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The objective of this study was to determine the effect of early weaning followed by a period of high-grain feeding on plasma acetate kinetics and signaling protein phosphorylation in LM tissue of growing steers. We hypothesized that early grain feeding would result in altered cell signaling and acetate use to support observed improvements in carcass gain and marbling. Fall-born Angus × Simmental steers were weaned at 106 ± 4 d of age (early weaned [EW]; n = 6) and fed a high-grain diet for 148 d or remained with their dams (normal weaned [NW]; n = 6) on pasture until weaning at 251 ± 5 d of age. Both treatments were subsequently combined and grazed on mixed summer pasture to 394 ± 5 d of age followed by a feedlot ration until harvest at 513 ± 5 d of age. Longissimus muscle tissue biopsies were collected at 253 ± 5 and 394 ± 5 d of age and at harvest. Total and phosphorylated forms of 5' adenosine monophosphate-activated protein kinase (AMPK) and downstream proteins of the mammalian target of rapamycin signaling pathway were determined by western blotting. Eight steers were used to assess acetate clearance at different age points via a bolus infusion of acetate (4 mmol/kg of BW). Early weaned steers had greater (P < 0.05) ADG than NW steers during the early grain feeding period. Phosphorylated to total ratios of ribosomal protein S6 (rpS6) and ribosomal protein S6 kinase 1 (S6K1) were significantly different during the early grain feeding period. Phosphorylated to total ratios of S6K1, rpS6, acetyl-CoA carboxylase, and 4E binding protein 1 and the absolute amount of phosphorylated AMPK were correlated with ADG, explaining 46% of the variance. Acetate clearance rates were less (P < 0.05) and synthesis rates were greater (P = 0.06) in EW steers during early grain feeding. Acetate synthesis rates were also greater (P < 0.05) in NW steers at harvest, suggesting a permanent shift in the gut microflora or gut function in response to the treatment. Neither treatment nor acetate infusion significantly affected plasma glucose or insulin concentrations. Plasma β-hydroxybutyric acid concentrations increased with acetate infusion (P < 0.05). Based on these results, altered cell signaling during the early grain feeding period likely mediated increased protein deposition, leading to increased carcass weights, but observed changes in acetate appearance and clearance rates do not appear to explain the observed differences in intramuscular fat deposition during the terminal feeding period.

The objectives of this study were to assess the effects of early grain feeding on acetate and glucose turnover rates and acetate and glucose preference for palmitate synthesis by subcutaneous fat (SCF), intramuscular fat (IMF), and visceral fat (VF) in finishing steers. Sixteen Angus × Simmental steers were used in the study; 8 were early weaned (EW) and fed a high-grain diet immediately after weaning for 100 or 148 d, and 8 remained with their dams on pasture until weaning at 202 ± 5 or 253 ± 5 d of age. Normal weaned (NW) and EW animals were combined and grazed to 374 ± 5 or 393 ± 5 d of age, when they were placed on a corn silage-based finishing ration until they achieved a SCF thickness of 1.0 to 1.2 cm (494 ± 17 d of age for EW steers and 502 ± 12 d of age for NW steers). Immediately before harvest, steers were continuously infused for 12 h with [2H3] acetate (1.63 mmol/min; n = 8) or [U-13C6] glucose (0.07 mmol/min; n = 8). Blood samples were collected before initiation of infusions and at the end of the infusion from 8 animals or at 1-h intervals for the first 11 h and at 15-min intervals for the last hour of infusion for the other 8 animals. Adipose tissue samples from SCF, IMF, and VF depots were collected at harvest, and lipids were extracted. Plasma enrichments of acetate and glucose and palmitate enrichment in each depot were used to calculate plasma turnover rates and fractional synthesis rates (FSR; % per h) of palmitate from each isotope. Early weaned steers had greater marbling scores compared to NW steers ( P< 0.05). Plasma turnover rates and FSR for EW and NW steers were similar except for SCF, where a greater FSR from acetate was observed for EW steers. It is possible the greater FSR for SCF was due to harvesting the animals at a slightly more advanced stage of conditioning as evidenced by the trend for greater 12th rib fat (P = 0.07). Plasma acetate turnover and palmitate FSR from acetate were much greater (P < 0.05) than the corresponding rates from glucose, supporting the primary role of acetate as an energy source and the primary substrate for lipid synthesis across fat depots. However, FSR from acetate and glucose were not different among depots, suggesting that any potential effects of dietary starch on differential deposition of energy in SCF and IMF are not substrate driven.

Brahman × British crossbred steers (n = 40 and 38 in yr 1 and 2, respectively) were used to evaluate the effects of calf management systems following early weaning (EW) on growth performance, muscle gene expression, and carcass characteristics. On the day of EW (d 0), steers were stratified by BW and age (95 ± 14 kg; 74 ± 14 d) and randomly assigned to a control treatment that was normally weaned (NW) on d 180 (n = 10 steers/yr) or to 1 of 3 EW treatments: 1) EW and limit fed a high-concentrate diet at 3.5% of BW (as-fed basis) in drylot until d 180 (EW180; n = 10 steers/yr), 2) EW and limit fed a high-concentrate diet at 3.5% of BW (as-fed basis) in drylot until d 90 and then grazed on bahiagrass pastures until d 180 (EW90; n = 10 steers/yr), or 3) EW and grazed on annual ryegrass pastures until d 60 (yr 1; n = 10 steers) or 90 (yr 2; n = 8 steers) and then on bahiagrass pastures until d 180 (EWRG). Early-weaned steers on ryegrass and bahiagrass pastures were supplemented with high-concentrate diet at 1.0% of BW (as-fed basis) until d 180. From d 180 to 270 (yr 1), all EW steers remained in their respective treatments, whereas NW steers were provided high-concentrate diet at 1.0% of BW (as-fed basis) on bahiagrass pastures. In yr 1, feedlot finishing period began on d 270. In yr 2, the study was terminated on d 180. In both years, EW180 steers were heaviest (P < 0.0001) on d 180. On d 180 of yr 1, EWRG steers were lightest (P < 0.0001) and EW90 steers were heavier (P = 0.05) than NW steers, whereas EW90, EWRG, and NW steers had similar BW on d 180 of yr 2 (P ≥ 0.14). On d 90, muscle PPARγ mRNA expression tended (P = 0.07) to be greater for EW180 steers and was greater (P = 0.008) for EW90 vs. EWRG steers but similar (P = 0.25) between EW180 and NW steers. On d 180, PPARγ mRNA was greater (P ≤ 0.06) for EW180 vs. NW, EW90, and EWRG steers. From d 274 to 302, EW180 steers had the least ADG (P ≤ 0.09), whereas EW90 steers had similar (P = 0.19) ADG compared with EWRG steers but greater (P = 0.03) ADG than NW steers. At slaughter, carcass characteristics did not differ (P ≥ 0.22) among treatments. In summary, EW steers provided a high-concentrate diet in drylot for at least 90 d were heavier at the time of normal weaning than NW steers and EW steers grazed on ryegrass pastures for 60 to 90 d and supplemented with concentrate at 1.0% of BW. Feeding a high-concentrate diet immediately after EW enhanced the muscle PPARγ expression but did not enhance marbling at slaughter.

Early weaning of calves to a high concentrate diet results in greater fat deposition and suggests early postnatal metabolic imprinting events may be exploited as a management tool to improve cattle value. Our objective was to implement a short, high energy dietary intervention before a typical grazing period to manipulate intramuscular fat deposition in finishing cattle. Fall-born, Angus-sired steer calves (n = 24) were stratified by sire and randomly assigned to normal weaned (NW) or metabolic-imprinted (MIP) treatments. At 105 ± 6d (135kg), MIP calves were transitioned to a diet containing 20% CP and 1.26 Mcal/kg NEg. Metabolic-imprinted calves were fed ad libitum as a group. Normal weaned calves remained on their dam until 253 ± 6 d of age. At this time, treatment groups were combined and grazed for 156 d on a mixed summer pasture. Following the grazing phase, steers were adapted to a corn silage-based feedlot diet and performance was monitored on 28-d intervals. Calves were staged for harvest based on backfat endpoint (target 1.0 to 1.2 cm). Metabolic-imprinted calves were heavier (P < 0.05) than NW calves (341 vs. 265 ± 4.2 kg) at normal weaning age. During the grazing phase, NW steers gained more weight than (P < 0.05) MIP steers (0.69 vs. 0.35 ± 0.03 kg/d). Feedlot performance and USDA yield grade were similar (P > 0.20) between treatments. However, MIP steers produced heavier (P < 0.05) carcasses (564 vs. 524 ± 5.6 kg) with higher (P < 0.001) marbling scores (645 vs. 517 ± 23). Therefore, calves consuming a high concentrate diet for 148 d after early weaning produced higher quality carcasses. This suggests early weaning and feeding a high concentrate before grazing is a viable strategy to increase marbling deposition compared with a traditional production system.

The objective of this study was to evaluate the influence of dietary protein and fat from distillers dried grains with solubles (DDGS) on feedlot performance, carcass characteristics, and meat quality in finishing steers. Angus-cross steers (n = 105; 443 +/- 20 kg of BW) were blocked by BW and randomly assigned to 1 of 5 dietary treatments: 1) corn-based diet with DDGS included at 25% of DM (CON), 2) CON with DDGS included at twice the amount of CON (50% of DM; 50DDGS), 3) CON with added corn protein to equal the CP in the 50DDGS diet (CON+CP), 4) CON with added vegetable oil to equal the fat in the 50DDGS diet (CON+VO), and 5) CON with protein and fat added to equal the CP and fat in the 50DDGS diet (CON+CPVO). Steers were fed to a common 12th-rib fat depth endpoint (1.3 +/- 0.2 cm; 68 to 125 d on trial). Loins and rounds were collected from 44 carcasses for Warner-Bratzler shear force (WBSF), ether extract, and case-life analyses. Data were analyzed using the MIXED procedure of SAS. Contrasts between 1) CON vs. elevated CP diets (50DDGS, CON+CP, and CON+CPVO; EP), 2) CON vs. elevated fat diets (50DDGS, CON+VO, and CON+CPVO; EF) and 3) CON vs. diets with elevated CP and fat (50DDGS and CON+CPVO; EPF) were analyzed. There were no differences in days on feed or DMI among treatments. Steers fed CON had greater ADG (P or= 0.06). Steers fed the CON diet had greater marbling scores (P or= 0.44). However, CON steers had greater (P = 0.02) L* values than EF-fed steers and greater b* values than EP, EF, and EPF steers (P

Muscle shortening and postmortem proteolysis both influence beef tenderness, but their interacting effects on tenderness are relatively unknown. Inherent myofibril structure and the extent of overlap between myosin and actin filaments are hypothesized to affect the availability of substrates for degradation by calpains. The objective of this study was to determine the influence of sarcomere length on the extent of calpain-induced proteolysis of bovine myofibrils in vitro. Bovine semitendinosus muscles were excised within 20 min postmortem and dissected into strips, which were stretched and attached to applicator sticks or allowed slack to generate samples with different sarcomere lengths upon rigor completion. Samples were allowed to undergo rigor in a neutral pH buffer containing a protease inhibitor. Myofibrils were isolated and incubated at room temperature with excess exogenous mu-calpain at a ratio of 1:800 (wt/wt; enzyme:myofibrillar protein) at pH 6.8 for 0, 2, 60, 1,440, or 2,880 min. Purified troponin was subjected to the same digestion conditions. Proteolysis of troponin T (TnT) was monitored using SDS-PAGE and Western blotting. Sarcomere length was greater (P < 0.0001) in stretched versus shortened samples (2.99 microm +/- 0.03 vs. 2.12 +/- 0.03 microm, respectively, means +/- SE). Western blots for both stretched and shortened samples exhibited bands corresponding to intact TnT and TnT fragments. The abundance of intact TnT decreased (P < 0.0001) with incubation time across both treatments. At 1,440 and 2,880 min, less (P < 0.05) intact TnT was detected in samples with long sarcomeres. These data indicate proteolysis of TnT occurs to a greater extent in samples with longer sarcomeres, possibly due to easier access of proteases to their targeted substrates. Degradation patterns of TnT were qualitatively similar between myofibrils and purified troponin after incubation with mu-calpain. Therefore, it is unlikely that the mechanism by which proteolysis is limited in short sarcomeres involves an actomyosin-mediated interference of TnT.

The interaction between sarcomere length and postmortem proteolysis as related to meat tenderness is not clear. The extent of thick and thin filament overlap alters actomyosin binding and may alter substrate availability during aging-induced tenderization. The objective of this study was to determine the influence of sarcomere length on proteolytic degradation in beef. Strips from bovine semitendinosus were either stretched 40% and restrained or allowed to shorten unrestrained in an ice bath. After rigor completion, 0.6-cm cross sections were fabricated and were randomly assigned to 2, 4, 7, or 10 d of aging treatments. Myofibrils were isolated for sarcomere length determination. Samples were collected and frozen for shear force analysis, and muscle proteins were extracted for SDS-PAGE and Western blotting analyses to determine troponin T (TnT) proteolysis. Sarcomere length was greater (P < 0.01) in stretched muscle samples compared with shortened samples (2.57 vs. 1.43 microm, respectively). Correspondingly, shear force values were greater (P < 0.05) in shortened samples than stretched samples. Western blots revealed the presence of 3 major intact TnT bands that diminished with time postmortem and 4 bands (TnT degradation products) that accumulated during postmortem storage. Quantification of intact TnT showed increased (P < 0.05) proteolysis at 4 and 7 d postmortem in samples with long sarcomeres. By 10 d, only traces of the greatest molecular weight intact TnT band were evident in both shortened and stretched samples, suggesting this TnT band may be more susceptible to proteolysis than other intact TnT bands. Degradation products of TnT appeared earlier postmortem in samples with long sarcomeres. The 30-kDa TnT fragment appeared after 7 d of postmortem storage in samples with long sarcomeres but not until 10 d in muscle containing short sarcomeres. Collectively, these data show that postmortem TnT proteolysis is sarcomere length-dependent and suggest that thick and thin filament overlap may influence the postmortem aging process in beef.

Objectives were to determine 1) effects on traits measured from birth to slaughter in F2 cross calves from sire breeds that differ in potential for lean tissue growth but have similar mature BW and 2) the gene action of the mutant Piedmontese myostatin allele. Hereford (normal muscling, H), Limousin (moderate increase in muscling, L), and Piedmontese (muscular hypertrophy, P) sires (20 to 25 per breed) were bred at random to crossbred cows to produce F1 calves that were inter se-mated within sire breed to produce F2 calves that were grown out, finished, and slaughtered. Piedmontese-cross calves were genotyped for the G-A transition mutation at the myostatin locus characteristic of P (msP). Genotypes were classified on the basis of having zero (P0), one (P1), or two (P2) copies of msP (H, n = 227; L, n = 207; P0, n = 40; P1, n = 107; and P2, n = 37). Limousin-cross F2 calves had heavier birth (but dystocia was not affected) and weaning weights, gained faster, had more muscle, less fat, larger pelvic area, and more efficient feed conversion than Hereford-cross F2 calves. Normal-muscled Piedmontese-cross F2 calves (P0) were similar to Hereford-cross F2 calves except that they required less assistance at birth in heifer dams, had less fat, gained slower, were less efficient, and had larger pelvic area. Addition of msP alleles (P1 and P2) consistently increased muscle through hyperplasia, decreased fat, and increased adjusted efficiency, but many of those changes were not linear. Residual variances for breed were heterogeneous for most traits related to muscularity. This heterogeneity was caused by increased variances for L and P and(or) lower variances for H. Accounting for the msP alleles decreased the variance for P in most traits, but heterogeneity remained for most traits among the five genotypes because L remained high, H was low, and(or) P2 was low. We conclude that differences in muscularity affect most traits, and when differences in muscularity include the msP allele, there is an incremental, but not equal, change in most traits with the addition of each copy of the msP allele. Advantages of L could be captured through normal crossbreeding and selection schemes but with some caution because of potential problems from increased variability. Advantages of P could be best captured through more complex breeding and selection programs that would lessen potential negative impacts and through marketing systems that do not penalize for very low fat.

The effect of testosterone on sexual dimorphism is evident by differential growth of forelimb and neck muscles in bulls and steers. Divergent hormone sensitivites may account for the differential growth rates of individual muscles. Therefore, the objective of this study was to compare androgen receptor (AR) expression in three different muscles of bulls and steers at various ages and growth rates. Thirty Montbéliard bulls and 30 steers were assigned to four slaughter age groups. Four or five animals of each sex were slaughtered at 4 and 8 mo of age. Animals in the remaining two slaughter groups (12 and 16 mo) were divided into groups of either restricted (R) or ad libitum (AL) access to feed. Five animals of each sex and diet were slaughtered at the end of the restricted intake period at 12 mo of age. To simulate compensatory growth, the remaining animals (R and AL) were allowed ad libitum access to feed until slaughter at 16 mo of age. Total RNA was extracted from samples of semitendinosus (ST), triceps brachii (TB), and splenius (SP) muscles. Androgen receptor mRNA was quantified in 200-ng total RNA preparations using an internally standardized reverse transcription (RT) PCR assay. Data were analyzed using 18S ribosomal RNA concentrations as a covariable. Steers had higher AR mRNA levels per RNA unit than bulls (P < .01). Androgen receptor mRNA levels differed between muscles (P < .05), with lowest expression in the SP. The pattern of AR expression differed (P < .05) for each muscle with increasing age. Between 4 and 12 mo of age, AR mRNA levels increased (P < .05) in SP but remained unchanged in the ST and TB. Feeding regimen had no effect on muscle AR expression, but steers exhibiting compensatory growth had higher AR mRNA levels than AL steers (P < .01) or bulls (P < .01). Our results show that AR expression is muscle-specific and may be modulated by circulating testicular hormones. These data suggest that the regulation of AR expression may be linked to allometric muscle growth patterns in cattle and compensatory gain in steers.

Skeletal muscle IGF-I and alpha-actin mRNA responses to increased amino acid availability were investigated in young, rapidly growing steers. Four Holstein steers (208 kg BW) were surgically implanted with an abomasal cannula and jugular catheters and allowed 2 wk to recover. Steers were offered hourly a 43:57 forage:concentrate diet at 95% of ad libitum intake supplemented with continuous abomasal infusion of glucose (to replace 12.5% of metabolizable ad libitum energy intake) for 13 d before the start of abomasal infusion of 67 g of casein N/d. Biopsies of the liver and both semimembranosus muscles were removed and frozen in liquid N, and casein infusion was begun. Muscle biopsies were collected at 8, 16, 24, and 48 h, and on d 7 and 14. Nitrogen balance increased from 23.6 to 71.5 g/d (P < .001) within 24 h and remained elevated (mean = 58.4 g/d) during the 14 d of casein infusion. Plasma urea N increased from 4 to 9.5 mg/dL at 24 h and remained unchanged to d 14. Muscle IGF-I mRNA abundance increased to 215% of basal values at 16 h (P < .01), 244% of basal values at 24 h, and 222% of basal values at 48 h after initiation of casein infusion. Values reached a maximum of 274% of basal values on d 7 and then declined to near preinfusion levels on d 14. The IGF-I mRNA abundance was approximately 100 times higher in liver than in skeletal muscle and was not different on d 0 and 14. Although plasma IGF-I concentrations were numerically higher during the first 24 h of abomasal casein infusion, they were not significantly higher during the chronic phase of treatment. Plasma IGF binding protein (BP)-2 concentrations were higher at 16, 24, and 48 h after casein infusion was begun, but IGFBP-3 concentrations were not altered at these sampling times. Neither acute (first 24 h) nor chronic (daily) plasma insulin concentrations were altered by abomasal casein infusion. Plasma somatotropin concentrations were lower (P = .008) at 24 h of casein infusion and beyond. Results suggest that enhanced amino acid availability may modulate skeletal muscle protein synthesis and accretion through an autocrine or paracrine IGF-I influence.

Double-muscled fetuses develop more muscle fibers than normal-muscled fetuses. To examine whether serum growth factors modulate muscle development in cattle, twin pregnancies were induced in eight Holstein heifers using embryos from Belgian Blue and Holstein genetics representing heavy (HM) and light (LM) muscled cattle, respectively. Twin combinations were 1) two pairs of Belgian Blue fetuses that were designated as HM (HM), 2) two pairs of Holstein fetuses that were designated as LM (LM), and 3) four pairs of mixed fetuses; the four Holstein fetuses were designated as LM (HM) and the four Belgian Blue fetuses were designated as HM (LM). Pregnancies were terminated at 175 +/- 5 d after conception and fetuses, with evidence of vascular anastomosis, were dissected. Carcass weights were greatest (P < .05) for HM fetuses. Total bone and individual femur weights were greatest (P < .05) for LM (LM) fetuses. Total skeletal muscle mass and mass of semitendinosus, quadriceps femoris, infraspinatus, and longissimus muscles were in the order of HM (HM) > HM (LM) > LM (HM) = LM (LM) (P < .05). Estimated apparent muscle fiber number determined from a cross-section of semitendinosus muscle was in the order of HM (LM) > HM (HM) > LM (HM) = LM (LM) (P < .05). These data show that the presence of a co-twinned fetus with a lower genetic propensity for muscle development reduces the capacity of heavily muscled fetuses to develop muscle mass by 175 d after conception and strongly support the idea that blood-borne factors regulate muscle hypertrophy in fetal cattle.

Double muscled (DM) cattle possess nearly 40% more muscle fibers than normal muscled (NM) beef or dairy cattle. Previous work showed that serum from DM fetuses stimulated proliferation of L6 myoblasts to a greater extent than serum from NM fetuses. Although the exact role of insulin-like growth factor-II (IGF-II) in the regulation of fetal myogenesis is unknown, it has been shown to serve as a autocrine-acting growth factor during terminal differentiation of myoblasts in mitogen-depleted culture media. Delay of IGF-II expression may alter the ultimate number of muscle fibers formed during fetal development. To investigate this, forty-seven skeletal muscle and twenty-nine liver samples were collected from NM fetuses representing fetuses grouped by crown-rump lengths (CRL) of < 20, 21-30, 31-40, 41-50, 51-60, 61-70 or > 70 cm. Twelve DM fetuses representing 20, 40, 50, and 85 cm were compared to NM groups with the same CRL. Total RNA preparations from these samples were subjected to northern and dot blot analysis using rat IGF-II and beta actin cDNAs and a human 28S rRNA oligomer. IGF-II transcripts of 4.5, 3.6, 2.75, 2.5, and 1.15 kilobases (kb) were detected in liver and muscle RNA from both DM and NM fetuses. Liver IGF-II expression increased (P < 0.05) in both DM and NM fetuses with CRL. Mean concentrations of muscle IGF-II mRNA initially increased (P < 0.05), then decreased (P < 0.05) with CRL in DM and NM fetuses. Muscle IGF-II mRNA was greater (P < 0.05) for NM fetuses compared to DM fetuses at 20 cm CRL, whereas at 53 cm CRL, DM muscle IGF-II was greater (P < 0.05) than that of NM fetuses. These results show that the maximum expression of muscle IGF-II is delayed in DM fetuses compared to NM fetuses. This delayed expression may play an explicit role in controlling myogenesis in the development of double muscle cattle.

Insulin-like growth factor-II (IGF-II) modulates myogenesis in muscle cell cultures, in utero. IGF-II gene expression is developmentally regulated in several tissues including muscle. Determining whether IGF-II is expressed by developing muscle cells or by neighbouring cells in developing muscle tissue is crucial for determining whether IGF-II exerts a paracrine or an autocrine affect on myogenesis. Semitendinosus muscle samples from 12 bovine fetuses ranging from 60 to 274 days post conception (pc) were analysed for the amount and localization of muscle IGF-II mRNA using Northern, dot blot and in situ hybridization analyses. Northern blot analysis revealed multiple IGF-II transcripts of 5.1, 3.7, 2.6, 2.0, 1.7 and 1.1 kb in developing bovine muscle tissue. The relative amount of muscle IGF-II mRNA increased (P < 0.05) until 162 days pc, then decreased (P < 0.01) to near undetectable levels by the end of gestation (approximately 284 days pc). Between 60 and 162 days pc, in situ hybridization revealed that the majority of the IGF-II transcripts were localized to developing muscle cells rather than connective tissue. After 162 days pc the IGF-II hybridization signal shifted away from muscle cells and greater accumulation was observed in the connective tissue at 274 days pc. These data confirm that the expression of IGF-II in developing bovine muscle tissue is primarily localized in muscle cells and support the claim that IGF-II acts as an autocrine-acting growth factor during myogenesis in vivo.

Double-muscled (DM) cattle possess more muscle fibers than do normal-muscled (NM) beef or dairy cattle. Serum-borne growth factors have been shown to modulate myogenesis. Media containing serum from 12 DM and 60 NM fetuses grouped by crown-rump lengths (CRL) of < or = 25, 26 to 50, 51 to 75, or > 75 cm were used to test the effect of fetal serum on L6 myoblast proliferation. Because DM and NM fetuses were similar in CRL at ages corresponding to 20- to 60-cm CRL, CRL was used to determine fetal age. Normal-muscled fetal serum-induced thymidine incorporation in L6 myoblasts was greater (P < .05) at CRL > 50- than at < or = 50-cm CRL. Mean incorporation tended to increase with CRL. Thymidine incorporation was 56, 41, and 41% greater (P < .05) with serum from DM fetuses than with that from NM fetuses at CRL of < or = 25, 26 to 50, and 51 to 75 cm, respectively. Morphological examination of cross-sections of the semitendinosus muscle showed that apparent muscle fiber number was greater (P < .05) for DM fetuses than for NM fetuses. These results confirm greater apparent muscle fiber number in DM cattle and show the existence of greater growth factor activity in serum of DM fetuses during early fetal development. This greater growth factor activity may play a role in bovine muscle fiber hyperplasia.

Interrelationships among concentrations and maturation of intramuscular collagen, serum concentration of hydroxyproline and testosterone and meat tenderness were determined in growing bulls and steers. Sixty-four Charolais X Angus bulls were assigned to sex treatment groups (intact or castrate) and slaughter groups (9, 12, 15 or 18 mo of age). Animals were bled at 30-min intervals via intrajugular catheters between 0600 and 1400 beginning 48 h before slaughter. Serum concentrations of testosterone were determined in each sample from bulls and from four samples from steers; serum hydroxyproline was determined in the last sample from both sexes. Testosterone mean values for the collection period were calculated. Samples of the longissimus, semitendinosus and infraspinatus muscles secured within 45 min postmortem were analyzed for intramuscular collagen concentration, percent soluble collagen and collagen thermal shrinkage temperature. Tenderness of loin steaks was determined by Warner-Bratzler shear test. Serum concentrations of hydroxyproline and testosterone were higher (P less than .01) in bulls than steers. Age effects were noted for both hydroxyproline (P less than .01) and testosterone (P less than .06). Total intramuscular collagen was greater (P less than .01) in bulls than steers and was different (P less than .01) among muscles, but the muscle differences were not uniform over all ages (P less than .05). Percent soluble collagen declined (P less than .01) with age and was different (P less than .01) among muscles. Interaction of age and muscle (P less than .01) and age and sex (P less than .05) also were noted for percent soluble collagen.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscle shortening and postmortem proteolysis both influence beef tenderness, but their interacting effects on tenderness are relatively unknown. Inherent myofibril structure and the extent of overlap between myosin and actin filaments are hypothesized to affect the availability of substrates for degradation by calpains. The objective of this study was to determine the influence of sarcomere length on the extent of calpain-induced proteolysis of bovine myofibrils in vitro. Bovine semitendinosus muscles were excised within 20 mm postmortem and dissected into strips, which were stretched and attached to applicator sticks or allowed slack to generate samples with different sarcomere lengths upon rigor completion. Samples were allowed to undergo rigor in a neutral pH buffer containing a protease inhibitor. Myofibrils were isolated and incubated at room temperature with excess exogenous μ-calpain at a ratio of 1:800 (wt/wt; enzyme:myofibrillar protein) at pH 6.8 for 0, 2, 60, 1,440, or 2,880 mm. Purified troponin was subjected to the same digestion conditions. Proteolysis of troponin T (TnT) was monitored using SDS-PAGE and Western blotting. Sarcomere length was greater (P < 0.0001) in stretched versus shortened samples (2.99 μm ± 0.03 vs. 2.12 ± 0.03 pm, respectively, means ± SE). Western blots for both stretched and shortened samples exhibited bands corresponding to intact TnT and TnT fragments. The abundance of intact TnT decreased (P < 0.0001) with incubation time across both treatments. At 1,440 and 2,880 min, less (P < 0.05) intact TnT was detected in samples with long sarcomeres. These data indicate proteolysis of TnT occurs to a greater extent in samples with longer sarcomeres, possibly due to easier access of proteases to their targeted substrates. Degradation patterns of TnT were qualitatively similar between myofibrils and purified troponin after incubation with μ-calpain. Therefore, it is unlikely that the mechanism by which proteolysis is limited in short sarcomeres involves an actomyosin-mediated interference of TnT. [ABSTRACT FROM AUTHOR]