Review: the cellular mechanisms underlying mammary tissue plasticity during lactation in ruminants.

The mammary tissue is characterized by its capacity to adapt in response to a wide variety of changing conditions. This adaptation capacity is referred to as the plasticity of mammary tissue. In dairy ruminants, lactation is challenged by modifications that can either be induced on purpose, such as by modifying management practices, or occur involuntarily, when adverse environmental constraints arise. These modifications can elicit both immediate changes in milk yield and composition and carryover effects that persist after the end of the challenge. This review focuses on the current knowledge concerning the cellular mechanisms underlying mammary tissue plasticity. The main mechanisms contributing to this phenomenon are changes in the activity and number of mammary epithelial cells (MECs). Changes in the number of these cells result from variations in the rates of cell proliferation and death as well as changes in the rate MEC exfoliation. The number of MECs also depends on the number of resident adult mammary stem cells and their progenitors, which can regenerate the pools of the various mammary cells. Several challenges, including changes in milking frequency, changes in level of feed supply and hormonal manipulations, have been shown to modulate milk yield together with changes in mammary cell activity, turnover and exfoliation. Epigenetic changes may be an additional mechanism of adaptation. Indeed, changes in DNA methylation and reductions in milk yield have been observed during once-daily milking and during mastitis in dairy cows and may affect cell activity persistently. In contrast to what has been assumed for a long time, no carryover effect on milk yield were observed after feed supply challenges in dairy cows and modification of milking frequency in dairy goats, even though the number of mammary cells was affected. In addition, mammary tissue plasticity has been shown to be influenced by the stage of lactation, health status and genetic factors. In conclusion, the cellular mechanisms underlying mammary tissue plasticity are diverse, and the mammary tissue either does or does not show elastic properties (with no permanent deformation), in response to environmental changes.

Response profiles of dairy cows to a single 24-h milking interval in relation with milk proteolysis, udder expansion and immune traits.

An extended milking interval of 24 h (24-h milking interval (24h-MI)) constitutes the acute phase of cow adaptation to once-daily milking (ODM). A recent trial including 724 24h-MI challenges demonstrated that milk yield responses to this acute phase of ODM are highly variable (from+22% to -52% of milk yield when switching to the 24h-MI, mean=-25.3%) and that factors such as stage of lactation parity and milk yield level influenced cows' responses but did not account for all individual variability. Additional traits related to physiological, immune and behavioural adaptation were measured on a subset (96 observations) of this data set. This study aimed to determine (1) the relationship of these traits with cows' milk yield responses, (2) their ability - combined with previously identified traits - to help predict milk yield responses to 24h-MI (adaptive profiles). The 24h-MI challenge consisted of three successive periods: one control week of twice-daily milking (cTDM), one single day of 24h-MI and then 13 days of TDM (pTDM). Milk yield responses to the 24h-MI (corrected for effects of stage of lactation, parity, milk yield level and milk yield) were related to physiological traits measured during cTDM (milk flow rate, presence or absence of interleukin-8) and to their changes during the 24h-MI (absolute increase in milk flow rate and relative udder distension). Analysis of associations between milk yield responses, stage of lactation, parity, milk yield level, proteolysis, udder expansion and immune traits found three adaptive cow profile clusters. Cows in cluster 1 had a less compliant udder than cows in cluster 2, and they lost more milk during the 24h-MI than cluster-2 and cluster-3 cows. After resuming twice daily-milking (TDM), cluster-2 cows fully recovered the milk they had lost during the 24h-MI. On the opposite, cluster-3 cows did not recover the milk they lost, likely due to udder inflammation during cTDM, as suggested by elevated concentrations of interleukin-8 in their milk. These results combining new traits with stage of lactation, parity and milk yield level constitute a first step towards predicting individual cow responses to a 24h-MI.

Genetic parameters of milk production traits in response to a short once-daily milking period in crossbred Holstein × Normande dairy cows.

Despite its potential utility for predicting cows' milk yield responses to once-daily milking (ODM), the genetic basis of cow milk trait responses to ODM has been scarcely if ever described in the literature, especially for short ODM periods. This study set out to (1) estimate the genetic determinism of milk yield and composition during a 3-wk ODM period, (2) estimate the genetic determinism of milk yield responses (i.e., milk yield loss upon switching cows to ODM and milk yield recovery upon switching them back to twice-daily milking; TDM), and (3) seek predictors of milk yield responses to ODM, in particular using the first day of ODM. Our trial used 430 crossbred Holstein × Normande cows and comprised 3 successive periods: 1 wk of TDM (control), 3 wk of ODM, and 2 wk of TDM. Implementing ODM for 3 wk reduced milk yield by 27.5% on average, and after resuming TDM cows recovered on average 57% of the milk lost. Heritability estimates in the TDM control period and 3-wk ODM period were, respectively, 0.41 and 0.35 for milk yield, 0.66 and 0.61 for milk fat content, 0.60 and 0.80 for milk protein content, 0.66 and 0.36 for milk lactose content, and 0.20 and 0.15 for milk somatic cell score content. Milk yield and composition during 3-wk ODM and TDM periods were genetically close (within-trait genetic correlations between experimental periods all exceeding 0.80) but were genetically closer within the same milking frequency. Heritabilities of milk yield loss observed upon switching cows to ODM (0.39 and 0.34 for milk yield loss in kg/d and %, respectively) were moderate and similar to milk yield heritabilities. Milk yield recovery (kg/d) upon resuming TDM was a trait of high heritability (0.63). Because they are easy to measure, TDM milk yield and composition and milk yield responses on the first day of ODM were investigated as predictors of milk yield responses to a 3-wk ODM to easily detect animals that are well adapted to ODM. Twice-daily milking milk yield and composition were found to be partly genetically correlated with milk yield responses but not closely enough for practical application. With genetic correlations of 0.98 and 0.96 with 3-wk ODM milk yield losses (in kg/d and %, respectively), milk yield losses on the first day of ODM proved to be more accurate in predicting milk yield responses on longer term ODM than TDM milk yield.

Effects of DGAT1 K232A polymorphism and milking frequency on milk composition and spontaneous lipolysis in dairy cows.

Milk spontaneous lipolysis (SL) of milk triglycerides is induced by the lipoprotein lipase, a milk native enzyme, and leads to an accumulation of partial glycerides and free fatty acids that are responsible for the deterioration of the taste of milk products. The gene coding for diacylglycerol acyltransferase 1 (DGAT1), an enzyme implicated in triglycerides synthesis, has an important polymorphic site at the K232A locus. This gene is well known to modulate milk composition. No data are available on the effects of DGAT1 on SL. Thus, a trial was carried out to evaluate the effects of DGAT1 K232A polymorphism on milk SL upon milking frequency variations [once- (ODM) and twice-daily milking (TDM)]. Twenty-one cows were divided into 3 groups according their DGAT1 K232A genotype: 8 cows had the KK genotype of DGAT1 (KK cows), 8 had the KA genotype (KA cows), and 5 had the AA genotype (AA cows). The trial consisted in 3 successive periods: 3 wk of TDM, 3 of ODM, and 4 of TDM. Samples were collected for fat and protein contents, SL, fatty acid, and protein profiles determinations. The KK cows presented higher fat and protein contents, lower milk production, and higher κ-casein percentage. No significant difference in fatty acid composition was noted between groups. The SL was twice as high for KK cows in TDM situations (1.13 vs. 0.59 and 0.63mEq/100g of fat, respectively, for KK, KA, and AA cows during the first period of TDM, and 0.46 vs. 0.25 and 0.21mEq/100g of fat, respectively, for KK, KA, and AA during the second period of TDM). The SL remained lower in TDM2 than in TDM1. During ODM, no difference in SL was found between groups and SL remained below 0.2mEq/100g of fat. These results demonstrate the existence of a correlation between DGAT1 genotypes and spontaneous lipolysis, in interaction with an environmental factor, milking frequency, although it has not been possible to clarify the causal mechanism at this stage.

Individual responses of dairy cows to a 24-hour milking interval.

Some dairy farmers opt to omit one milking, either incidentally or weekly, without changing other milking times. This practice entails an extended milking interval of 24h (24h-MI), which is associated with a decrease in milk yield. This decrease varies among cows and could be partly due to factors such as stage of lactation and milk yield level. The aim of this study was to describe the average and individual responses in terms of loss and carryover effects of a 24h-MI on milk yield. The influence of factors such as parity, stage of lactation, and milk yield potential were investigated, together with response repeatability. Our trial used 292 Holstein-Friesian cows, and consisted of 3 successive periods: 1 wk of twice-daily milking (TDM) as a control, one 24h-MI, and then 13d of TDM. The number of observations per cow ranged from 1 to 9, with no more than three 24h-MI per lactation. The 24h-MI reduced milk yield by 23% (7.8 kg on average) and milk lactose content by 2.6g/kg on the 24h-MI day. Milk fat and protein content, and somatic cell score increased by 3.0 g/kg, 0.5 g/kg, and 0.4 units, respectively. No significant carryover effect was found of a 24h-MI on milk yield or milk composition 2 wk after resumption of TDM. Milk yield loss and recovery varied widely (coefficient of variation 62%), and the relationship between milk loss and milk recovery showed substantial variation (residual standard deviation 2.1 kg/d). Cows with a greater milk potential level lost more milk yield but recovered more milk, with no influence on recovery:loss ratio. Cows in early lactation recovered the lost milk yield faster. Repeatability of the responses to a 24h-MI was 44% for milk yield loss (kg/d), 57% for relative milk yield loss (%), 33% for milk yield recovery (kg/d), and 0% for milk recovery:loss ratio (%), suggesting a genetically determined ability to limit loss when one milking is omitted. To conclude, a 24h-MI caused higher milk yield losses than reported in previous studies. Stage of lactation, estimated potential milk yield level, and parity explained the cows' response to the 24h-MI, but did not account for all the individual variability.

Effects of oligomycins on adenosine triphosphatase activity of mitochondria isolated from the yeasts Saccharomyces cerevisiae and Schwanniomyces castellii.

Functional mitochondria with respiratory control were isolated from the yeasts Saccharomyces cerevisiae and Schwanniomyces castellii. The presence of site I in Schw. castellii was indicated by higher ADP/O ratio than in S. cerevisiae where this site is absent. The ATPase Vmax was higher in S. cerevisiae than in Schw. castellii mitochondria. The latter was increased by the DR12 nuclear mutation. Nevertheless, the stimulation by heat and the inhibition profile of oligomycins on mitochondrial F1-F0 ATPase activities were similar in all three tested strains. In S. cerevisiae and Schw. castelli wild type or mutant mitochondria, the well-known inhibition of F1-F0 ATPase activity by low concentrations of oligomycins is abolished at high inhibitor concentrations near 60microg/ml suggesting uncoupling of F1 activity. At still higher oligomycin concentration the ATPase activity of both species and mutant is again strongly inhibited, suggesting an inhibitory effect on yeast F1 activity not detected so far.

[Detection of exercise-induced bronchospasm in sixth-grade students: impact of health education]. / Détection d'un bronchospasme à l'effort chez des élèves de sixième: impact d'une éducation pour la santé.

UNLABELLED: Underdiagnosis of asthma is frequent in children and may be improved by the development of school-based health programs. MATERIAL AND METHODS: We developed an educational program in 11-year-old schoolchildren who participated in a screening test for exercise-induced asthma (EIA). All children were given an asthma questionnaire before and after two educational sessions. RESULTS: Mean score for asthma knowledge quiz increased from 63 to 85% (p < 0.001). Thirty-six children (3.7%) were initially considered as asthmatic and showed better responses than non-asthmatic children. Peak expiratory flow (PEF) was measured before and after an outside running exercise. A fall in PEF of at least 15% was considered an abnormal result. Seventy-height children (8%) had a decrease in PEF, including 65 children initially not recognized as asthmatic. For these latter, a medical evaluation was recommended to parents. Only 28 of these children gave informations on follow-up: 19 had lung function tests, two received a treatment without preliminary function test, and seven had neither lung function test nor treatment. Among the children who realized lung function tests, five had criteria for airway obstruction at baseline, and ten had significant bronchial hyperreactivity. CONCLUSIONS: However, a new screening test revealed that only a small minority of children initially not recognized as asthmatic but having decreased their PEF, were consequently considered as asthmatic by their practitioner, even in case of positive lung function test.

[The value of peak expiratory flow after exertion]. / Intérêt du débit expiratoire de pointe aprés effort.

We have measured peak expiratory flow after a 15-20 minutes run in cold weather in 777 pupils of the sixth year of Val-de Marne. The aim of this measurement was to validate the peak expiratory flow as a means of detecting the early state of respiratory anomalies. A questionnaire was distributed to 920 families before the endurance test. A fall of peak expiratory flow greater than 15% was seen in 77 pupils (9.9%). Twelve of 35 asthmatic pupils who participated in the run lowered their peak expiratory flow compared to 65 out of 742 who were not known as asthmatic (chi 2 = 24; p < 0.001). Complementary respiratory function tests could not be obtained in a systematic fashion so the significance of this lowering remains difficult to interpret. In our study, the level of scholastic delay in asthmatics was close to that of the non asthmatics. The role of the service for the promotion of health for the benefit of pupils (SPSEFE) concentrates as much on the early stages as on the follow up and education of young asthmatics at school in partnership with the families and the treating physician. Writing out a welcoming individualised protocol enable young asthmatics the best possible integration into school.