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Insulin-like growth factor-1 (IGF1) is crucial for regulating post-natal growth and, along with myostatin (MSTN), regulates muscle size. Here, we sought to clarify the roles of these two genes in regulating sexually dimorphic growth of body and muscle mass. In the first study, we established that Igf1 mRNA was increased to a greater extent and Igf1 receptor mRNA increased earlier in male, than in female, gastrocnemius muscles during the rapid phase of growth (from 2 to 6 weeks) were unchanged, thereafter, to 32 weeks of age in WT mice (P < 0.001). In the second study, we sought to determine if supplemental IGF1 could overcome the sexual dimorphism of muscle and body mass, when myostatin is absent. We crossed myostatin null (Mstn–/–) mice with mice over-expressing Igf1 in skeletal muscle (Igf1+) to generate six genotypes; control (Mstn+/+), Mstn+/–, Mstn–/–, Mstn+/+:Igf1+, Mstn+/–:Igf1+ and Mstn–/–:Igf1+ (n = 8 per genotype and sex). In both sexes, body mass at 12 weeks was increased by at least 1.6-fold and muscle mass by at least 3-fold in Mstn–/–:Igf1+ compared with Mstn+/+ mice (P < 0.001). The abundance of AKT was increased in muscles of mice transgenic for Mstn, while phosphorylation of AKTS473 was increased in both male and female mice transgenic for Igf1+. The ratio of phosphorylated to total AKT was 1.9-fold greater in male mice (P < 0.001). Thus, despite increased growth of skeletal muscle and body size when myostatin was absent and IGF1 was in excess, sexual dimorphism persisted, an effect consistent with greater IGF1-induced activation of AKT in skeletal muscles of males.

Insulin-like growth factors (IGFs) and myostatin have opposing roles in regulating the growth and size of skeletal muscle, with IGF1 stimulating, and myostatin inhibiting, growth. However, it remains unclear whether these proteins have mutually dependent, or independent, roles. To clarify this issue, we crossed myostatin null (Mstn−/−) mice with mice overexpressing Igf1 in skeletal muscle (Igf1+) to generate six genotypes of male mice; wild type (Mstn+/+), Mstn+/−, Mstn−/−, Mstn+/+:Igf1+, Mstn+/−:Igf1+ and Mstn−/−:Igf1+. Overexpression of Igf1 increased the mass of mixed fibre type muscles (e.g. Quadriceps femoris) by 19% over Mstn+/+, 33% over Mstn+/− and 49% over Mstn−/− (P Mstn and addition of Igf1. Myostatin regulated the number, while IGF1 regulated the size of myofibres, and the deletion of Mstn and Igf1+ independently increased the proportion of fast type IIB myosin heavy chain isoforms in T. anterior (up to 10% each, P Mstn−/− mice, while phosphorylation of AKTS473 was increased in Igf1+ mice (Mstn+/+:Igf1+, Mstn+/−:Igf1+ and Mstn−/−:Igf1+). Our results demonstrate that a greater than additive effect is observed on the growth of skeletal muscle and in the reduction of body fat when myostatin is absent and IGF1 is in excess. Finally, we show that myostatin and IGF1 regulate skeletal muscle size, myofibre type and gonadal fat through distinct mechanisms that involve increasing the total abundance and phosphorylation status of AKT and rpS6.

Undernutrition suppresses the growth of skeletal muscles and alters the expression of insulin-like growth factor 1 (IGF1), a key mitogen, and myostatin, a potent inhibitor of myogenesis. These changes can explain, at least in part, the reduced growth of skeletal muscles in underfed lambs. We have recently identified a myostatin splice variant (MSV) that binds to and antagonizes the canonical signaling of myostatin. In the present study, we hypothesized that the expression of MSV would be reduced in conjunction with myostatin and IGF1 in response to underfeeding in skeletal muscles of sheep. Young growing ewes were fed either ad libitum or an energy-restricted diet (30% of maintenance requirements) for 28 d. This regime of underfeeding resulted in a 24% reduction in body mass (P < 0.001) and a 36% reduction in the mass of the semitendinosus muscles relative to controls (P < 0.001) by day 28. The concentrations of MSV and IGF1 messenger RNA (mRNA) were reduced (both P < 0.001), but myostatin mRNA was not altered in semitendinosus muscles. Unlike the reduced expression of mRNA, the abundance of MSV protein was increased (P < 0.05) and there was no change in the abundance of myostatin protein. Our results suggest that undernutrition for 28 d decreases the signaling of myostatin by increasing the abundance of MSV protein. Although this action may reduce the growth inhibitory activity of myostatin, it cannot prevent the loss of growth of skeletal muscles during undernutrition.

The growth and differentiation factor-11 (GDF-11) gene is thought to code for a single protein that plays a crucial role in regulating the development of multiple tissues. In this study, we aimed to investigate if the GDF-11 gene has another transcript and, if so, to characterise this transcript and determine its tissue-specific and developmental expression. We have identified a novel transcript of GDF-11 in mouse muscle, which contains the 3' region of intron 1, exon 2, exon 3 and 3'UTR, and has two transcription initiation sites and a single termination site. We named the novel transcript GDF-11ΔEx1 because it does not contain exon 1 of canonical GDF-11. The GDF-11ΔEx1 transcript was expressed in the skeletal muscles, heart, brain and kidney, but was undetectable in the liver and gut. The concentration of the GDF-11ΔEx1 transcript was increased in gastrocnemius muscles from three to 6 weeks of age, a period of accelerated muscle growth, steadily declined thereafter and was higher in male than female mice (P < 0.001 for age and sex). GDF-11ΔEx1 cDNA was predicted to code for a putative N-terminal-truncated propeptide and the canonical ligand for GDF-11. However, propeptide-specific antibodies could not identify proteins of the expected size in skeletal muscle. Interestingly, in silico analysis of the GDF-11ΔEx1 RNA predicted a secondary structure with the potential to coordinate multiple protein interactions as a molecular scaffold. Therefore, we postulate that GDF-11ΔEx1 may act as a long non-coding RNA to regulate the transcription of canonical GDF-11 and/or other genes in skeletal muscle and other tissues.

Growth and differentiation factor-11 (GDF-11) is a secreted protein that is closely related to myostatin, a known inhibitor of skeletal muscle development. The role of GDF-11 in regulating skeletal muscle growth remains unclear and the pattern of expression during post-natal growth has not been reported. Therefore, we sought to determine the expression of GDF-11 during post-natal growth and its effect on myoblast proliferation and differentiation. We collected gastrocnemius muscles from male and female mice at 2, 3, 4, 6, 12, 20 and 32 weeks of age (n = 6 per sex and age). In addition, gastrocnemius muscles were col- lected from male wild-type and myostatin knockout mice at 4, 6, 12 and 20 weeks of age (n = 6 per age and genotype). RNA was extracted and reverse tran- scribed. Northern analysis identified an expected 4.4 kb mRNA transcript for GDF-11 in gastrocnemius muscles of mice. The concentration of GDF-11 mRNA, as determined by quantitative PCR, was increased in gastrocnemius muscles from 2 to 6 weeks—a period of rapid postnatal muscle growth—and remained higher in male than female mice from 4 to 20 weeks of age (P gastrocnemius muscles of myostatin knockout compared with wild-type mice (P < 0.05), which may suggest a compensatory mecha- nism for the lack of myostatin. In support, recombi- nant GDF-11 inhibited differentiation of C2C12 mur- ine myoblasts and those isolated from myostatin knockout and wild-type mice (P < 0.05). Inhibited dif-ferentiation of C2C12 myoblasts was associated with decreased mRNA expression of early and late mo- lecular markers of differentiation (MyoD, myogenin, IGF-II, desmin and MyHC, P < 0.05). Our results suggest that GDF-11 regulates growth of skeletal muscles by inhibiting myoblast differentiation in an autocrine/paracrine manner and, perhaps, also plays a role in regulating sexually dimorphic growth.

Myostatin inhibits myogenesis and there is reduced abundance of the mature protein in skeletal muscles of adult male compared with female mice. This reduction probably occurs after translation, which suggests that it is a regulated mechanism to reduce the availability of myostatin in males. Reduced myostatin may, thereby, contribute to the development of sexually dimorphic growth of skeletal muscle. Our first objective was to determine if the decrease in mature myostatin protein occurs before the linear growth phase to aid growth, or afterwards to maintain the mass of adult muscle. Mice were killed from 2 to 32 weeks and the gastrocnemius muscle was excised. Myostatin mRNA increased from 2 to 32 weeks and was higher in males than females (P < 0.001). In contrast, mature protein decreased in males after 6 weeks (P < 0.001). Our second objective was to determine if growth hormone (GH) induces the decrease in mature myostatin protein. GH increased myostatin mRNA and decreased the abundance of mature protein in hypophysectomised mice (P < 0.05). Our final objective was to determine if the decrease in mature protein occurs in skeletal muscles of male Stat5b−/− mice (Stat5b mediates the actions of GH). As expected, mature myostatin protein was not reduced in Stat5b−/− males compared with females. However, myostatin mRNA remained higher in males than females irrespective of genotype. These data suggest that: (1) the decrease in mature myostatin protein is developmentally regulated, (2) GH acting via Stat5b regulates the abundance of mature myostatin and (3) GH acts via a non-Stat5b pathway to regulate myostatin mRNA.

This study tested whether administration of insulin-like growth factor-II (IGF-II) enhances muscle regeneration. Rat biceps femoris muscle was damaged with notexin and then IGF-II was administered for up to 7 days. Results show that the proportion of nuclei containing or surrounded by immunoreactivity to MyoD, myogenin, and developmental myosin heavy chain (dMHC) is less in the IGF-II treatment group relative to the control group on days 1 (p=0.057), 2 (p=0.034), and 3 (p=0.047), respectively. This indicates a delay in muscle precursor cell (MPC) proliferation and differentiation with IGF-II administration. This effect was not associated with decreased binding capacity of the type 1 IGF receptor, as determined by receptor autoradiography in day 1 muscle sections (NS), but was associated with inhibition of phagocytic processes. The cross-sectional area of regenerating muscle fibers was significantly greater in the IGF-II treatment group than in the control group by day 7 (p=0.0092). The enhancing effect of IGF-II on late muscle regeneration, when the main process taking place is fiber enlargement, coincides with the period in which IGF-II is normally expressed by regenerating muscle, indicating that greater endogenous production of IGF-II would be associated with improved regeneration.

The IGF axis is nutritionally sensitive in vivo and IGFs stimulate myoblast proliferation and differentiation in vitro, while myostatin inhibits these processes in vitro. We hypothesised that underfeeding would reversibly inhibit the myogenic activity of satellite cells in vivo together with decreased IGF-I and increased myostatin in muscle. Satellite cell activity was measured indirectly from the expression of proliferating cell nuclear antigen (PCNA) and the myogenic regulatory factors (MRFs), MyoD, Myf-5 and myogenin. Young sheep were underfed (30% of maintenance) and some killed after 1, 4, 12, 17, 21 and 22 weeks. Remaining underfed animals were then re-fed a control ration of pellets and killed after 2 days, and 1, 6 and 30 weeks. Expression of PCNA and MRFs decreased during the first week of underfeeding. This coincided with reduced IGF-I and myostatin mRNA, and processed myostatin. Subsequently, Myf-5, MyoD, myostatin mRNA and processed myostatin increased, suggesting that satellite cells may have become progressively quiescent. Long-term underfeeding caused muscle necrosis in some animals and IGF-I and MRF expression was increased in these, indicating the activation of satellite cells for muscle repair. Re-feeding initiated rapid muscle growth and increased expression of PCNA, IGF-I and the MRFs concurrently with decreased myostatin proteins. In conclusion, these data indicate that IGF-I and myostatin may work in a coordinated manner to regulate the proliferation, differentiation and quiescence of satellite cells in vivo.

Myostatin inhibits skeletal muscle development. Therefore, we sought to determine whether larger body and muscle mass in male mice was associated with lower mRNA and protein expression of myostatin compared with females. Ten male and ten female mice of the C57 strain were killed at 16–18 wk of age, and their biceps femoris, gastrocnemius, and quadriceps femoris muscles were collected. Body and muscle masses were 40% heavier ( P < 0.001) in males than in females. Northern analysis showed no difference in mRNA between males and females. In contrast, Western analysis showed that processed myostatin (26 kDa) was 40–60% lower ( P < 0.001) in males compared with females. These data show first that decreased processed myostatin is a posttranscriptional and posttranslational event and, second, that decreased abundance of processed myostatin is associated with increased body mass and skeletal muscle mass in male compared with female mice.

Myostatin, a member of the TGF-beta superfamily, is a key negative regulator of skeletal muscle growth. The role of myostatin during skeletal muscle regeneration has not previously been reported. In the present studies, normal Sprague-Dawley and growth hormone (GH)-deficient (dw/dw) rats were administered the myotoxin, notexin, in the right M. biceps femoris on day 0. The dw/dw rats then received either saline or human-N-methionyl GH (200microg/100g body weight/day) during the ensuing regeneration. Normal and dw/dw M. biceps femoris were dissected on days 1, 2, 3, 5, 9 and 13, formalin-fixed, then immunostained for myostatin protein. Immunostaining for myostatin revealed high levels of protein within necrotic fibres and connective tissue of normal and dw/dw damaged muscles. Regenerating myotubes contained no myostatin at the time of fusion (peak fusion on day 5), and only low levels of myostatin were observed during subsequent myotube enlargement. Fibres which survived assault by notexin (survivor fibres) contained moderate to high myostatin immunostaining initially. The levels in both normal and dw/dw rat survivor fibres decreased on days 2-3, then increased on days 9-13. In dw/dw rats, there was no observed effect of GH administration on the levels of myostatin protein in damaged muscle. The low level of myostatin observed in regenerating myotubes in these studies suggests a negative regulatory role for myostatin in muscle regeneration.

In post-natal animals, plasma concentrations of IGF-I are tightly regulated by nutritional status. The current study reports that plasma levels of IGF-II in sheep are also regulated by nutrition, but whether plasma IGF-II is increased, decreased or remains the same, depends on the age of the animal. Ewe lambs, ranging in age from 2 days to 2 years, were fed or fasted for lengths of time between 24 and 72 h. Blood samples were taken at intervals of 24 h throughout the treatment period and immediately before slaughter. Plasma concentrations of IGF-I increased with advancing age in fed animals (P

The enlarged muscles of certain breeds of cattle, such as the Belgian Blue, have been shown to result from a marked increase in the number of normal sized muscle fibers. Originally insulin-like growth factors (IGFs) were implicated in this myofiber hyperplasia, as IGFs have been shown to stimulate myoblast proliferation as well as maintain fiber differentiation. Recently it has been reported that mice lacking a myostatin gene, a member of the TGFbeta superfamily, have enhanced skeletal mass resulting from increased muscle fiber number and size. Mutations in this gene have been found in double-muscled cattle, indicating that myostatin is an inhibitor of muscle growth. Myostatin is expressed early in gestation and then maintained to adulthood in certain muscles. Myostatin expression in bovine muscle is highest during gestation when muscle fibers are forming and some of the myogenic regulatory factors have elevated expression over the same period as myostatin. Molecular expression of the IGF axis does not differ between Belgian Blue and normal muscled cattle, and IGF-II mRNA is increased throughout formation of secondary fibers in both breeds. However, myostatin and MyoD expression in muscle differ between normal and hypertrophied muscle cattle breeds. This evidence strongly suggests that lack of myostatin is associated with an increase in fiber number which then results in a marked increase in potential muscle mass in double-muscled cattle.

Pelage growth cycles are regulated by circulating prolactin in many mammals, but the intercellular mediators of this signaling are unknown. Binding sites for insulin-like growth factors (IGFs) were examined in sheep skin to show changes in distribution and abundance of IGF receptors associated with a prolactin stimulus and the subsequent hair follicle growth cycle. Follicle cycles were induced in New Zealand Wiltshire ewes by a surge in plasma prolactin following a 4-month period of prolactin suppression with bromocriptine. Eight treated and three control sheep were slaughtered at intervals over 43 days during the follicle growth cycle. At 12-20 days after the elevation of prolactin, wool follicles passed through brief catagen and telogen phases, followed by a return to anagen. IGF binding sites were localized in skin sections by incubation with 125I-IGF-I or 125I-IGF-II. Displacement with competitive binding inhibitors (unlabeled IGF-I, IGF-II, des(1-3)IGF-I, des(1-6)IGF-II, or insulin) and affinity cross-linking showed that these binding sites were predominantly IGF type 1 and type 2 (mannose-6-phosphate) receptors. The radioligands bound especially to follicle germinal cells and prekeratinocytes. Increases in specific binding of both radioligands were observed after the rise in prolactin, but prior to anatomical changes in follicles associated with cessation of growth. For IGF-I, highest binding density was observed during catagen in the germinal matrix and dermal papilla cells. For IGF-II, peak density occurred during late anagen/early catagen in the germinal matrix and during telogen in the dermal papilla. These cycle associated changes in receptor availability suggest that IGF receptors are involved in control of the wool growth.

The insulin-like growth factors (IGFs) are considered to have a role in the regulation of renal growth and development. The purpose of the present study was to evaluate the effect of nutritional stress on IGF binding in ovine kidney at different postnatal ages. Binding of IGF-I and IGF-II to kidneys of fed and fasted sheep was characterised using histological autoradiography, competitive binding assays, and SDS-PAGE. Nutritional regulation of IGF-I binding was restricted to cells of the proximal tubules of two and 14-day-old lambs where we identified an IGF binding protein which was upregulated in response to fasting and where IGF-II binding was also slightly enhanced. Ontogenetic changes occurred in the glomeruli where IGF-I binding peaked at 6 months (P < or = 0.001), and IGF-II binding increased to 4 months and then plateaued (P < or = 0.01). In the medulla, IGF-II binding was highest at 4 and 6 months (P < or = 0.05). From these studies, we conclude that the IGF axis may play a role in the regulation of the metabolic response to fasting in the kidney of young lambs. Furthermore, the changes with age which are described may reflect a transition period at 4-6 months, from an initial promotion of kidney growth and development in young lambs to establishment of the metabolic and clearance functions in the adult animal.

Myostatin plays a fundamental role in regulating the size of skeletal muscles. To date, only a single myostatin gene and no splice variants have been identified in mammals. Here we describe the splicing of a cryptic intron that removes the coding sequence for the receptor binding moiety of sheep myostatin. The deduced polypeptide sequence of the myostatin splice variant (MSV) contains a 256 amino acid N-terminal domain, which is common to myostatin, and a unique C-terminus of 65 amino acids. Western immunoblotting demonstrated that MSV mRNA is translated into protein, which is present in skeletal muscles. To determine the biological role of MSV, we developed an MSV over-expressing C2C12 myoblast line and showed that it proliferated faster than that of the control line in association with an increased abundance of the CDK2/Cyclin E complex in the nucleus. Recombinant protein made for the novel C-terminus of MSV also stimulated myoblast proliferation and bound to myostatin with high affinity as determined by surface plasmon resonance assay. Therefore, we postulated that MSV functions as a binding protein and antagonist of myostatin. Consistent with our postulate, myostatin protein was\ud co-immunoprecipitated from skeletal muscle extracts with an MSV-specific antibody. MSV over-expression in C2C12 myoblasts blocked myostatin-induced Smad2/3-dependent signaling, thereby confirming that MSV antagonizes the\ud canonical myostatin pathway. Furthermore, MSV over expression increased the abundance of MyoD, Myogenin and MRF4 proteins (P,0.05), which indicates that MSV stimulates myogenesis through the induction of myogenic regulatory factors. To help elucidate a possible role in vivo, we observed that MSV protein was more abundant during early post-natal muscle development, while myostatin remained unchanged, which suggests that MSV may promote the growth of skeletal muscles. We conclude that MSV represents a unique example of intra-genic regulation in which a splice variant directly antagonizes the biological activity of the canonical gene product.

The relative abundance and location of type 1 IGF receptors in sheep muscles have been measured to determine whether changes occur during post-natal growth and nutritional stress. Using the technique of histological autoradiography, specific binding of 125I-IGF-I in muscle fibre and connective tissue of M. biceps femoris and M. gastrocnemius was demonstrated, as was specific binding to the tendon of M. gastrocnemius and the surrounding connective tissue. The binding site in both muscles was characterised as the type 1 IGF receptor in membrane preparations using competitive binding assay and SDS-PAGE. Type 1 receptors were more abundant in connective tissue than muscle fibre or tendon (P≤0·001). Levels changed significantly with age in all tissues (P=0·054 to P≤ 0·001), while change as a result of fasting was limited to a receptor increase in the connective tissue of M. gastrocnemius (P=0·034). IGF-I mRNA in M. bicepsfemoris, as assessed by in situ hybridisation, showed changes in expression with increasing age (P≤ 0·025) but no change with fasting. These data indicate that the distribution, relative abundance and nutritional sensitivity of type 1 receptors are related to cell type in vivo. The overall decline of receptors with increasing age may be a feature of transition from linear animal growth to cell maintenance in adult animals. Connective tissue appears to be more sensitive than muscle fibre to nutrition, possibly allowing the reduction of non-essential metabolism during fasting. Journal of Endocrinology (1996) 148, 337–346

Red deer antler tips in the growing phase were removed 60 days after the recommencement of growth for autoradiographical studies and RRAs. Sections were incubated with radiolabeled GH or insulin-like growth factor-I (IGF-I), with or without excess competing unlabeled hormones, and were analyzed autoradiographically. There was negligible binding of [125I]GH in any histological zone of antler sections. [125I]IGF-I showed highest specific binding in the chondroblast zone to a receptor demonstrating binding characteristics of the type 1 IGF receptor. The lowest specific binding of [125I]IGF-I was to prechondroblasts. RRAs on antler microsomal membrane preparations RRAs on antler microsomal membrane preparations confirmed the absence of GH receptors and the presence of type 1 IGF receptors found by autoradiography. These findings suggest that IGF-I may act in an endocrine manner in antler growth through a receptor resembling the type 1 IGF receptor. The presence of type 1 receptors in the chondroblast zone implicates IGF-I involvement in cartilage formation through matrixogenesis. There is no support for IGF-I having a major role in mitosis in the antler.

The effect on young lambs of 0·25 mg recombinant bovine GH (bGH)/kg per day on plasma concentrations of insulin-like growth factor-I (IGF-I), glucose, specific hepatic GH binding and body composition changes was examined at two levels of nutrition (lucerne pellets; 3 and 1·7% of body weight/day). Lambs on low levels of nutrition had low plasma IGF-I (P < 0·001). Plasma concentrations of IGF-I were increased by bGH treatment at both levels of nutrition, with the high nutrition group showing the greatest IGF-I response after 3 and 40 days of bGH treatment. Plasma glucose, after 40 days, was higher overall (P < 0·05) in lambs on high nutrition. bGH treatment increased plasma glucose, with the response being greater in the well-fed lambs. Specific binding of GH to liver membranes was highest in lambs on high nutrition and on bGH treatment; no significant interaction between nutrition and bGH treatment was detected, indicating that specific binding of GH was increased proportionally by bGH at both nutritional levels. The major change in body composition was the reduced level of fatness in lambs treated with bGH. There was no significant effect of bGH on body weight although bGH treatment tended to increase weight gain of well-fed lambs and decreased weight loss of poorly nourished lambs. The results show that, although there was a significant (P < 0·05) bGH/nutrition interaction for IGF-I there was no such interaction for body weight/components or specific GH binding to the liver. The results indicate that an increase in plasma IGF-I does not necessarily result in increases in growth or changes in carcass composition. Journal of Endocrinology (1991) 128, 181–186

This study examined the effect of genotype on prenatal muscle development in both normal-muscled (NM) animals and in double-muscled (DM) animals harboring a mutation in the gene for myostatin that results in the production of a functionally inactive protein. The following muscle development parameters were analyzed at four gestational ages: muscle weight, fiber type, by both enzyme histochemistry and myosin heavy-chain (MHC) immunocytochemistry, and average fiber area. The weights of both M. vastus lateralis and M. vastus medialis were greater throughout prenatal development in the DM animals compared to NM. The percentage of type 1 muscle fibers initially declined with gestational age and subsequently increased in both NM and DM. The percentage of type 1 fibers was consistently lower in DM than in NM. A pattern of MHC isoform localization was shown in DM muscle that is indicative of a delay in muscle development relative to NM. Muscle fiber size was differentially regulated in NM and DM, depending on fiber type. Type 1 fibers were smaller in DM than NM in late gestation, while type 2 fibers were smaller throughout gestation. This study suggests that the inactivating myostatin mutation in DM animals may be associated with changes in both skeletal muscle fiber type and fiber size during bovine muscle development.

Myostatin inhibits myogenesis. Therefore, we sought to determine if mice lacking the myostatin gene [ Mstn(-/-)] would lose less muscle mass than wild-type mice during 7 days of hindlimb suspension (HS). Male Mstn(-/-) and wild-type (C57) mice were subjected to HS or served as ground-based controls ( n = 6/group). Wild-type mice lost 8% of body mass and ∼13% of wet mass from biceps femoris, quadriceps femoris, and soleus, whereas the mass of extensor digitorum longus (EDL) was unchanged after HS. Unexpectedly, Mstn(-/-) mice lost more body (13%, P < 0.05) and quadriceps femoris (17%, P < 0.05) mass than wild-type mice and lost 33% of EDL mass ( P < 0.01) after HS. Protein expression of myostatin in biceps femoris and quadriceps femoris was not altered, whereas expression of MyoD, Myf-5, and myogenin increased in wild-type mice and tended to decrease in muscles of Mstn(-/-) mice. These data suggest that HS induced myogenesis in wild-type mice to counter atrophy, whereas myogenesis was not induced in Mstn(-/-) mice, thereby resulting in a greater loss of muscle mass.

GH enhances skeletal muscle growth, and IGF-II peptide is highly expressed during regeneration. We have therefore investigated the effect of GH administration on IGF-II binding and expression in regenerating rat skeletal muscle using the techniques of receptor autoradiography and in situ hybridisation. Notexin, a myotoxin, was injected into the right M. biceps femoris (day 0), causing affected fibres to undergo necrosis followed by rapid regeneration. Animals were administered either GH (200 μg/100 g body weight) or saline vehicle daily. Contralateral muscles were used as regeneration controls. GH administration during regeneration resulted in significant increases in body weight, and damaged and undamaged muscle weights (PP< 0·001). Specifically, IGF-II expression in regenerating fibres was elevated relative to control and survivor fibres after day 3 (PP125I-IGF-II binding in regenerating muscle was examined in the same fibre types as well as in connective tissue. 125I-IGF-II binding in regenerating fibres was higher (P125I-IGF-II binding in all damaged muscle tissues on day 5 (PP Journal of Endocrinology (1996) 149, 81–91