Adipocyte differentiation is probably controlled by transcriptional and post-transcriptional regulation. Longissimus lumborum from Angus steers (aged 155 d; seven animals per diet) fed high-starch or low-starch diets for 112 d (growing phase) followed by a common high-starch diet for an additional 112 d (finishing phase) was biopsied at 0, 56, 112 and 224 d for transcript profiling via quantitative PCR of twenty genes associated with adipogenesis and energy metabolism. At 56 d steers fed high starch had greater expression of PPARγ as well as the lipogenic enzymes ATP citrate lyase (ACLY), glucose-6-phosphate dehydrogenase (G6PD), fatty acid synthase (FASN), fatty acid binding protein 4 (FABP4), stearoyl-CoA desaturase (SCD), glycerol-3-phosphate acyltransferase, mitochondrial (GPAM), and diacylglycerol O-acyltransferase homologue 2 (DGAT2), and the adipokine adiponectin (ADIPOQ). Expression of insulin-induced gene 1 (INSIG1) was also greater with high starch at 56 d. Steers fed low starch experienced a marked increase in FASN, FABP4, SCD, DGAT2 and thyroid hormone-responsive (SPOT14 homologue, rat) (THRSP) between 56 and 112 d of feeding. A greater expression of the transcription factors sterol regulatory element-binding transcription factor 1 (SREBF1) and MLX interacting protein-like (MLXIPL) was observed at 224 d in steers fed high starch, suggesting a nutritional imprinting effect. Carryover effects of low starch feeding were discerned by greater expression at 224 d of THRSP, FABP4, SCD and DGAT2. These steers also had greater PPARγ at 224 d. Despite these responses, low starch led to greater expression at 224 d of nuclear receptor subfamily 2, group F, member 2 (NR2F2), a known repressor of rodent adipocyte differentiation through its negative effects on PPARγ, ADIPOQ and FABP4. Results suggested that early exposure to high starch induced precocious intramuscular adipocyte proliferation and metabolic imprinting of lipogenic transcription regulators. Low starch might have blunted the PPARγ-driven adipogenic response through up-regulation of NR2F2 but the endogenous ligand for this nuclear receptor remains unknown.

Hydration of feeds and bacterial attachment to feed particles are thought to play major roles in rumen digestion of fibrous feedstuffs. The objective of the present study was to integrate these phenomena in a mechanistic model that could be used for data analysis. The proposed model was based on the conversion of biomass, where digestion end-products can be used for the synthesis of bacterial mass. Digestion of the potentially digestible fraction and subsequent accumulation of bacterial mass was based on a sequential, three-compartment model. These compartments represented substrate undergoing hydration, digestion, and bacterial mass accumulation. A fraction of the substrate was used for synthesis of bacterial mass. It was assumed that these bacteria associate either temporarily or permanently with the remaining substrate. Dacron bags containing either dry or fully-hydrated lucerne (Medicago sativa), maize (Zea mays) cobs, orchard grass (Dactylis glomeratd), and wheat straw were incubated in the rumen of a steer that was infused continuously with (15NH4)2SO4. The 15N-enrichments of isolated particle-associated bacteria and residue remaining in the bags were used to estimate bacterial attachment. Substrate remaining and microbial mass accumulation were analysed simultaneously. Hydration did not appear to limit digestion. Fractional rate of digestion and appearance of attached bacterial mass was fastest for lucerne. For lucerne, 5 % of the digestion end-products were used for synthesis of bacteria that associated with the substrate, whereas for maize cobs, orchard grass, and wheat straw this was 16, 14, and 19% respectively. Less than 2% of digestion end-products were used for synthesis of bacteria that permanently remained associated with the substrate. Permanent association can occur only with the indigestible fraction, and probably represents bacterial debris. Lysis and/or detachment of bacterial cells was highest for lucerne, and was indicative of the rapid dynamics of lucerne digestion.