Heat Stress Increases Mitochondrial Complex I Capacity in Female Pigs but Favors Reliance on Complex II in Males

Abstract

Heat stress (HS) negatively impacts animal health and impairs growth, but little is known about sex‐specific responses of skeletal muscle mitochondria to HS. To test the hypothesis that HS would have more adverse effects on mitochondria in females than males, samples were collected from the oxidative portion of the semitendinosus muscle of 3‐month‐old female and castrated male pigs under thermoneutral (TN) conditions or after 1 or 7 d of HS at 39.4°C (n=8/sex/group). Mitochondrial volume density and function were determined via citrate synthase (CS) and cytochrome c oxidase activities. Mitochondrial oxidative phosphorylation (P) and electron transfer (E) capacities were evaluated by high resolution respirometry. Data were analyzed using mixed linear models in SAS v9.4 with treatment, sex, and treatment × sex as fixed effects. Overall, integrative (relative to tissue weight) maximal P with complex I and II (PCI+II) and E (ECI+II) were greater after 1 and 7 d of HS compared to TN conditions (P≤0.05). The contribution of PCI+II to maximal E (flux control ratio; FCRPCI+II) was greater in TN pigs compared to either HS group (P≤0.03), suggesting decreased efficiency of mitochondrial energy production following HS. However, there appeared to be some adaptation to heat over time as FCRPCI+IIwas greater in pigs that were heat stressed for 7 d compared to 1 d (P=0.05). As hypothesized, some measures differed by sex. Specifically, in females, intrinsic (relative to CS activity) complex I‐supported P (PCI) tended to be greater following 7 d of HS compared to TN (P=0.07); PCI was unaffected by heat in males, resulting in females having greater intrinsic PCI than males following 7 d of HS (P=0.05). The contribution of PCI to total E (FCRPCI) was lesser with 7 d HS than 1 d or TN conditions in male pigs (P≤0.01) but unchanged in females, resulting in females also having greater FCRPCIthan males following 7 d of HS (P=0.008). Conversely, the contribution of electron transfer supported by complex II to total E (FCRECII) was greater in TN compared to 1 d of HS in males (P=0.004) but then increased at 7 d of HS (P=0.02) so that TN and 7 d HS were similar. In females, FCRECII tended to be lesser following 7 d of HS compared to TN, resulting in a trend for males to have greater FCRECII than females after 7 d of HS (P=0.08). Finally, males tended to have greater FCRPCI+IIthan females (P≤0.08). These results suggest HS increased mitochondrial utilization of complex I in females but reliance on complex II in males, resulting in more efficient mitochondria following HS in males than in females. Therefore, skeletal muscle in females may be more susceptible to negative cellular impacts of HS.