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It is generally accepted that the primary mechanisms governing skeletal muscle hypertrophy are satellite cell activation, proliferation, and differentiation. Specific growth factors and hormones modulate satellite cell activity during normal muscle growth, but as a consequence of resistance exercise additional regulators may stimulate satellite cells to contribute to gains in myofiber size and number. Present knowledge of the regulation of the cellular, biochemical and molecular events accompanying skeletal muscle hypertrophy after resistance exercise is incomplete. We propose that resistance exercise may induce satellite cells to become responsive to cytokines from the immune system and to circulating hormones and growth factors. The purpose of this paper is to review the role of satellite cells and growth factors in skeletal muscle hypertrophy that follows resistance exercise.

In the present manuscript, the methods required to generate purified cultures of mature adipocytes, as well as stromal vascular cells, from the same isolation are detailed. Also, we describe the in vitro conditions for the dedifferentiation of the isolated mature adipocytes. These two types of cells may be used to reevaluate differences between presently available cellular models for lipogenesis/lipolysis and might provide a new cellular physiological system for studies utilizing the proliferative progeny from mature adipocyte dedifferentiation. Alternative possibilities to the dedifferentiation phenomenon are proposed, as this new area of research is novel.Abbreviations: DMEM -Dulbecco's modified Eagle medium; DMEM/F12 -1:1 ratio; Dulbecco's modified Eagle medium + Ham's F12; FBS -fetal bovine serum; HBSS -Hank's balanced salt solution; HS -horse serum; PBS -phosphate buffered saline, pH 7.08; PSG -pigskin gelatin; SC -satellite cell

Bovine adipofibroblasts, 3T3-L1 cells, L-6 myogenic cells, and sheep satellite cells were allowed to proliferate for 48 h. Oil red-O (ORO) was dissolved in three different solvents isopropanol, propylene glycol and triethyl phosphate. At 48 h, the proliferative cultures were stained with the three stains. ORO stain prepared in both propylene glycol and triethyl phosphate resulted in bright red droplets appearing in all cultures, whereas ORO dissolved in isopropanol was not taken up by any of the cells. These data suggest that certain preparations of ORO may stain cells in non-adipogenic lineages as well as undifferentiated preadipocytes. Caution must be exercised when choosing solvents for ORO in differentiation studies using cells of the fat/adipose lineage.

Adipose tissue metabolism is an essential factor in establishment of a successful lactation, and we have a good understanding of changes in metabolic flux in relation to lactation, parity, and diet. However, the mechanisms of control of flux are less well understood. To continue our investigations into the control of adipose tissue metabolism, we conducted a transcriptomic analysis of adipose tissue of dairy cattle in late pregnancy and early lactation. Our objective was to determine the changes in gene expression in adipose tissue between 30 d prepartum and 14 d in milk in first-lactation animals, and to determine if changes in expression were related to practical production variables. Animals were Holstein heifers fed the same diet to National Research Council requirements, and adipose tissue was biopsied at 30 d prepartum and 14 DIM. Total RNA was extracted and used to determine gene expression on a bovine gene array. Genes that code for proteins controlling fatty acid transport were highly expressed including fatty acid binding proteins (FABP4 and FABP5) and lipoprotein lipase. Among those genes increasing in expression were those controlling lipolysis, including ADRB2 (52%) and LIPE (23%). Many genes coding for enzymes controlling lipogenesis decreased, including SREBP (-25%), TSHSP14 (-30.8%), LPL (-48.4%), and ACACA (-63.9%). This gene expression array analysis in adipose tissue of lactating dairy cattle identifies several key genes that are components of the adaptation to lactation that can be incorporated into models of nutritional efficiency and may be amenable to genetic or dietary manipulation.

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Creatine in the monohydrate form induced differentiation of myogenic satellite cells. Other agents examined did not increase satellite cell proliferation or differentiation. These results provide initial evidence for a mechanistic understanding of observed effects in vivo of increased muscular size and strength from creatine supplementation.

The responsiveness of progency of sheep-derived unilocular fat cells (adipofibroblasts) to dexamethasone, insulin, insulinlike growth factor I (IGF-I), growth hormone (GH), and basic fibroblst growth factor (FGF) was determined in a clonal culture system. Primary cultures of mature adipocytes were obtained from intermuscular adipose tissue (semimembranosus/semitendinosus seam depot) of sheep by ceiling culture techniques. Following degeneration of unilocular fat droplets and re-establishment of fibroblasticlike adipofibroblasts, all adipofibroblasts adhering to upper flask surfaces were collected and isolated away from fibroblasts (which had no multilocular vesicles) by Percoll gradient centrifugation. Progeny derived from a single adipofibroblast were isolated and tested for the ability to proliferate, differentiate, and accumulate lipids. Stock cultures of adipofibroblasts reached confluence in 5 d and were induced to differentiate from 7 to 9 d with dexamethasone-methyl isobutylxanthine-insulin (DMI). Incubation with insulin, IGF-I, GH, or FGF prior to confluence followed by induction with DMI produced no direct (priming) effect on subsequent differentiation. When substituted individually in place of DMI during the 2 d differentiation/induction period, all factors induced differentiation of cultured adipofibroblasts as determined by lipogenesis (P < .05) and lipoprotein lipase activity (P < .05). Thus, isolated adipofibroblasts from sheep muscle may be induced by hormones and growth factors to display mature adipocyte morphology in cell culture. Further definition of the adipofibroblast culture system may aid in the identification of mechanisms regulating adipocyte development in sheep skeletal muscle, as well as in the study of intercommunication between fat and muscle cells.