggest a post-transcriptional regulation of LPL in these nutritional conditions. Underfeeding did not modify GLUT4 contents in perirenal AT and muscles, while refeeding increased it only in perirenal AT (+250 %). Our present results contrast with previous results in rats, where LPL is regulated in opposite directions in AT and muscles, and GLUT4 is generally increased by fasting and decreased by refeeding in skeletal muscles. The present results highlight the bovine specificity of the response, which probably arises in part from peculiarities of ruminant animals for nutrient digestion and absorption.

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Dietary CLA isomers were shown to reduce adipose tissues in growing animals, mainly in mice, but their effects in adult animals remain unclear. This study was conducted to determine whether these effects depend on the isomer fed, on physical activity, or on the initial level of body fat. Male Wistar rats (4 mo old) were fed for 6 wk diets containing either no CLA, the cis-9, trans-11 CLA isomer (10 g/kg), the trans-10, cis-12 CLA isomer (10 g/kg), or both isomers (10 g/kg each). Half of the rats were assigned to exercise by treadmill running (1 h/d, 22 m/min). The initial body fat level was normal (12.7%) in a first trial, and high (18.9%) in a second trial. Chemical and anatomical body compositions were determined by chemical analysis and organ dissection. In both trials, the CLA diets, whatever the isomer, had no effect on food intake and body weight changes, on body chemical composition (fat, protein and water contents or gains), or on the body anatomical composition (weights or gains in epididymal and perirenal adipose tissues, in liver and in 4 muscles). There was no interaction between CLA treatment and physical activity. In conclusion, adult male rats do not appear to be responsive to the fat-to-lean partitioning effect of CLA described in growing rats. This was not affected by exercise or initial body fat level.

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Trans -18:1 and 18:2 isomer composition in ruminal fluid during the daily feeding cycle was examined in 3 cows fed a high concentrate diet (35:65) with 5% (DM basis) sunflower oil (SO), 5% linseed oil (LO), or 2.5% fish oil (FO) in a 3×3 Latin square with 3 4-wk periods. Grass hay and concentrate mixtures were fed at 0900, 1300, and 1700h daily. Ruminal fluid was collected at 0900, 1100, 1300, 1500, 1700, 2000, and 0000h. Feeding SO resulted in the greatest mean concentrations (% of total fatty acids) of trans 10, cis 12-18:2 and cis 9, trans 11-18:2. In particular, trans 10, cis 12-18:2 with SO was greater at 1500 (0.29%), 2000 (0.34%), and 0000h (0.25%) relative to 0900h (0.07%). Cis 9, trans 11-18:2 concentration increased from 0.47% at 0900h to a peak of 2.06% at 1100h; it remained greater than the percentage determined at 0900h at 1300 (1.4%) through 0000h (1.1%). Concentration of trans 11, cis 15-18:2 was greatest with LO, ranging from 3.3% (0900h) to a peak of 11.4% at 2000h. Mean trans 10-18:1 concentration ranked by diet was SO>FO>LO. Peak trans 10-18:1 with SO was observed at 1700h (14.9%) compared with 0900h (5.1%). Trans 11-18:1 did not differ with diet or time. Stearic acid decreased over time with all diets reaching minimum concentrations at 1700 to 2000h relative to 0900h. Feeding FO, however, decreased mean 18:0 concentration 4-fold compared with LO or SO. The moderate effect on concentration of trans -18:1 coupled with accumulation of 18:2 intermediates and the decrease of 18:0 over time suggest that oils reduced the biohydrogenation of 18:2 isomers to trans -18:1.

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International audience; The leptin receptor (LEPR) gene consists of 20 exons divided over 1.75 Mb. Parts of bovine LEPR exon 4 (79 bp), exon 11 (95 bp) and exon 20 (513 bp) of 20 cows (Holstein-Friesian) were sequenced (AJ580799; AJ580800; AJ580801) in an attempt to find polymorphisms. In exons 4 and 11 no SNPs were found. In exon 20, a T to C missense mutation was found at nucleotide 115, which causes an amino acid substitution at residue 945 (T945M). Frequencies for alleles C and T were 0.93 and 0.07 respectively, in a population of 323 Holstein-Friesian cows and TT animals were not detected. Using genotypes of these cows an association study was performed for leptin concentrations during late pregnancy and lactation. Leptin concentrations were determined by radioimmunoassay (RIA). The T945M mutation showed an association with circulating leptin concentrations only during late pregnancy (P < 0.05) but not during lactation (P > 0.05). The CC genotype had higher concentrations than the CT genotype during this period. A combined effect with previously described leptin polymorphisms on prepartum leptin concentrations was observed, with one genotype combination having significantly lower levels of leptin up to 50 days, but interaction effects were not significant. The T945M polymorphism may have induced a structural change in the intracellular domain of the LEPR, which may have influenced the signal transduction pathway. However, the effect was found only for the heterozygous genotype because the TT genotype was not detected in this population of 323 Holstein-Friesian cows.

The effect of linseed oil (LSO) supplementation on total-tract and ruminal nutrient digestibility, N metabolism, and ruminal fluid characteristics was investigated in dairy cows fed diets containing different forage to concentrate ratios (F:C). The experimental design was a 4 x 4 Latin square with 2 x 2 factorial arrangement of treatments. Four lactating Holstein cows were fed a forage-rich diet without LSO (F; F:C = 65:35, dry matter basis), a forage-rich diet with LSO (FO; F:C = 65:32, 3% LSO), a concentrate-rich diet without LSO (C; F:C = 35:65), or a concentrate-rich diet with LSO (CO; F:C = 35:62, 3% LSO). Total-tract digestibility of DM and OM was greater with supplemental LSO. A tendency for greater total-tract digestibility of NDF and ADF also was observed in cows fed LSO. Ruminal digestibility of NDF or ADF decreased when CO was fed compared with C. In contrast, feeding FO increased NDF or ADF digestibility compared with F. Although ruminal starch digestion was nearly complete with all diets, digestibility was greater when cows were fed C or CO compared with F or FO. Bacterial N flow to the duodenum decreased when FO was fed compared with F. In contrast, feeding CO increased bacterial-N flow compared with C. Neither F:C nor LSO supplementation affected ruminal pH or total VFA concentration in ruminal fluid. However, molar proportion of propionate was greater with C or CO compared with F or FO and increased with LSO supplementation regardless of F:C. Molar proportion of n-butyrate decreased with LSO supplementation. Total protozoal numbers in ruminal fluid decreased markedly only when CO was fed. Overall, data show that feeding LSO had no negative effects on total-tract digestion in dairy cows but may decrease ruminal fiber digestibility when fed with high-concentrate diets. The widely spread idea that LSO decreases digestibility, arising from studies with sheep, did not seem to apply to lactating cows fed 3% LSO.

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International audience; The respective effects and interactions of supplementation with two conjugated linoleic acid (CLA) isomers and exercise on plasma metabolic profile, activity of lipogenic enzymes and cellularity in two adipose tissue sites. those of the liver and heart were examined in adult Wistar rats. Rats that were either sedentary or exercise-trained by treadmill running were fed one of four diets: a diet without CLA. a diet with either 1% cis 9, trans 11 CLA or 1% trans 10, cis 12 CLA; or a mixture of both isomers (1% of each) for 6 weeks. We ob-served that the exercise decreased lipogenic enzyme activities in epididymal and perirenal adipose tissue. Plasma cholesterol. insulin, and leptin concentrations were lower in exercise-trained rats than in sedentary rats. The ingestion of either CLA mixture or the trans 10, cis 12 CLA increased lipogenic enzyme activities in epididymal tissue and more markedly in perirenal adipose tissue, especially in sedentary rats, and without affecting adipose tissue weight or cellularity. A similar effect of trans 10, cis 12 CLA was observed in regard to malic enzyme activity in the liver. In addition, this isomer decreased plasma lipid and urea concentrations and increased plasma 3-hydroxybutyrate levels. The ingestion of cis 9, trans 11 CLA increased fatty acid synthase activity in perirenal adipose tissue in sedentary rats and decreased plasm cholesterol and leptin concentrations. These results show that isomers of CLA decrease plasma lipids and stimulate adipose tissue lipogenesis without changing adipose weight in adult sedentary or exercise-trained rat, thus suggesting a stimulation of adipose tissue turnover.

Dans une représentation systémique du système d’élevage, l’éleveur incarne la composante décisionnelle du système, tandis que l’animal constitue, avec la ressource, sa composante biotechnique. L’animal lui-même peut être considéré comme un système biologique soumis à un environnement contraignant, dont les aptitudes à survivre, croître, se reproduire et s’adapter jouent un rôle fondamental dans la pérennité du système d’élevage. Les femelles reproductrices tiennent une place particulière dans cette représentation car non seulement elles déterminent une large part de la productivité du système par leur propre niveau de production (production laitière, productivité numérique), mais elles en assurent également sa reproductibilité au cours du temps (investissement reproductif).A l’échelle de l’individu et de sa durée de vie, les modalités d’adaptation reposent sur des processus de régulations comportementales et physiologiques. De tels processus ont été particulièrement mis en avant par les études portant sur l’aptitude des femelles de ruminants à s’adapter à la contrainte nutritionnelle(comportement alimentaire au pâturage et aptitude à constituer et mobiliser les réserves adipeuses). L’adaptation à la sous-nutrition revêt un intérêt particulier pour la pérennité des systèmes d’élevage en milieux difficiles dans la mesure où les processus de régulation du métabolisme énergétique interviennent sur la fonction de reproduction et sont par ailleurs susceptibles d’informer la femelle, à des moments-clés de son cycle de production, du risque associé à l’enclenchement d’une nouvelle gestation. Si les mécanismes digestifs, métaboliques et hormonaux impliqués dans l’adaptation à la sous-nutrition sont identiques chez les ruminants, les seuils de réponse varient selon le génotype (espèce, race), révélant ainsi des différences de potentiel adaptatif. Par ailleurs, si certains échecs biotechniques peuvent être imputés à une moindre efficacité de la réponse adaptative, d’autres peuvent en revanche se révéler cohérents du point de vue du fonctionnement biologique de l’organisme et de la gestion de priorités telles que la survie de l’individu ou l’investissement maternel. Ainsi, lorsqu’il s’agit d’évaluer l’aptitude des systèmes d’élevage des zones difficiles à être pérennes, parleur capacité à assurer un certain niveau de production, et/ou durables, par leur aptitude à répondre à des enjeux d’entretien du milieu et de conservation de la ressource, il convient en particulier d’avoir une bonne connaissance des potentiels adaptatifs des animaux, From a systemic point of view, the breeder can be considered as the decisional component of the livestock farming system, whereas animals are usually depicted to be part of the biotechnical component. The animal itself can be considered as an autonomous biological system subjected to environmental constraints, whose abilities to survive, to grow, to reproduce and, finally to cope with the environnement and the livestock practices, play a major role in the ability of the livestock system to sustain. In such a conceptual representation of the system, the reproductive females play a major role as they determine in a great part the productivity and the durability of the system through their abilities to maintain their own production level (milk production, numeric productivity) and to save their reproductive efficiency (repeated pregnancies and lactations) over years. Considering the animal level and its lifespan, it is clear that the capacities to adapt rely on behavioural and physiological regulatory processes. Such processes are particularly revealed along studies in which biological mechanisms are involved to face the nutritional constraint (grazing behaviour, body reserves’ dynamics). Focusing on the feeding constraint is quite interesting as regulatory processes implied in the energetic metabolism may interfere directly or indirectly with the reproductive function and thus, may play on the durability of the livestock system. A biological significance of such relationships between nutrition and reproduction is given that they allow the female to be informed about the associated risk of entering a productive process facing the uncertainty of the nutritional context. If the general mechanisms implied in the adaptability to cope with the underfeeding constraint are conserved in ruminants, the thresholds (or priorities) may largely differ according to the breed within the same species. Breeds are more or less able to cope with a new herd management or new nutritional conditions. In such a context, we point out that failures in reproduction, that might be interpreted by the breeder as an inability of the female to adapt, might have a sense on a biological point of view in regards to the adaptation to the nutritional risk. Hence, in order to evaluate the ability of the ruminants livestock farming systems to perpetuate in hard environments (maintaining their production levels) or to assess sustainable objectives (opening bushy landscapes by increasing grazing pressure), animals’ inherent adaptive potentialities have to be well known

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This study aimed to evaluate metabolic and endocrine adaptations to energy intake in multiparous Holstein cows (n = 90; mean 9434 kg energy-corrected milk yield/305 d) over the first 20 wk postpartum and to assess the association of leptin with metabolic, endocrine, and zootechnical traits. Concentrates were fed automatically for 24 h at 30% (C30) or 50% (C50) of total dry matter intake (DMI) from wk 1 to 10 postpartum and at linearly reduced amounts thereafter. Roughage was fed for ad libitum intake. The DMI was measured over 24 h; milk yield and body weight (BW), twice/d; milk composition, 4 times/wk; and milk acetone, weekly. Blood samples for determination of metabolite, hormone, and electrolyte concentrations and enzyme activities were obtained at wk 2 prepartum, and at wk 1 to 16 and at wk 20 postpartum from 0730 to 0900. Body condition scores (BCS) and backfat thickness were measured postpartum and during wk 1, 4, 8, 12, 16, and 20. Energy balance (EB) was considerably lower, but milk yield only slightly lower, in C30 than C50. Metabolic stress was more marked in C30 than C50, expressed by lower, glucose, insulin, insulin-like growth factor-1 (IGF-1), triiodothyronine, milk protein, and lactose concentrations, higher nonesterified fatty acid, β-hydroxybutyrate, growth hormone, and milk acetone concentrations, and an accelerated decrease in BCS and backfat thickness. Nevertheless, C30 adapted successfully and thus maintained high milk yields despite negative EB. Leptin concentrations were lower in C30 than in C50 over the first 20 wk postpartum and were positively associated with BCS, EB, BW, cholesterol, albumin, insulin, and IGF-1; negatively associated with DMI and triiodothyronine; and were higher in cows calving in spring than in fall. Leptin is one among several factors involved in the regulation of energy metabolism and may be important for overall homeostatic and homeorhetic control of metabolism and thus for maintenance of performance.

Relationship between voluntary food intake (VFI) and gene expression for appetite-regulating peptides was examined in the brains of Soay rams under contrasting photoperiods. Two groups ( n = 8) were subjected to alternating block long-day (LD) and short-day photoperiods (SD) over a period of 42 wk to entrain long-term cycles in VFI. Five animals from each group were killed 18 wk into LD or SD, and the brains were collected for in situ hybridization studies. VFI was fourfold higher under LD compared with SD. Body weight, abdominal fat, or plasma leptin levels were similar under LD and SD. LD animals were in positive energy balance and sexually inactive, and SD animals were in negative energy balance and sexually active. Neuropeptide Y (NPY) mRNA levels were higher in the arcuate nucleus (ARC) under LD, and pro-opiomelanocortin expression was lower under LD. Leptin receptor (Ob-Rb) was higher in the ARC under LD. We conclude that photoperiod-induced increase in VFI correlates with expression of NPY, but not with expression of genes for other putative orexigenic peptides. Ob-Rb gene expression is regulated by photoperiod.

International audience; Milk fat has a large effect on nutritional, technological and sensorial properties of milk products. The milk fat content and composition are modulated by genetics and nutritional factors and imply a large number of enzymes. The regulation of their gene expression in the mammary gland still needs to be clarified. An association between the extensive polymorphism at the $\alpha_{s1}$-casein ($\alpha_{s1}$-Cas) locus and both the lipid content and the characteristics of this fraction in caprine milk has been demonstrated. In order to decipher the mechanism responsible for this impact, a quantification of the transcripts of four lipogenic key enzymes (acetyl-CoA carboxylase, fatty acid synthase, lipoprotein lipase and stearoyl-CoA desaturase)was performed using real-time RT-PCR, suggesting an absence of association between the $\alpha_{s1}$-Cas genotype and expression variability of the studied genes. This approach has been completed by a more global analysis using a first generation of ruminant macroarray gathering 400 gene probes. The comparison of the expression profiles of lactating goat $\alpha_{s1}$-Cas A/A (strong allele) and F/F (defective allele) mammary gland allowed to confirm the expected variability in the expression of known genes (such as those encoding the $\alpha_{s1}$-casein) in ruminant mammary tissues as well as to identify up- and down-regulated genes. A second generation of ruminant cDNA macroarray extended to a few thousands of genes is currently in progress and will be applied to study different factors such as the nutritional regulation of gene expression in themammary gland.

International audience; Leptin is a hormone produced by adipocytes, and its expression is regulated by body fatness and energy balance. This study describes the association of four leptin gene polymorphisms in dairy cows (R4C, A59V, RFLP1, and BM1500) with circulating leptin concentrations during the periparturient period. A59V is located at a between-species conserved region of leptin, and R4C might have effect on the tertiary structure of the leptin protein because of the presence of an extra cystein. RFLP1 is an intronic SNP and BM1500 is a microsatellite located 3.6 kb downstream of the leptin locus. The four polymorphisms were genotyped in 323 HF heifers with known pedigree. Leptin concentrations were determined biweekly from 30 days before until 80 days after parturition. The effect of genotype on leptin concentrations was modeled by fitting a spline in ASREML describing leptin concentrations as a function of days relative to parturition for each genotype/allele. Surprisingly, associations were found during pregnancy, but not during lactation. This indicates that the polymorphism could be more effective during pregnancy. If further studies demonstrate that more leptin-binding protein (Ob-Re) is present in this stage, it is hypothesized that a structural difference in the leptin protein could cause a sub-optimal binding stringency to Ob-Re. Free leptin could be cleared faster than bound leptin, and this could result in lower leptin concentrations during pregnancy for the polymorphism. The effects found might be ascribed to R4C. However, more study on the Ob-Re receptor, like binding stringencies between R4C and wild-type leptin and glycosylation during pregnancy, would provide more insight in the results found.

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This study aimed to estimate individual and herd-level energy balance (EB) using blood and milk traits in 90 multiparous high-yielding Holstein cows, held on a research farm, from wk 1 to 10 postpartum (p.p.) and to investigate the precision of prediction with successively decreased data sets simulating smaller herd sizes and with pooled samples. Dry matter intake, milk yield, and BW were measured daily from parturition through wk 10 p.p. Milk composition was determined 4 times per week, and milk acetone was measured weekly. Blood samples for the determination of metabolites, hormones, electrolytes, and enzyme activities were taken weekly from wk 1 to 10 p.p. between 0730 and 0900. Body condition scores and ultrasonic measurements of backfat thickness and fat depth in the pelvic area were evaluated in wk 1, 4, and 8 p.p. Concentrations of glucose, cholesterol, urea, insulin, insulin-like growth factor-1, triiodothyronine, and thyroxine (T4) in blood plasma and of lactose and urea in milk were positively correlated with EB, whereas concentrations of nonesterified fatty acids (NEFA), creatinine, albumin, beta-hydroxybutyrate, and growth hormone and enzyme activities in blood, and concentrations of fat, protein, fat:lactose ratio, and acetone in milk were negatively correlated with EB. Leptin concentration was not correlated to EB over the first 10 wk p.p. To estimate EB linear mixed-effects, models were developed by backward selection procedures. The most informative traits for estimation of EB were the fat:lactose ratio in milk and NEFA and T4 concentrations in blood. The precision of estimation of EB in individual cows was low. Using blood in addition to milk traits did not result in higher precision of estimation of herd-level EB, and decreasing sample sizes considerably lowered the precision of EB prediction. Estimation of overall mean herd-level EB over the first 10 wk p.p. using pooled samples was precise even with small sample sizes, but does not consider the level of EB in particular weeks. In conclusion, estimation of herd-level EB at individual weeks using milk traits only has practical implication with herd sizes of > or = 100 cows if calving is highly seasonal and of or = 400 cows if calving is uniformly distributed. Using blood in addition to milk traits does not improve precision of estimation of herd-level EB, regardless of sample size.

Conversely to rodents, the involvement of hypothalamic neuropeptide Y (NPY) neurons in the control of nutrition is poorly understood in ruminants such as sheep. Therefore, the aim of this work was to describe the NPY neurons of the diencephalon in ewes submitted to different diets. In colchicine-treated animals, large populations of NPY-immunoreactive (-ir) neurons were observed in a ventral and a lateral subpopulation of the infundibular nucleus (IN), in the median eminence, the pituitary stalk, and the dorsomedian and dorsocaudal nuclei. No labeled perikaryon was observed in the magnocellular neurons of the hypothalamus, although numerous labeled fibers were noted in the neural part of the pituitary. The pattern of distribution of NPY-ir neurons in the sheep hypothalamus is similar in many ways to those of rodents, but it presents also many specific characteristics that have not been previously described. In ewes that were fasted for 24 hours, or fed ad libitum, the number of NPY-ir neurons was the same whatever the hypothalamic structures. In underfed ewes (40% of maintenance for 24 weeks), the lateral subpopulation of the IN presented a higher number of NPY-ir neurons than observed in the 100% fed ewes. Conversely, in the ventral subpopulation, the animals refed ad libitum (at least 150% of maintenance for 4 days) presented a lower number of NPY-ir neurons than the other groups. The other NPY neuronal populations of the hypothalamus were not significantly modified by the dietary treatments. For the first time, we demonstrated the presence of two functionally distinct subpopulations of NPY neurons in the sheep IN. The variations of labeled neurons were correlated with plasma nonesterified fatty acid levels but not with leptinemia.

We examined the ability of the ovine mammary gland to synthesize leptin throughout pregnancy and lactation. Leptin gene expression was assayed by real-time reverse transcription and polymerase chain reaction in mammary gland from ewes at 15, 80, 106, 112, or 141 d of pregnancy and at 0 (30 min after parturition), 3, 48, or 70 d of lactation. Leptin mRNA level was high at the beginning (the first 80 d) and at the end of pregnancy and was lower at mid-pregnancy and throughout lactation. Furthermore, during these periods of mammary leptin expression, the location of leptin protein, as determined by immunohistochemical analysis, changed within mammary tissue. It was located in adipose cells during early stages of pregnancy, in epithelial cells after full cell differentiation just before parturition, and in myoepithelial cells after parturition. These data, compared with published data on leptin receptor gene expression, provide evidence that leptin could be produced by different cell types of the mammary gland and could act as a paracrine factor on mammary cell growth and differentiation via adipose-epithelial cells and myoepithelial-epithelial cell interactions.

Effets de la traite une fois par jour combinee au niveau d'alimentation pendant les trois premieres semaines de la lactation sur la production et les activites enzymatiques du lait, et l'etat nutritionnel, chez des vaches Holstein. Vingt-quatre vaches multipares ont ete reparties des le velage en 4 lots, selon un schema factoriel 2 x 2: 2 frequences de traite (1 traite par jour pendant les 3 premieres semaines de lactation, puis 2 traites par jour vs. 2 traites par jour en permanence) x 2 concentrations energetiques de la ration (normale ou basse). L'essai a dure en moyenne 30 semaines et s'est termine au pâturage, le 28 mai 2000. Pendant la periode hivernale, les vaches etaient logees ensemble dans une stabulation libre a logettes et recevaient a volonte une ration complete. La traite omise etait celle du soir. Les vaches non traites ont emis significativement plus de vocalisations que les temoins autour de la traite omise, et elles ont ete plus nombreuses a perdre du lait. La traite 1 fois par jour a progressivement reduit la quantite de lait secretee par rapport a celle produite par les vaches traites 2 fois par jour, l'ecart atteignant 8,5 kgj -1 (P 0,10) pour l'ensemble de l'essai. La traite 1 fois par jour n'a pas affecte la teneur du lait en matieres grasses, en proteines (vraies), en albumine serique, et en lactose, ni le nombre de cellules somatiques ou l'activite de la plasmine et celle induite par l'activation du plasminogene. Elle a accru la teneur en immunoglobulines G1 (P = 0,02; mesure a la semaine 3) et a diminue l'activite de la lipoproteine lipase, ce qui n'avait pas encore ete rapporte. Elle a ameliore l'etat nutritionnel en debut de lactation (pertes de poids vif et d'etat corporel reduites, bilan energetique calcule accru) et n'a eu aucun effet sur la quantite de ration ingeree. La ration de concentration energetique normale a entraine, par rapport a celle de concentration energetique basse, les effets attendus: accroissement significatif des quantites ingerees (1,2 kg MSj -1 ), de la quantite de lait produite (3,1 kgj -1 ), de son taux proteique (2,0 gkg -1 ) et des index nutritionnels, notamment. La reduction de quantite de lait produite au cours des 3 premieres semaines de la lactation entrainee par la traite 1 fois par jour a ete de 5,5 kgj -1 et 8,5 kgj -1 respectivement pour les lots recevant les rations de concentration energetique faible ou normale (interaction non significative; P > 0,10).

This experiment investigates the changes in the plasma leptin levels of sheep fed a diet of three energy levels (low, moderate and high). Four mature wethers were used for this experiment. For the first 4 weeks, the sheep were fed diets to provide 1.2 times the maintenance metabolizable energy requirements, low energy levels (LE). During the second 4 weeks, the sheep were fed diets to provide 1.5 times the maintenance metabolizable energy requirements, moderate energy levels (ME). During third 4-week period, the sheep were fed a diet to provide 1.8 times the maintenance metabolizable energy requirements, high energy levels (HE). Body weight was determined every week. Blood samples were collected prior to the morning meal at 3 days intervals throughout the experiment, and plasma leptin, insulin and nonesterified fatty acid (NEFA) concentrations were assayed. Body weight decreased in week 1 after the start of the experiment, it continued to decrease during the LE feeding, but it gradually increased until the end of HE period. Similarly, plasma leptin concentration decreased during LE feeding, but increased during the HE feeding. Additionally, positive correlation was obtained between leptin and insulin or glucose concentrations, whereas no clear relationship with circulating NEFA was observed. In conclusion, it was suggested that plasma leptin concentrations were affected by the metabolizable energy in feed.

Conjugated linoleic acids (CLA) found in ruminant milk fat are a byproduct of incomplete biohydrogenation of lipids by ruminal bacteria. We examined the effect of different dietary fat supplements and processing methods on CLA. In trial 1, dietary supplements of Ca salts of fatty acids from canola oil, soybean oil, and linseed oil increased CLA content of milk fat by three- to fivefold over the control diet. Trials 2 and 3 examined the effect of processing methods for heat treatment of full fat soybeans. In trial 2, extrusion, micronizing, and roasting resulted in two- to threefold greater concentrations of CLA in milk fat than the control diet (raw ground soybeans). In trial 3, different temperatures of extrusion (120, 130, and 140 degrees C) increased the CLA content of milk fat to a similar extent; CLA averaged 19.9 mg/g of fatty acids for the extrusion treatments compared with 4.2 mg/g of fatty acids for the control diet (raw ground soybeans). Fish oil (200 and 400 ml/d) was examined in trial 4 and both levels resulted in CLA concentrations in milk fat that were about threefold greater than the control diet. In trial 5, grain and silage from a high oil corn hybrid increased the CLA content of milk fat; however, responses were modest with the CLA concentration (mg/g of fatty acids) averaging 4.6 and 2.8 for diets with high oil hybrid and normal hybrid, respectively. Similarly, dietary supplements of animal fat byproducts (tallow plus yellow grease; trial 6) resulted in modest increases in the CLA content of milk fat. Overall, several dietary manipulations involving lipid sources and processing methods were identified that allow for a marked increase in the conjugated linoleic acid content of milk fat.

International audience; A 5-kb long transcript encoding the stearoyl-CoA desaturase (SCD) was identified by Northern-blot analysis of poly(A)+ mRNA from caprine lactating mammary gland. Complete sequencing of the SCD cDNA (5123 bp) revealed that the coding region (1080 nt) is followed by an unusually long (3.8 kb) 3'-UTR sequence, deriving from a single exon, in which a polymorphism, due to the deletion of a nucleotide triplet, was detected. The complete structural organization of the relevant gene has been determined. The transcription unit was shown to span a 15-kb region and to consist in six exons varying in size from 131 (3rd exon) to 4047 bp (6th exon), and five introns varying in size from 600 to 3700 bp. Using the fluorescence in situ hybridization technique, the SCD gene was localized to bovine and caprine chromosomes 26q21, and ovine chromosome 22q21.

Après un rappel des voies métaboliques et des flux de nutriments qui concourent à la lipogenèse mammaire, cet article est consacré aux principaux effets de l’alimentation sur la composition en acides gras (AG) du lait de ruminant, en particulier les AG polyinsaturés, les AG monoinsaturés trans et l’acide linoléique conjugué (CLA). Les principaux facteurs alimentaires étudiés sont la nature des fourrages (dont l’herbe pâturée), le suif, protégé ou non, et la supplémentation des rations avec des huiles végétales ou marines (poisson ou algues), protégées ou non. La supplémentation en suif augmente la sécrétion d’acide oléique. La teneur du lait en C18:2 est comprise entre 2 et 3 % avec les rations non supplémentées en lipides, elle n’est augmentée que de 1,5 point lors d’un apport de graines ou d’huiles riches en C18:2, en raison d’une hydrogénation ruminale poussée. Les rations à base d’herbe (riches en C18:3) augmentent légèrement (0,5 à 1 point) la proportion de C18:3 dans le lait par rapport aux rations à base d’ensilage de maïs. La graine de lin permet un accroissement d’environ 0,3 point. La teneur en C20:5 (EPA) et en C22:6 (DHA) est accrue jusqu’à 0,5 % des AG totaux par l’addition d’huiles marines à la ration. Le taux butyreux du lait peut être fortement diminué par les régimes pauvres en fibres et riches en céréales et/ou par l’administration d’huiles marines ou végétales riches en AG insaturés. Les C18:1 trans, notamment les C18:1 trans 11 et trans 10, mais aussi le trans 10, cis 12 CLA, pourraient jouer un rôle dans cette diminution en inhibant la lipogenèse mammaire. Les régimes alimentaires augmentant les proportions des CLA et des C18:1 trans dans le lait sont ceux qui apportent des précurseurs lipidiques pour la formation de ces AG, ou qui modifient l’activité microbienne associée à l’hydrogénation ruminale des AG polyinsaturés ou l’activité de la désaturase mammaire. L’influence de l’alimentation sur les différents isomères du C18:1 et du CLA du lait a été peu étudiée., After a survey of metabolic pathways and nutrient fluxes involved in mammary lipogenesis, this review summarises the known effects of diet on ruminant milk fatty acid (FA) composition, and particularly polyunsaturated FA, trans monounsaturated FA and conjugated linoleic acid (CIA). The main dietary factors which were studied are the nature of forages (including pasture), and the supplementation of dairy rations with protected or unprotected tallow, and protected or unprotected vegetable or marine oils (fish or algae). Tallow supplementation increases the secretion of oleic acid. Milk fat percentage of linoleic acid varies from 2 to 3% in basal diets. It is increased by only 1.5 point when the diet is supplemented with oils or seeds rich in C18:2, because of an extensive ruminal biohydrogenation. Milk fat from grass diets (rich in C18:3) is higher (0.5 to 1 point) in linolenic acid than milk fat from maize silage diets. Linseed increases the milk fat percentage of linolenic acid by 0.3 point. Milk fat percentage of C20:5 (EPA) and C22:6 (DHA) is enhanced up to 0.5 % of total FA by the supply in the diet of marine oils. Milk fat content can be greatly decreased by low fibre, high grain diets and/or by feeding unprotected marine oils or vegetable oils rich in unsaturated FA. A role of trans-C18A isomers, including trans-11 and trans-10 C18:1, but also of trans 10, cis 12 CLA, in the inhibition of mammary gland lipogenesis is suggested. Diets which increase milk trans C18:1 or CLA percentage in milk fat are those which provide lipid substrates for the formation of CLA or trans C18:1 in the rumen, or those which change the microbial activity associated with ruminal biohydrogenation of polyunsaturated FA, or those which change the mammary desaturase activity. The influence of dietary factors on the different C18:1 and CLA isomers in milk fat is not well known.

Changes in the amount and metabolism of adipose tissue (AT) occur in underfed ruminants, and are amplified during lactation, or in fat animals. The fat depot of the tail of some ovine breeds seems to play a particular role in adaptation to undernutrition; this role could be linked to its smaller adipocytes and high sensitivity to the lipolytic effect of catecholamines. Glucocorticoids and growth hormone probably interact to induce teleophoretic changes in the AT responses to adenosine and catecholamines during lactation. Fat mobilization in dry ewes is related both to body fatness and to energy balance. Thein vivoβ-adrenergic lipolytic potential is primarily related to energy balance, whereas basal postprandial plasma non-esterified fatty acids (NEFA) are related to body fatness, and preprandial plasma NEFA is the best predictor of the actual body lipid loss. Several mechanisms seem to be aimed at avoiding excessive fat mobilization and/or insuring a return to the body fatness homeostatic set point. As well as providing the underfed animal with fatty acids as oxidative fuels, AT acts as an endocrine gland. The yield of leptin by ruminant AT is positively related to body fatness, decreased by underfeeding, β-adrenergic stimulation and short day length, and increased by insulin and glucocorticoids. This finding suggests that the leptin chronic (or acute) decrease in lean (or underfed respectively) ruminants is, as in rodents, a signal for endocrine, metabolic and behavioural adaptations aimed at restoring homeostasis.

National audience; Although the effect of lactation stage is similar, the responses of milk yield and composition (fat and protein contents) to different types of lipid supplements differ greatly. between goats and cows. Milk fat content increases with almost all studied fat supplements in goats but not in cows. However, the response of milk fatty acid (FA) composition is similar, at least for major FA, including conjugated linoleic acid (CLA) in goats and cows supplemented with either protected or unprotected lipid supplements. Goat milk CLA content increases sharply after either vegetable oil supplementation or fresh grass feeding, but does not change markedly when goats receive whole untreated oilseeds. Important interactions are observed between the nature of forages and of oil supplements on trans-10 and trans-11 C18:1 and CLA. Peculiarities of goat milk FA composition and lipolytic system play an important role in the development of either goat flavor (release of branched, medium-chain FA) or rancidity (excessive release of butyric acid). The lipoprotein lipase (LPL) activity, although lower in goat than in cow milk, is more bound to the fat globules and better correlated to spontaneous lipolysis in goat milk. The regulation of spontaneous lipolysis differs Widely between goats and cows. Goat milk lipolysis and LPL activity vary considerably and in parallel across goat breeds or genotypes, and are low during early and late lactation, as well as when animals are underfed or receive a diet supplemented with protected or unprotected vegetable oils. This could contribute. to decreases in the specific flavor of goat dairy products with diets rich in fat.

A specific leptin RIA was developed to assess concentrations of leptin in ovine plasma, and was shown to be efficient with bovine and caprine plasma. A specific, high-affinity antibody was generated against recombinant ovine leptin which, when used in a competitive leptin RIA, provided valid estimates of linearity (r=+0.989-0.998), recovery (102%), repeatability (13%) and limit of sensitivity (0.83 ng/ml for 100 microl sample size). Serial dilutions of five ovine, bovine or caprine plasma samples showed good linearity and parallelism with the recombinant ovine leptin standard curve. A comparison of this RIA was made with a commercial 'multi-species' RIA kit using 56 ovine plasma samples. Major differences were found in assay sensitivity. Non-lactating, non-pregnant, ovariectomized ewes were fed a ration for 65 days which provided 90+/-9% (control; n=12) or 39+/-2% of maintenance energy requirements (underfed; n=16) in order to analyse the respective effects of body fatness (estimated by either an in vivo dilution technique or body condition scoring) and of nutritional status on plasma leptin concentration. There was a significant positive correlation between body fatness or body condition score and plasma leptin levels (r=+0.68, P

International audience; After a brief survey of metabolic pathways and nutrient fluxes involved in mammary lipogenesis, this review summarises the known effects of diet on ruminant milk fat composition. Special attention is given to fatty acids that could play a positive role for human health, such as butyric acid, oleic acid, C18 to C22 polyunsaturated fatty acids and conjugated linoleic acid (CLA). The efficiency of the transfer of C18:2, C18:3, C20:5, C22:5 and C22:6, from the duodenum to the milk, is reviewed. The main dietary factors taken into account are the nature of forages, including pasture, and the supplementation of dairy rations with protected or unprotected vegetable or fish oils. Dose-response curves of milk CLA are reviewed for different fat supplements, as well as the non-linear relationship between milk CLA and trans C18:1. The potential of dietary factors to increase the mean CLA content in cow milk fat is about 300% above basal values. There is, however, a need to evaluate how the different feeding strategies could change the other aspects of milk fat quality.; Plasticité de la matière grasse du lait de ruminant : contrôle nutritionnel des acides gras saturés, polyinsaturés, trans et conjugués. Après un bref rappel des voies métaboliques et des flux de nutriments qui concourrent à la lipogenèse mammaire, cette revue est consacrée aux principaux effets des facteurs alimentaires sur la composition des lipides du lait de ruminant. Un intérêt particulier est porté aux acides gras qui peuvent avoir des effets positifs sur la santé humaine, tels que les acides butyrique, oléique, linoléique conjugué (CLA) et les acides gras polyinsaturés, de 18 à 22 atomes de carbone. L'efficacité du transfert des C18:2, C18:3, C20:5, C22:5 et C22:6, du duodénum au lait est estimée à partir des données de la bibliographie. Les principaux facteurs alimentaires considérés sont la nature des fourrages, dont l'herbe pâturée, et la supplémentation des rations pour vaches laitières avec des huiles végétales ou de poisson, protégées ou non. L'augmentation potentielle de la teneur en CLA de la matière grasse du lait de vache par les facteurs alimentaires est d'environ 300 % au-dessus du niveau de base. Des courbes dose-réponse du CLA du lait sont établies à partir des différentes publications sur les effets de suppléments lipidiques, ainsi que la relation non linéaire qui existe entre le CLA et le C18:1 trans du lait. Il reste toutefois à évaluer les conséquences des différentes stratégies alimentaires sur les autres aspects de la qualité du lait.

The aim of this study was to analyse the effects of the intravenous infusion of a non-selective beta-agonist (beta-A) (isoproterenol, ISO), and selective beta1-A (dobutamine, D) or beta2-A (terbutaline, T) on body fat mobilisation in non-pregnant heifers, in a 2 x 2 crossover design with two treatments (underfeeding or overfeeding) and two periods. The effect of the duration of submission to each energy level on basal and ISO-induced fat mobilisation was also studied. ISO had a high and significant lipolytic effect whatever the energy level. Nevertheless, the response area of non-esterified fatty acids (NEFA) to ISO in underfed cows was 1.7 times greater than that in overfed cows. T had a slight but significant lipolytic effect on NEFA plasma response in the underfed group. D had no lipolytic effect. Basal and ISO-stimulated plasma NEFA levels were similar after 1 or 4 weeks of underfeeding.

Three methods for estimating in vivo body fat in sheep, by the assessment of body water using the deuterium oxide (Ddilution technique, adipose cell size (ACS) measurement and body condition scoring (BCS), were studied in mature ewes and compared to results obtained after slaughter and chemical analyses. Twenty Lacaune dairy ewes were slaughtered at three stages of lactation (30 days (n = 8), 60 days (n = 8) and 110 days (n = 4)), with a mean body weight (BW) of 72.7 ± 7.6 kg, 16.0 t 4.7 kg of body lipids (LIP) and 42.7 ± 3.8 kg of total body water. Body fatness ranged from 10.3 to 28.8 % of empty body weight. Although the ewes were relatively fat, the mean adipose cell volume of subcutaneous pericaudal or sternal adipose tissue (209 or 287 pl, respec- tively) was in the lower range of previously reported values, while cells from omental adipose tissue had a larger mean volume (804 pl) than has been previously reported. Due to differences in the frame of ewes, prediction of body lipids through D,O, ACS and BCS was improved by including body weight in the model. The best equations using ACS were obtained with subcutaneous fat tissues (sternal tissue: residual coefficient of variation (rCV) = 13.9 % and pericaudal: rCV = 17.8 %), fol- lowed by internal tissue (omental: rCV = 19.5 %). The rCVs of the prediction of body lipids with these in vivo methods were in general agreement with published works on cattle or sheep i.e. D 8.8 %, pericaudal ACS: 13.9 % and BCS: 16.7 %. The interest and limitations of the methods studied are dis- cussed in terms of different experimental objectives and constraints. (© Elsevier / Inra)

Les effets respectifs de l’état nutritionnel et de la photopériode sur le métabolisme lipidique et le partage de nutriments entre le tissu adipeux et le muscle sont récapitulés chez le ruminant, en particulier pour ce qui concerne l’expression des gènes spécifiant les enzymes lipogéniques et la lipoprotéine-lipase chez la vache et la brebis. La restriction alimentaire diminue la synthèse de novo d’acides gras et l’activité lipoprotéine-lipasique dans les tissus adipeux des ruminants adultes ou en croissance. Ces effets sont partiellement ou totalement inversés par la réalimentation. Chez les ovins et les bovins adultes, l’activité lipoprotéine-lipasique est diminuée par la restriction dans le muscle cardiaque, oxydatif, inchangée dans le muscle Longissimus thoracis, glycolytique, et augmentée par la réalimentation dans ces deux muscles. Les activités lipoprotéine-lipasiques du tissu adipeux et du muscle de ruminant répondent donc dans le même sens à des variations de l’état nutritionnel, contrairement aux données généralement publiées chez le rat. De plus, les taux d’ARNm de la lipoprotéine-lipase dans le tissu adipeux et le muscle varient de manières identiques à son activité chez les différentes espèces, ce qui suggère une régulation, au moins en partie, pré-traductionnelle de l’expression du gène spécifiant cette enzyme. La photopériode exerce une influence directe sur l’activité métabolique des tissus adipeux et musculaires de brebis. Ainsi, les capacités lipogéniques du tissu adipeux et l’activité lipoprotéine-lipasique du muscle glycolytique sont augmentées par les jours longs. Inversement, la lipomobilisation est augmentée par les jours courts. Ces adaptations métaboliques sont probablement impliquées dans les adaptations physiologiques des ovins aux variations saisonnières des disponibilités alimentaires., The respective effects of feeding level and photoperiod variations on lipid metabolism and nutrient partitioning between ruminant adipose tissue and muscle are reviewed, with a particular interest in lipogenic cow and the ewe. Underfeeding decreases de novo fatty acid synthesis and lipoprotein lipase activity in adult and growing ruminant adipose tissues, Refeeding partially or totally reverses these effects, The lipoprotein lipase activity is not modified bg underfeeding in the glycolytic Longissimus thoracis muscle, but is decreased in the oxidative cardiac muscle of adult ovine tmd bovine animals. Refeeding increases lipoprotein lipase activity in these two muscle types. Feeding level alters lipoprotein lipase in the same way in adipose tissues and muscles of ruminants, contrary to observations usually made in the rat. However, variations in lipoprotein lipase mRNA level in adipose tissue and muscle are parallel to those of lipoprotein lipase activity in the different animal species. These results at least partly suggest a pretranslational regulation in the expression of the lipoprotein lipase gene. Photoperiod directly affects adipose tissue and muscle metabolic activities in ewes. Indeed, Lipogenic capacities in adipose tissue and lipoprotein lipase activity in glycolytic muscle are increased by long days. Inversely, lipomobilisation is increased by short days. These metabolic adaptations are probably involved in physiological adaptations of ewes to seasonal changes in food availability.

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Leptin is secreted by adipose tissue and plays a pivotal role in regulating both body energy homeostasis and reproductive function in rodents. Among livestock, sheep is a seasonal breeder whose reproductive period is initiated by short daylength. We show that plasma leptin and leptin gene expression in perirenal adipose tissue were decreased when ovariectomized Lacaune ewes were exposed to short days (8 versus 16 h light/d). This effect of the photoperiod occurred despite the nutritional status, with leptin levels lower in underfed than in refed ewes, and without significant changes in perirenal fat mass and adipocyte size. Plasma prolactin and leptin followed a similar pattern suggesting a relationship between the two hormones. These findings indicate, for the first time, that adipose tissue leptin is modulated by daylength independently of food intake, body fatness and gonadal activity. Furthermore, plasma non-esterified fatty acids of underfed ewes subjected to short days were more elevated than for underfed ewes on long days. On the other hand, refed ewes placed under long daylength tended to have a higher adipose tissue lipogenic activity than refed ewes on short days. We propose that these adaptations of leptin production and lipogenic activity with long photoperiod are of physiological significance for body fat deposition, which naturally occurs during long days when food is abundant. Conversely, a low leptin level during short days may enhance the sensitivity to food deprivation during the natural reproductive season, where any food shortage would decrease plasma leptin under a threshold critical for reproduction. Furthermore, in this situation, the observed enhanced ability to mobilize body fat may be related to the necessity to cope with energy shortage.

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In response to undernutrition, short- (days) and medium-term (weeks) adaptations are more pronounced for splanchnic organs than for other tissues. For the latter, the long-term response involves a sequential mobilization (fat > muscle > bone) with relative priorities differing among anatomical sites. Among chemical components, the body lipids are extensively used (up to 80%) in underfed animals, while the range of protein utilization is limited (up to 15-20%). The decrease in energy expenditure during undernutrition is mostly due to a short- and medium-term decrease in feeding activity, and in the mass and activity of splanchnic tissues. In the long-term, energy expenditure and tissue masses decrease concomitantly, but there does not appear to be a significant decrease in expenditure per unit tissue weight. Nitrogen losses decrease in response to decline in nitrogen fluxes and also due to sparing by renal activities and sometimes by urea recycling. However, ruminants do not seem to be able to compensate for a low level of intake (below maintenance) by an increase in digestive efficiency. Numerous hormones (insulin, growth hormone, insulin-like growth factor I, catecholamines, thyroid hormones, cortisol, leptin, etc.) are involved in the changes during undernutrition of nutrient fluxes between tissues, either through control of synthesis and/or degradation in peripheral tissues (adipose tissue and muscle), or through hepatic conversions of substrates (gluconeogenesis, ureagenesis and ketogenesis), in order to maintain the constancy of the internal environment (homeostasis) and/or to sustain productive functions (teleophoresis). However, reproductive process may be blocked in underfed animals.