Biological sex impacts oxidative stress in skeletal muscle in a porcine heat stress model

Oxidative stress contributes to heat stress (HS)-mediated alterations in skeletal muscle; however, the extent to which biological sex mediates oxidative stress during HS remains unknown. We hypothesized muscle from males would be more resistant to oxidative stress caused by HS than muscle from females. To address this, male and female pigs were housed in thermoneutral conditions (TN; 20.8 ± 1.6 °C; 62.0 ± 4.7% relative humidity; n = 8/sex) or subjected to HS (39.4 ± 0.6 °C; 33.7 ± 6.3% relative humidity) for 1 (HS1; n = 8/sex) or 7 days (HS7; n = 8/sex) followed by collection of the oxidative portion of the semitendinosus. While HS increased muscle temperature, by 7 d, muscle from heat-stressed females was cooler than muscle from heat-stressed males (0.3 °C; p<0.05). Relative protein abundance of 4-HNE-modified proteins increased in HS1 females compared to TN (p=0.05). Further, MDA-modified proteins and 8-OHdG concentration, a DNA damage marker, was increased in HS7 females compared to TN females (p=0.05). Enzymatic activities of catalase and SOD remained similar between groups; however, GPX activity decreased in HS7 females compared to TN and HS1 females (p≤0.03) and HS7 males (p=0.02). Notably, HS increased skeletal muscle Ca2+ deposition (p=0.05) and was greater in HS1 females compared to TN females (p<0.05). Heat stress increased SERCA2a protein abundance (p<0.01); however, Ca2+ ATPase activity remained similar between groups. Overall, despite having lower muscle temperature, muscle from heat-stressed females had increased markers of oxidative stress and calcium deposition than muscle from males following identical environmental exposure.

Heat stress alters hematological parameters in barrows and gilts

The purpose of this investigation was to establish the role biological sex plays on circulating factors following heat stress (HS). Barrows and gilts (36.8 ± 3.7 kg body weight) were kept in either thermoneutral (TN; 20.8 ± 1.6 °C; 62.0 ± 4.7 % relative humidity; n = 8/sex) conditions or exposed to HS (39.4 ± 0.6 °C; 33.7 ± 6.3 % relative humidity) for either 1 (HS1; n = 8/sex) or 7 (HS7; n = 8/sex) days. Circulating glucose decreased as a main effect of environment (P=0.03). Circulating non-esterified fatty acid (NEFA) had an environment × sex interaction (P<0.01) as HS1 barrows had increased NEFA compared to HS1 gilts (P=0.01) and NEFA from HS7 gilts increased compared to HS1 gilts (P=0.02) and HS7 barrows (P=0.04). Cortisol, insulin, glucagon, T3 and T4 were reduced as a main effect of environment (P≤0.01). Creatinine was increased in HS1 and HS7 animals compared to TN (P≤0.01), indicative of decreased glomerular filtration rate. White blood cell populations exhibited differential patterns based on sex and time. Neutrophils and lymphocytes had an environment × sex interaction (P≤0.05) as circulating neutrophils were increased in HS1 barrows compared to TN and HS7 barrows, and HS1 gilts (P≤0.01) and HS7 barrows had less neutrophils compared to TN barrows (P=0.01), whereas they remained similar in gilts. In contrast, barrow lymphocyte number was similar between groups, but in HS7 gilts they were decreased compared to TN and HS1 gilts (P≤0.04). In total, these data demonstrate that HS alters a host of circulating factors and that biological sex mediates, at least in part, the physiological response to HS.

Effects of heat stress on markers of skeletal muscle proteolysis in dairy cattle

Heat stress (HS) markedly affects postabsorptive energetics and protein metabolism. Circulating urea nitrogen increases in multiple species during HS and it has been traditionally presumed to stem from increased skeletal muscle proteolysis; however, this has not been empirically established. We hypothesized HS would increase activation of the calpain and proteasome systems as well as increase degradation of autophagosomes in skeletal muscle. To test this hypothesis, lactating dairy cows (∼139 d in milk; parity ∼2.4) were exposed to thermal neutral (TN) or HS conditions for 7 d (8 cows/environment). To induce HS, cattle were fitted with electric blankets for the duration of the heating period and the semitendinosus was biopsied on d 7. Heat stress increased rectal temperature (1.3°C) and respiratory rate (38 breaths per minute) while it decreased dry matter intake (34%) and milk yield (32%). Plasma urea nitrogen (PUN) peaked following 3 d (46%) and milk urea nitrogen (MUN) peaked following 4 d of environmental treatment and while both decreased thereafter, PUN and MUN remained elevated compared with TN (PUN: 20%; MUN: 27%) on d 7 of HS. Contrary to expectations, calpain I and II abundance and activation and calpain activity were similar between groups. Likewise, relative protein abundance of E3 ligases, muscle atrophy F-box protein/atrogin-1 and muscle ring-finger protein-1, total ubiquitinated proteins, and proteasome activity were similar between environmental treatments. Finally, autophagosome degradation was also unaltered by HS. Counter to our hypothesis, these results suggest skeletal muscle proteolysis is not increased following 7 d of HS and call into question the presumed dogma that elevated skeletal muscle proteolysis, per se, drives increased AA mobilization.

Environment-induced heat stress causes structural and biochemical changes in the heart

Prolonged exposure to heat can lead to environment-induced heat stress (EIHS), which may jeopardize human health, but the extent to which EIHS affects cardiac architecture and myocardial cell health are unknown. We hypothesized EIHS would alter cardiac structure and cause cellular dysfunction. To test this hypothesis, 3-mo old female pigs were exposed to thermoneutral (TN; 20.6 ± 0.2 °C; n = 8) or EIHS (37.4 ± 0.2 °C; n = 8) conditions for 24 h, hearts were removed and dimensions measured, and portions of the left ventricle (LV) and right ventricle (RV) were collected. Environment-induced heat stress increased rectal temperature 1.3 °C (P < 0.01), skin temperature 11 °C (P < 0.01) and respiratory rate 72 breaths per minute (P < 0.01). Heart weight and length (apex to base) were decreased by 7.6% (P = 0.04) and 8.5% (P = 0.01), respectively, by EIHS, but heart width was similar between groups. Left ventricle wall thickness was increased (22%; P = 0.02) and water content was decreased (8.6%; P < 0.01) whereas in RV, wall thickness was decreased (26%; P = 0.04) and water content was similar in EIHS compared to TN. We also discovered ventricle-specific biochemical changes such that in RV EIHS increased heat shock proteins, decreased AMPK and AKT signaling, decreased activation of mTOR (35%; P < 0.05), and increased expression of proteins that participate in autophagy. In LV, heat shock proteins, AMPK and AKT signaling, activation of mTOR, and autophagy-related proteins were largely similar between groups. Biomarkers suggest EIHS-mediated reductions in kidney function. These data demonstrate EIHS causes ventricular-dependent changes and may undermine cardiac health, energy homeostasis, and function.

The effect of Mitoquinol (MitoQ) on heat stressed skeletal muscle from pigs, and a potential confounding effect of biological sex

Heat stress (HS) poses a major threat to human health and agricultural production. Oxidative stress and mitochondrial dysfunction appear to play key roles in muscle injury caused by HS. We hypothesized that mitoquinol (MitoQ), would alleviate oxidative stress and cellular dysfunction in skeletal muscle during HS. To address this, crossbred barrows (male pigs) were treated with placebo or MitoQ (40 mg/d) and were then exposed to thermoneutral (TN; 20 °C) or HS (35 °C) conditions for 24 h. Pigs were euthanized following the environmental challenge and the red portion of the semitendinosus (STR) was collected for analysis. Unexpectedly, malondialdehyde concentration, an oxidative stress marker, was similar between environmental and supplement treatments. Heat stress decreased LC3A/B-I (p < 0.05) and increased the ratio of LC3A/B-II/I (p < 0.05), while p62 was similar among groups suggesting increased degradation of autophagosomes during HS. These outcomes were in disagreement with our previous results in muscle from gilts (female pigs). To probe the impact of biological sex on HS-mediated injury in skeletal muscle, we compared STR from these barrows to archived STR from gilts subjected to a similar environmental intervention. We confirmed our previous findings of HS-mediated dysfunction in muscle from gilts but not barrows. These data also raise the possibility that muscle from gilts is more susceptible to environment-induced hyperthermia than muscle from barrows.