Organ and cellular contributions to heat production in Brahman and Angus steers

Abstract Reductions in basal metabolism and growth rate appear to contribute to decreased heat production of Bos indicus breeds. Lower metabolic rate may be due to smaller organ size and lower metabolic activity. Liver and heart account for high percentage of metabolic activity relative to their weight, and at a cellular level, mitochondria are responsible for producing energy for cellular maintenance. Our objective was to determine liver and heart weight and evaluate expression of key mitochondrial proteins in Bos indicus (80–100% Brahman) and Bos taurus (80–100% Angus) steers. Steers (n = 14 per breed) were harvested at the university meat laboratory. After evisceration, livers and hearts were weighed, and samples were frozen in liquid nitrogen for further analyses. Western blotting was used to determine expression of proteins in mitochondrial energy production: citrate synthase, a marker of mitochondria content; succinate dehydrogenase B (SDH-B), complex II; cytochrome c oxidase subunit IV, complex IV; and ATP synthase subunit 5A (ATP5A), complex V. Data were analyzed using an unpaired t-test in SAS-JMP. Although steers were similar age at slaughter (P = 0.84), Brahman steers weighed less (P < 0.0001) and thus produced lighter carcasses (P < 0.0001) and smaller livers and hearts (P < 0.0001). On a relative basis (kg organ/kg BW), Brahman tended to possess smaller hearts (P = 0.06) and smaller livers (P = 0.03). Expression of mitochondria proteins in heart did not differ between breeds (P > 0.3). However, Brahman exhibited lower expression of SDH-B (P = 0.005) but tended to have higher expression of ATP5A (P = 0.07) in liver. Altogether, this supports that smaller organ size is a contributing factor to lower heat production in heat-tolerant Brahman steers, and shifts in mitochondria protein expression in liver may impact energy metabolism at the cellular level.