NO is not substantially involved in afferent signalling in rat muscle spindles

The nitric oxide synthase-1 and nitric oxide synthase-3/nitric oxide signalling systems in the heart of wild type mice and mouse mutants

Recently, we have shown that nitric oxide synthase-1 (NOS-1) and thus its product NO are present in the sarcolemma region of a subpopulation of atrial cardiomyocytes in the rat heart. In order to find out whether this newly discovered sarcolemma-associated NOS/NO system represents a general signalling mechanism in the murine rodent heart and whether its properties are comparable to those in skeletal muscle fibres, immunohistochemical and catalytic histochemical methods (including image analysis) were applied to the heart and extensor digitorum longus (EDL) and tongue muscles of wild type and mutant mice. In different strains of wild type mice and NOS-3 knockouts, urea-resistant (and therefore specific) NOS NADPH diaphorase histochemistry and NOS-1 immunohistochemistry revealed that NOS-1 activity and protein were present in the sarcolemma region of a subpopulation of atrial and ventricular working cardiomyocytes, but not in those of the impulse conducting system. Using image analysis, NOS-1 showed similar activities in the sarcolemma region of cardiomyocytes and in EDL type I myofibres. In mdx and NOS-1 knockout mice, NOS-1 was absent from the sarcolemma region of atrial and ventricular cardiomyocytes and of EDL and tongue muscle fibres, whereas NOS-1 was present in the hearts of NOS-3 knockouts. Atrial natriuretic peptide immunohistochemistry identified part of the atrial NOS-1-expressing cardiomyocytes as myoendocrine cells. In mdx mice as well as in NOS-1 - and NOS-3-deficient animals, the peptide was found in greater abundance than in wild type mice. These data suggest that NOS-1 is expressed in a subpopulation of working cardiomyocytes in the murine rodent heart, that the myoendocrine cells may be negatively modulated by NOS-1 - and NOS-3-produced NO, and that the anchoring mechanisms for NOS-1 in these cells (i.e. their confinement to the sarcolemma region) are comparable to those in skeletal muscle fibres.

NO message from muscle

The synthesis of the free radical gas nitric oxide (NO) is catalyzed by the enzyme NO synthase (NOS). NOS converts arginine and molecular oxygen to NO and citrulline in a reaction that requires NADPH, FAD, FMN, and tetrahydrobiopterin as cofactors. Three types of NOS have been identified by molecular cloning. The activity of the constitutively expressed neuronal NOS (nNOS) and endothelial NOS (eNOS) is Ca(2+)/calmodulin-dependent, whereas that the inducible NOS (iNOS) is Ca(2+)-insensitive. The predominant NOS isoform in skeletal muscle is nNOS. It is present at the sarcolemma of both extra- and intrafusal muscle fibers. An accentuated accumulation of nNOS is found in the endplate area. This strict sarcolemmal localization of nNOS is due its association with the dystrophin-glycoprotein complex, which is mediated by the syntrophins. The activity of nNOS in skeletal muscle is regulated by developmental, myogenic, and neurogenic influences. NO exerts several distinct effects on various aspects of skeletal muscle function, such as excitation-contraction coupling, mitochondrial energy production, glucose metabolism, and autoregulation of blood flow. Inside the striated muscle fibers, NO interacts directly with several classes of proteins, such as soluble guanylate cyclase, ryanodine receptor, sarcoplasmic reticulum Ca(2+)-ATPase, glyceraldehyde-3-phosphate dehydrogenase, and mitochondrial respiratory chain complexes, as well as radical oxygen species. In addition, NO produced and released by contracting muscle fibers diffuses to nearby arterioles where it acts to inhibit reflex sympathetic vasoconstriction.

Association of soluble guanylate cyclase with the sarcolemma of mammalian skeletal muscle fibers

Previous investigations have shown that NO-producing nitric oxide synthase (NOS)-1 and CO-generating heme oxygenase (HO-2) are associated with the sarcolemma of skeletal muscle fibers in many mammalian species. Despite numerous roles ascribed to NO and possibly also CO in skeletal muscle, a specific receptor for both gases has hitherto not been found in myofibers. Therefore, in the present work the appearance of the alpha1, beta1 and beta2 subunits of soluble guanylate cyclase (sGC), the most commonly known receptor for NO and potentially also CO, was analysed in mammalian skeletal muscles using immunoblotting and immunohistochemistry. Immunoblotting with an antibody against the beta1 subunit of sGC revealed a band of 70 kDa corresponding to the molecular weight of this protein. Immunohistochemistry with antibodies against the alpha1, beta1 and beta2 sGC subunits showed that the larger part of positivity was present in the sarcolemma region of skeletal muscle fibers and colocalized with NOS-1 mainly in type II myofibers and with HO-2 in type I and type II myofibers. For the first time, sarcolemmal association of sGC and its colocalization with NOS-1 generating the sGC-activator NO and with HO-2 producing the potential sGC upregulator CO have been demonstrated in the present study. These results enable a better understanding of the role of NO and CO in myofibers and suggest a so far unknown molecular mechanism for the interaction of sGC with the sarcolemma.

Caveolin-3 and nitric oxide synthase I in healthy and diseased skeletal muscle

Recently, it has been shown for mouse skeletal muscle that caveolin-3 is localized in the sarcolemma and cofractionates with the original dystrophin complex (DC). In order to find out whether caveolin-3 is a further component of the recently established and enlarged nitric oxide synthase (NOS) I-DC and whether members of this complex interact with and are potentially regulated by caveolin-3, mammalian and non-mammalian healthy and diseased (dystrophic) skeletal muscles were investigated using caveolin-3, NOS I, DC components and myosin immunohistochemistry as well as NOS I-associated diaphorase histochemistry. In healthy mammalian skeletal muscle, caveolin-3 was colocalized with the DC components in all extra- and intrafusal fibers. By contrast, NOS I was absent in type I extrafusal fibers of certain species. In patients with Duchenne muscular dystrophy and mdx mice the components of the NOS I-DC were not detected in all extra- and intrafusal fiber types, while caveolin-3 was found unchanged. In healthy non-mammalian skeletal muscle, i.e. of birds, reptiles and fishes, caveolin-3 immunoreactivity was lacking in the sarcolemma as was alpha-sarcoglycan; the other NOS I-DC components were either present or absent. In conclusion, although caveolin-3 is localized in the sarcolemma of mammalian myofibers, there are differences in the microarchitecture of the components of the DC complex and of caveolin-3 which does not appear to be linked with the NOS I-DC. Potential regulatory interactions between caveolin-3 and NOS I may nevertheless exist in those fibers where both molecules are colocalized. The absence of caveolin-3 and alpha-sarcoglycan immunoreactivities in non-mammalian myofibers may suggest that the functions of these proteins are subserved by other components of NOS I-DC complex.