Distribution of adenosine A1, A2A and A2B receptors in human skeletal muscle
Many important physiological functions of skeletal muscle, such as glucose uptake, contraction and blood flow, have been proposed to be regulated via the action of adenosine on adenosine receptors. The cellular location of adenosine receptors in skeletal muscle is however, not known. The present study examined the distribution of A1, A2A and A2B adenosine receptors in human skeletal muscle using immunohistochemistry. All three receptor types were localized to vascular smooth muscle and endothelial cells, only the adenosine A2A and A2B receptors were observed in the plasma membrane and cytosol of the skeletal muscle. The finding was supported by results from western-blotting analysis. The cytosolic staining of the adenosine A2A receptor was slightly more intense in the type I muscle fibres, whereas the A2B receptor was almost absent in type I fibres. The present findings demonstrate for the first time, direct evidence for the existence of A2A and A2B adenosine receptors but absence of the A1 receptor in the sarcolemma and cytosol of skeletal muscle cells. The data also show existence of all three of the A1, A2A and A2B adenosine receptors in vascular cells of skeletal muscle tissue.
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