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Rising temperatures significantly impact cattle production economically, reducing feed intake, efficiency, and growth, leading to substantial losses. Despite these known effects, there remains a paucity of data on the specific responses of skeletal muscle and adipose tissue to heat stress conditions. Therefore, this study aimed to elucidate the effects of prolonged heat stress on the molecular mechanisms of skeletal muscle growth and adipose tissue deposition and physiological response. Thirty-two predominantly Angus beef steers [initial average body weight (BW) = 364 ± 4.60 kg] were subject to a 28-d heat stress period. Steers were randomly allotted to one of two treatment groups (n = 16 steers/treatment) 1) Control (CON), exposed to outdoor environmental temperature-humidity index (THI) of 60; or 2) Heat Stress (HS), subject to THI of 70 to 80 for 8 h then returned to control treatment THI to simulate typical cooldown periods exhibited in instances of HS. Feed intakes were recorded daily, while BW was measured weekly prior to feed delivery. Biopsies of the skeletal muscle from the Longissimus muscle (LM) at the area of the 13th rib and adipose tissue from the subcutaneous fat of the flank region were performed on d 1, 14, and 28 for each individual steer and used for molecular analysis. For gene expression, a repeated measures ANOVA was utilized, acknowledging the individual animal as the experimental unit. The analysis of both gene expression over time and growth performance indicators was conducted using repeated measures ANOVA, with post-hoc analysis via Tukey’s test for specific comparisons. Data presentation includes the standard error of means, with an alpha (α) level set at 0.05 for determining statistical significance. Behavioral signs of heat stress, elevated respiratory rates, were observed with HS steers exhibiting a respiratory rate of 92.92 ± 24.79 breaths per minute (bpm) compared with 60.48 ± 14.94 bpm in CON steers (P < 0.01). Heat stress decreased average daily gain (ADG) during d 0 to 7 (49.11%) and d 15 to 21 (59.14%) compared with the control group (P < 0.01). Dry Matter Intake (DMI; P > 0.1) and feed efficiency (P > 0.1) were not altered in CON vs. HS steers. In skeletal muscle, on d 28 of exposure to heat stress, there was a significant overexpression of Heat Shock Protein (HSP) isoforms 20, 27, and 90 (P < 0.05) relative to the control group, accompanied by increased Myf5 (P < 0.05) gene expression. In adipose tissue, there was upregulation of PPARγ and SCD (P < 0.05), both of which are implicated with adipose tissue differentiation and lipid deposition process. Our findings indicated that HS modifies the expression of genes associated with skeletal muscle growth and adipose tissue deposition, concurrently impairing overall animal growth predominantly through reduced feed intake. [ABSTRACT FROM AUTHOR]

Guanidinoacetic acid (GAA) is an amino acid-like compound that acts as a precursor to creatine (CRE) when either made in the body or digested. Creatine is directly used by skeletal muscle tissue as an energy source with the potential to act as a growthpromoting agent. The study objectives were to evaluate the effects of various CRE concentrations on bovine satellite cells for the rate of proliferation and myogenic differentiation. Satellite cells (SC) were isolated from the Longissimus muscle of 3-month-old Holstein bulls (n = 5). Cells were seeded at a density of 1 × 104 cells/mL on 96-well plates and incubated with 0 (CON), 2.5, 5, 7.5, and 10 mM of CRE. The absorbance rate was measured at 0, 24, 48, 72, and 96 h of incubation. Cells for gene expression were plated on 6-well plates at a density of 1 × 105 in 10% FBS/DMEM. Following 24 h of adhesion, SC were incubated with 5 and 7.5mM CRE for 48 h. After which, cells were harvested after 48 h for Pax3, Pax7, and Myf5 mRNA gene expression analysis. To identify the effects of CRE during myogenic differentiation, SCs were induced myogenic differentiation. Upon reaching approximately 90% confluency, the growth medium was replaced by differentiation media (2% horse serum/DMEM) containing 0 (CON), 5, and 7.5 mM CRE. A two-way ANOVA and Tukey's HSD test were conducted regarding cell proliferation. A one-way ANOVA and Tukey's HSD test were performed for gene expression. Interaction of treatment against time was noted (P < 0.01) in Con, 2.5, 5, 7.5, and 10 mM CRE. Cell proliferation rate was influenced by CRE treatment (P < 0.01). At 72 h of incubation, all CRE treatments increased cell proliferation rate compared with Con (P < 0.05). At 96 h of incubation, cells treated with 2.5, 5, and 7.5 mM of CRE increased proliferation compared with Con (P < 0.05). At 96 h, CRE treatment 2.5, 5, and 7.5 mM increased proliferation rate compared with 10mM CRE treated cells (P < 0.05). No treatment effect was found in the mRNA gene expression of Pax7, Pax3, and Myf5 (P > 0.05). Cells incubated with CRE showed no change in mRNA gene expression of IGF-1 and Myogenin (P > 0.05), and a tendency was noted for S6KB1 increase expression under treatment (P = 0.0643). S6KB1 is a protein kinase that, through mTOR pathway expression, leads to an increase in cell growth, protein synthesis, and cell proliferation. Results suggest that the treatment of BSCs with CRE impacted proliferation, CRE at 5mM vs. 7.5 mM concentrations treatment of differentiated muscle cells led to a trend in altering S6KB1 gene expression. [ABSTRACT FROM AUTHOR]

Understanding the myogenic and self-renewal capability of satellite cells under heat stress (HS) is fundamental to coping with global warming in the beef cattle industry. Study objectives were to determine transcriptomic changes and proliferation capability of bovine satellite cells (BSCs) under moderate versus acute levels of HS and exposure times. Satellite cells were isolated from four three-month-old Holstein calves (semitendinosus) and cultured with 10% FBS/DMEM. Following 24h of attachment period, BSCs were incubated under the following temperature conditions 1) control (38°C; Con), 2) moderate (39.5°C; MHS) and extreme (41°C; EHS) and for various exposure times (0, 1, 3, 6, 12, 24, and 48 h). Cells were harvested and used for mRNA gene expression via RT-qPCR at each time point. Cell proliferation was measured using the ELISA kit. Data were analyzed by two-way ANOVA and Tukey HSD test. The expression of HSP70 was greatest (P < 0.05) in cells treated EHS at 1 and 3 h. Cells treated EHS overexpressed (P < 0.05) HSPB1 at 3, 6, 12, and 24 h. A similar pattern was observed (P < 0.05) for MyoD expression at 24 h after heat exposure. Contrarily, Myf5 and PAX3 were decreased (P < 0.05) by EHS compared to the Con at 3 h. Cell proliferation rate was enhanced (P < 0.01) in both MHS and EHS treated BSCs compared withthe Con at 48 h. Our results suggest that extreme heat exposure stimulates the HSPs (HSP70 and HSPB1) expression while modulating muscle regeneration by regulating myogenic regulatory factors (MyoD, Myf5) and PAX3. The transcriptional effect peaked after 3h of heat exposure, but the long-term (48h) exposure may activate the proliferation of BSCs. [ABSTRACT FROM AUTHOR]

Creatine (CRE) is an energy-storing molecule of skeletal muscle and is reported as potential growth promotants. With Guanidinoacetic acid (GAA) being a precursor to creatine, the potential to evaluate supplemental GAA's role in the body from a skeletal muscle focus is needed within the beef industry. Study objectives were to evaluate the effects of CRE and GAA on bovine satellite cells (BSCs) proliferation and myogenic potential. BSCs were obtained from three-month-old Holstein calves (n=3), harvested, and incubated with 10% FBS/DMEM. Following 24h of cell adhesion, BSCs were incubated with treatments: 1) Control, 2) 10mM GAA, and 3) 5mM CRE. Cell proliferation rate was measured at 0, 24, 48, 72, and 96h. In order to identify the effects of CRE and GAA during myogenic differentiation, satellite cells were incubated in a differentiation medium (2% horse serum/DMEM) containing 10mM GAA and 5mM CRE. After 48h, cells were harvested and used for mRNA gene expression (Myh7, MyoG, IGF-1, and S6KB1). One-way ANOVA and Tukey's HSD test were used for gene expression. Two-way ANOVA and Tukey's HSD test were conducted for cell proliferation assay. No interaction effect was noted (P > 0.05) between the effects of treatment against time in end output of cell proliferation. Cell proliferation rate was not altered (P > 0.05), with proliferation rate time measurement having a greater result of cell proliferation (P < 0.001). The expression of IGF-1 was overexpressed (P < 0.05) in GAA treated BSC during the myogenic differentiation. CRE increased (P < 0.01) MyoG expression. GAA and CRE did not alter Myh7 orS6KB1 mRNA gene expression. Results suggest that the treatment of BSCs with creatine or GAA may improve myogenic differentiation and promote skeletal muscle growth via IGF-1, respectively. [ABSTRACT FROM AUTHOR]