THE THROMBIN RECEPTOR IS PRESENT IN MYOBLASTS AND ITS EXPRESSION IS REPRESSED UPON FUSION

Cultured myoblasts derived from limb muscle of newborn rats express th rombin receptor immunoreactivity on their surface. Receptor expression is repressed upon myoblast fusion. This is due at least in part to a decrease in the amount of the thrombin receptor mRNA. Addition of thro mbin triggers calcium transients only in mono- but not multinucleated muscle cells. Furthermore, thrombin increases the rate of myoblast pro liferation that coincides with an activation of mitogen-activated prot ein kinase. Northern analysis of thrombin receptor mRNA expression in skeletal muscle showed that the transcript is present at a relatively high level at birth, but is almost undetectable in the adult. By in si tu hybridization, the mRNA at birth appeared to be present mostly in m ononucleated cells grouped in clusters, but not in muscle fibers. Very few nuclei surrounded by a mRNA signal were present on muscle section s of rats 24 days postnatally. These results suggest that the thrombin receptor plays a role in muscle development.