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Message: <blockquote data-quote="Deleted member 43589" data-source="post: 262567" data-attributes="member: 43589">Sinha-Hikim I, Taylor WE, Gonzalez-Cadavid NF, Zheng W, Bhasin S. Androgen receptor in human skeletal muscle and cultured muscle satellite cells: up-regulation by androgen treatment. J Clin Endocrinol Metab. 2004 Oct;89(10):5245-55.Full Study[URL unfurl="true"]https://academic.oup.com/jcem/article/89/10/5245/2844716[/URL]AbstractAndrogens stimulate myogenesis, but we do not know what cell types within human skeletal muscle express the androgen receptor (AR) protein and are the target of androgen action. Because testosterone promotes the commitment of pluripotent, mesenchymal cells into myogenic lineage, we hypothesized that AR would be expressed in mesenchymal precursor cells in the skeletal muscle. AR expression was evaluated by immunohistochemical staining, confocal immunofluorescence, and immunoelectron microscopy in sections of vastus lateralis from healthy men before and after treatment with a supraphysiological dose of testosterone enanthate. Satellite cell cultures from human skeletal muscle were also tested for AR expression. AR protein was expressed predominantly in satellite cells, identified by their location outside sarcolemma and inside basal lamina, and by CD34 and C-met staining. Many myonuclei in muscle fibers also demonstrated AR immunostaining. Additionally, CD34+ stem cells in the interstitium, fibroblasts, and mast cells expressed AR immunoreactivity. AR expression was also observed in vascular endothelial and smooth muscle cells. Immunoelectron microscopy revealed aggregation of immunogold particles in nucleoli of satellite cells and myonuclei; testosterone treatment increased nucleolar AR density. In enriched cultures of human satellite cells, more than 95% of cells stained for CD34 and C-met, confirming their identity as satellite cells, and expressed AR protein. AR mRNA and protein expression in satellite cell cultures was confirmed by RT-PCR, reverse transcription and real-time PCR, sequencing of RT-PCR product, and Western blot analysis. Incubation of satellite cell cultures with supraphysiological testosterone and dihydrotestosterone concentrations (100 nm testosterone and 30 nm dihydrotestosterone) modestly increased AR protein levels. We conclude that AR is expressed in several cell types in human skeletal muscle, including satellite cells, fibroblasts, CD34+ precursor cells, vascular endothelial, smooth muscle cells, and mast cells. Satellite cells are the predominant site of AR expression. These observations support the hypothesis that androgens increase muscle mass in part by acting on several cell types to regulate the differentiation of mesenchymal precursor cells in the skeletal muscle.DISCUSSION FROM FULL STUDYIn summary, although multiple cell types within the human skeletal muscle express AR protein, satellite cells, and myonuclei are the predominant sites of AR expression. ARs aggregate within the nucleoli of satellite cells and myonuclei. Testosterone and DHT up-regulate AR expression in vivo and in vitro. These data are consistent with the proposal that androgens induce skeletal muscle hypertrophy by acting at multiple sites within the muscle through multiple mechanisms, including modulation of pluripotent stem cell commitment and differentiation and regulation of muscle protein synthesis; further studies are needed to elucidate the molecular basis of androgen action on human skeletal muscle.So when you build muscle using testosterone or other AAS obviously muscle is enlarged and new satellite cells are made therefore AR's are increased in numbers and existing AR are upregulated.Here is a much more simplified article that explains the same thing,[URL unfurl="true"]https://thinksteroids.com/articles/androgen-receptor-regulation/[/URL]</blockquote>

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