Hepatocyte growth factor (HGF) is present in skeletal muscle and facilitates skeletal muscle regeneration by activating quiescent satellite cells and stimulating their proliferation. However, possible involvement of HGF from non-muscle organs during muscle regeneration is still uncovered. Since liver injury induces HGF expression in distal HGF-producing organs such as lung, kidney and spleen, we examined if this is the case in muscle injury in analogy. In rat femoral muscle, HGF protein levels were elevated within 1 h after muscle injury, with a simultaneous proteolytic activation of HGF protein. Semiquantitative RT-PCR analysis revealed an elevation of HGF mRNA expression after muscle injury in the liver and spleen, and also an increase of HGF protein levels in the spleen, suggesting the presence of endocrine HGF-inducing factor(s) during muscle regeneration. Indeed, the sera from the rat with muscle regeneration were capable of inducing HGF mRNA expression when applied to primary cultured spleen cells from intact rats. These results indicated that skeletal muscle injury induces HGF expression in the non-muscle HGF-producing organs, especially in the spleen, and suggested the possible involvement of non-muscle organ-derived HGF in activation/proliferation of satellite cells during muscle regeneration. (C) 2002 Elsevier Science (USA).