Muscle satellite cells are quiescent precursors interposed between myofibers and a sheath of external lamina. Although their activation and recruitment to cycle enable muscle repair and adaptation, the activation signal is not known. Evidence is presented that nitric oxide (NO) mediates satellite cell activation, including morphological hypertrophy and decreased adhesion in the fiber-lamina complex. Activation in vivo occurred within 1 min after injury. Cell isolation and histology showed that pharmacological inhibition of nitric oxide synthase (NOS) activity prevented the immediate injury-induced myogenic cell release and delayed the hypertrophy of satellite cells in that muscle. Transient activation of satellite cells in contralateral muscles 10 min later suggested that a circulating factor may interact with NO-mediated signaling. Interestingly, satellite cell activation in muscles of mdx dystrophic mice and NOS-I knockout mice quantitatively resembled NOS-inhibited release of normal cells, in agreement with reports of displaced and reduced NOS expression in dystrophin-deficient mdx muscle and the complete loss of NOS-I expression in knockout mice. Brief NOS inhibition in normal and mdx mice during injury produced subtle alterations in subsequent repair, including apoptosis in myotube nuclei and myotube formation inside laminar sheaths. Longer NOS inhibition delayed and restricted the extent of repair and resulted in fiber branching. A model proposes the hypothesis that NO release mediates satellite cell activation, possibly via shear-induced rapid increases in NOS activity that produce "NO transients."
INTRODUCTIONAfter muscle injury, satellite cells are activated and recruited to cycle as precursors for new muscle formation. Between injury and proliferation in vivo, satellite cells express immediate early genes after 3-6 h (Weiss, 1994;Kami et al., 1995) and muscle regulatory genes after 6 h (Grounds et al., 1992) in concert with proliferating cell nuclear antigen (Chambers and McDermott, 1996). The expression of these genes, release of growth factors such as bFGF, and DNA synthesis 24 -30 h later are used to characterize muscle regeneration in injured and dystrophic muscle (Grounds and McGeachie, 1989;Anderson et al., 1995;Floss et al., 1997 Floss et al., , 1998. The timing and sequence of events are specific to those of repair (Megeney et al., 1996;Li et al., 1997;McIntosh et al., 1998) but similar to those of development (Rudnicki and Jaenisch, 1995;Yun and Wold, 1996).The fine structure of satellite cells, positioned intimately between the fiber sarcolemma and the external lamina (Mauro, 1961;Ishikawa, 1966), changes during their transition from quiescence to activation. Nuclei enlarge and become euchromatic. The typical attenuated organelle-poor cytoplasm expands, and organelles such as mitochondria and rough endoplasmic reticulum hypertrophy (Schultz, 1976;Snow, 1977;Schultz et al., 1978Schultz et al., , 1985. However, although activation is recognized as essential to repair and defined as precursor s...