Technical note: A muscle biopsy technique for stratifying cattle by skeletal muscle metabolic activity.

Tissue biopsy metabolic activity, assessed using the oxidation-reduction indicator resazurin, may serve as a proxy to assess energy expenditure associated with maintenance in nongrowing animals or growth rate in growing animals. Herein, we evaluate the repeatability, practicality, and sensitivity of a resazurin-based assay for ranking bovine skeletal muscle biopsies based on metabolic activity. Six yearling Holstein heifers (body weight = 330 ± 11.3 kg) were fed 4 dietary treatments consisting of high or low rumen-degradable starch and fiber arranged factorially in a partially replicated Latin square design. Periods were 18 d, consisting of 3 d for diet transition, 14 d for diet adaptation, and 1 d for sample collection. Semitendinosus biopsies were collected into ice-cold Dulbecco's modified Eagle medium (Fisher Scientific, Hampton, NH) from each heifer during each period. Analysis was initiated within an hour of sample collection. To assess tissue metabolic rate, biopsies were transferred to Dulbecco's modified Eagle medium with resazurin and incubated at 37°C. Fluorescence of each sample was read at time 0 and at 15-min intervals for 2 h. Change in fluorescence was representative of skeletal muscle reducing equivalent production. Fluorescent signal strength increased with time and relative rank of treatments did not change with time; accordingly, future studies may compare fluorescence at a single time point. Change in fluorescence at 120 min was used for analysis of the fixed effects of fiber, starch, and animal when accounting for a random effect of period. Samples collected when animals were on a high-ruminally degradable starch diet were more metabolically active than samples collected from animals on low-starch diets. Significant differences in metabolic activity among individual animals were also identified. Average relative fluorescence was correlated with dry matter intake, average daily gain, and feed-to-gain ratio. The relative fluorescence tended to correlate with average daily gain (r = 0.749) and feed-to-gain ratio (r = -0.783); change in fluorescence did not correlate with dry matter intake. Although evaluated on a small sample size, this technique shows promise as a potential means of ranking animals by growth or feed efficiency. Further work on a larger experimental population is needed to confirm the usefulness of this assay as a consistent and reliable predictor of these important phenotypic parameters.

A 100-Year Review: Stress physiology including heat stress.

Stress is an external event or condition that places a strain on a biological system. The animal response to a stress involves the expenditure of energy to remove or reduce the impact of the stress. This increases maintenance requirements of the animal and results in loss of production. The biological response to stress is divided into acute and chronic phases, with the acute phase lasting hours to a few days and the chronic phase lasting several days to weeks. The acute response is driven by homeostatic regulators of the nervous and endocrine systems and the chronic phase by homeorhetic regulators of the endocrine system. Both responses involve alterations in energy balance and metabolism. Thermal environment affects all animals and therefore represents the largest single stressor in animal production. Other types of stressors include housing conditions, overcrowding, social rank, disease, and toxic compounds. "Acclimation" to a stress is a phenotypic response developed by the animal to an individual stressor within the environment. However, under natural conditions, it is rare for only one environmental variable to change over time. "Acclimatization" is the process by which an animal adapts to several stressors within its natural environment. Acclimation is a homeorhetic process that takes several weeks to occur and occurs via homeorhetic, not homeostatic, mechanisms. It is a phenotypic change that disappears when the stress is removed. When the stress is severe and not relieved by acclimatization or management changes, the animal is considered chronically stressed and is susceptible to increased incidence of disease and poor health. Milk yield and reproduction are extremely sensitive to stress because of the high energy and protein demands of lactation and the complexity of the reproductive process and multiple organs that are involved. Improvements in protection of animals against stress require improved education of producers to recognize stress and methods for estimating degree of stress on animals.

Hepatic adaptations to maintain metabolic homeostasis in response to fasting and refeeding in mice.

BACKGROUND: The increased incidence of obesity and associated metabolic diseases has driven research focused on genetically or pharmacologically alleviating metabolic dysfunction. These studies employ a range of fasting-refeeding models including 4-24 h fasts, "overnight" fasts, or meal feeding. Still, we lack literature that describes the physiologically relevant adaptations that accompany changes in the duration of fasting and re-feeding. Since the liver is central to whole body metabolic homeostasis, we investigated the timing of the fast-induced shift toward glycogenolysis, gluconeogenesis, and ketogenesis and the meal-induced switch toward glycogenesis and away from ketogenesis. METHODS: Twelve to fourteen week old male C57BL/6J mice were fasted for 0, 4, 8, 12, or 16 h and sacrificed 4 h after lights on. In a second study, designed to understand the response to a meal, we gave fasted mice access to feed for 1 or 2 h before sacrifice. We analyzed the data using mixed model analysis of variance. RESULTS: Fasting initiated robust metabolic shifts, evidenced by changes in serum glucose, non-esterified fatty acids (NEFAs), triacylglycerol, and ß-OH butyrate, as well as, liver triacylglycerol, non-esterified fatty acid, and glycogen content. Glycogenolysis is the primary source to maintain serum glucose during the first 8 h of fasting, while de novo gluconeogenesis is the primary source thereafter. The increase in serum ß-OH butyrate results from increased enzymatic capacity for fatty acid flux through ß-oxidation and shunting of acetyl-CoA toward ketone body synthesis (increased CPT1 (Carnitine Palmitoyltransferase 1) and HMGCS2 (3-Hydroxy-3-Methylglutaryl-CoA Synthase 2) expression, respectively). In opposition to the relatively slow metabolic adaptation to fasting, feeding of a meal results in rapid metabolic changes including full depression of serum ß-OH butyrate and NEFAs within an hour. CONCLUSIONS: Herein, we provide a detailed description of timing of the metabolic adaptations in response to fasting and re-feeding to inform study design in experiments of metabolic homeostasis. Since fasting and obesity are both characterized by elevated adipose tissue lipolysis, hepatic lipid accumulation, ketogenesis, and gluconeogenesis, understanding the drivers behind the metabolic shift from the fasted to the fed state may provide targets to limit aberrant gluconeogenesis and ketogenesis in obesity.

Dietary restriction reduces the rate of estradiol clearance in sheep (Ovis aries).

Three experiments were designed to test the effect of dietary restriction on clearance of 17beta-estradiol (E(2)) in sheep. A preliminary experiment examined the effect of a 4-d fast on the rate of E(2) clearance in wethers. The second experiment tested the hypothesis that either long-term restriction (7 wk) or a 5-d fast would increase steroid-binding capacity of serum by increasing the concentration of sex hormone-binding globulin (SHBG) in the blood of ovariectomized ewes. In Exp. 3, we hypothesized that nutrition-dependent regulation of E(2) clearance by the liver would result in divergence in biliary extraction of E(2) in fed and fasted wethers receiving comparable levels of exogenous E(2). A marked difference in E(2) clearance between fed and fasted wethers was noted in the preliminary study. Relative to ad libitumfed wethers, a 4-d fast decreased E(2) clearance by 52%. Serum concentrations of SHBG were increased in long-term energy-restricted and fasted ewes, relative to the concentration in maintenancefed ewes (P = 0.015). Furthermore, a 5-d fast nearly doubled serum steroid-binding capacity in wethers. The E(2) concentration in bile was 2 times greater in fasted than in fed wethers. This fasting-dependent increase in biliary E(2) may be reflective of the increased serum E(2) in fasted animals, because each 1 pg/mL increase in serum E(2) increased bile E(2) by 0.86 +/- 0.12 pg/mL, independent of nutrition (P = 0.002). Our results demonstrate that the rate of clearance of E(2) is decreased during nutritional restriction. Additionally, these data indicate that altered SHBG expression, enterohepatic recirculation, or both are involved in the decreased E(2) clearance during dietary restriction.

Effects of age on body condition and production parameters of multiparous beef cows.

Pregnancy rate, calving interval, birth weight, weaning weight, and quarterly BCS were collected for 5 consecutive years on 454 fall-calving multiparous British crossbred cattle (3 to 10 yr of age) to evaluate associations of age with BCS and production parameters. Body weight and BCS were collected pre-calving, prebreeding, at weaning, and midway through the second trimester of pregnancy (August). Body condition score was correlated with age during all seasons (P < 0.01). At calving, breeding, and in August, 3-yr-old cows had the lowest BW and BCS, whereas 8-yr-old cows had the greatest. At weaning, these values were maximal in 10-yr-old cows. Pregnancy rate was near 80% up to 9 yr of age but decreased to 57% in 10-yr-old cows. The relationship of pregnancy rate with age appears to be correlated with the BCS decrease at breeding in the older cows, supported by the fact that inclusion of BCS at breeding in the statistical model eliminated the effect of age on pregnancy rate (P = 0.42). Calving interval was longer in 3-yr-old cows compared with 4- to 9-yr-old cows (P = 0.02); however, among older cows, there was little change in the calving interval. Birth weight reached a maximum at 8 yr of age (35 +/- 0.9 kg) and a minimum in 3-yr-old cows (32 +/- 0.7 kg). Birth weights of calves born to both 3- and 4-yr-old cows were lower than for those born to 5-, 6-, 7-, or 8-yr-old cows (P < 0.05). Ten-year-old cows weaned lighter calves (205-d adjusted weaning weight) than younger dams. Furthermore, 3-yr-old cows weaned calves 9 +/- 2.1 and 14 +/- 2.4 kg lighter than 4- and 5-yr-old cows, respectively (P < 0.001). Interpretation of the age analyses of calving interval, birth weight, and weaning weight was independent of the inclusion of BCS in the model. This study documents the effects of age on calving interval, birth weight, and weaning weight that are independent of BCS.