Fat mobilization in Pietrain and Large White pigs

1. Two experiments were done with Pietrain and Large White pigs (about 50 kg body-weight) to determine whether fat mobilization is enhanced in the stress-sensitive Pietrains (which also produce pale, soft, exudative (PSE) meat).2. In Expt 1, plasma glucose and insulin concentrations and free fatty acid (FFA) concentrations and composition were measured in five Pietrains and five Large Whites after feeding, during an infusion of norepinephrine (2.5 μg/kg body-weight per min), 16 and 21 h after the withdrawal of food and following insulin administration (0.3 IU/kg body-weight). The entry rate of oleic acid was measured 4 h after feeding. Body composition and longissimus dorsi pH measurements were made on killing the pigs 4 d after the experiment.3. After feeding, the Pietrains tended to have lower concentrations of glucose and insulin in plasma compared with the Large Whites, and higher concentrations of FFA. The Pietrains also had a faster entry rate of oleic acid into body tissues. In fasting (16 h) these differences in hormone and metabolite concentrations were also present and relationships between them and body development could be detected. Pietrains had higher concentrations of FFA and lower concentrations of insulin than Large Whites at a particular stage of the development of fat and muscle (subcutaneous fat weight÷longissimus muscle weight).4. The fatty acid composition of FFA resembled that of the backfat triglycerides in fasting and during norepinephrine infusion but not in the fed state. In particular the proportion of fatty acid 18:1 was low in the fed pigs and that of 18:2 was high. The contrast in FFA composition between the fed and stimulated state was greater in the Large Whites.5. The Pietrains were less sensitive to the antilipolytic action of insulin. Glucose and FFA concentrations were similar in both breeds during the norepinephrine infusion although insulin concentrations were increased to a larger extent in the Large Whites at the termination of the infusion.6. In Expt 2, anaesthetized Pietrain and Large White pigs were given constant infusions of norepinephrine alone (2.5 μg/kg body-weight per min) and norepinephrine+propranolol (2 and 10 μg/kg body-weight per min) or phentolamine (2 and 10 μg/kg body-weight per min). The aim was to determine whether the breeds differed in their sensitivity to norepinephrine when conscious responses to the hormone were prevented.7. When norepinephrine alone was infused, fat mobilization was greater in the Pietrain pigs and glucose concentrations were greater in the Large Whites. Propranolol markedly reduced the lipolytic action of norepinephrine, particularly in the Pietrain pigs studied, and phentolamine reduced its glycogenolytic action, particularly in the Large White pig.8. It seems that the leanness of stress-sensitive, PSE-susceptible Pietrain pigs may be due to an enhanced fat mobilization under various conditions, associated with an impairment in insulin metabolism and a greater sensitivity to the β-adrenergic action of catecholamines on body fat stores.