Effect of thyroid status in the perinatal period on oxidative capacities and mitochondrial respiration in porcine liver and skeletal muscle.
Regulatory thermogenesis is reduced in newborn piglets which have been made hypothyroid during late gestation by giving the sow a high glucosinolate rapeseed diet (test animals). Thereafter, the progressive increase in thermogenic capacity parallels the development of a marked postnatal hyperthyroid state. To explain these effects of thyroid hormones at the tissue and mitochondrial levels, we have examined both liver and skeletal muscle to determine possible underlying changes in (i) tissue oxidative capacities (cytochrome oxidase (CO) activity), between 80 d of gestation and 48 h after birth, and (ii) mitochondrial content and respiratory capacities at 24 h of life. In control piglets, CO activity increased sharply during late gestation and the first 2 d of life in liver and rhomboideus (RH) muscle (P < 0.01), whereas only a prenatal increase was observed in longissimus dorsi (LD) muscle. Test fetuses were hypothyroid and had lower CO activities than controls during late gestation in RH muscle (P < 0.06, at 110 d of gestation; P < 0.08, at birth) and in liver (P < 0.001, at birth). The postnatal increase in CO activity in RH muscle and liver was higher (P < 0.05) in test than in control piglets, and as a result the difference between the 2 groups was not significant by 24-48 h of life. There was no effect of treatment on LD muscle. At 24 h, hyperthyroid test piglets had lower amounts of mitochondrial proteins than controls (P < 0.05) in all three tissues, possibly reflecting reduced mitochondrial protein synthesis during fetal life and suggesting that high postnatal T3 levels did not bring about major increases in protein synthesis within 24 h. However, test piglets exhibited higher rates of mitochondrial respiration than controls in liver and RH muscle, as shown by increases in State III and FCCP-stimulated respirations (P < 0.05), and mitochondrial CO and creatine kinase activities (P < 0.05). In RH muscle, both subsarcolemmal and intermyofibrillar mitochondria showed the same trends. No changes were observed in LD muscle. Our results describe for the first time the effect of thyroid hormones on perinatal oxidative capacities and neonatal mitochondrial respiration in liver and skeletal muscle of the pig, through both the short-term regulation of mitochondrial respiration and the long-term control of mitochondrial biogenesis. The differential sensitivity of LD and RH muscles to thyroid hormones is discussed.