Effects of tri-iodothyronine administration on the disposal of oral [1-14C]triolein, lipoprotein lipase activity and lipogenesis in the rat during lactation and on removal of the litter

Hyperthyroidism affects lipid metabolism in lactating and suckling rats

Two per thousand pregnant women have hyperthyroidism (HT), and although the symptoms are attenuated during pregnancy, they rebound after delivery, affecting infant development. To examine the effects of hyperthyroidism on lactation, we studied lipid metabolism in maternal mammary glands and livers of hyperthyroid rats and their pups. Thyroxine (10 microg/100 g body weight/d) or vehicle-treated rats were made pregnant 2 wk after commencement of treatment and sacrificed on days 7, 14, and 21 of lactation with the litters. Circulating triiodothyronine and tetraiodothyronine concentrations in the HT mothers were increased on all days. Hepatic esterified cholesterol (EC) and free cholesterol (FC) and triglyceride (TG) concentrations were diminished on days 14 and 21. Lipid synthesis, measured by incorporation of [3H]H2O into EC, FC, and TG, fatty acid synthase, and acetyl CoA carboxylase activities increased at day 14, while incorporation into FC and EC decreased at days 7 and 21, respectively. Mammary FC and TG concentrations were diminished at day 14; incorporation of [3H]H2O into TG decreased at days 7 and 21, and incorporation of [3H]H2O into FC increased at day 14. In the HT pups, growth rate was diminished, tetraiodothyronine concentration rose at days 7 and 14 of lactation, and triiodothyronine increased only at day 14. Liver TG concentrations increased at day 7 and fell at day 14, while FC increased at day 14 and only acetyl CoA carboxylase activity fell at day 14. Thus, hyperthyroidism changed maternal liver and mammary lipid metabolism, with decreased lipid concentration in spite of increased liver rate of synthesis and decreases in mammary synthesis. These changes, along with the mild hyperthyroidism of the litters, may have contributed to their reduced growth rate.

Alterations in myocardial lipid metabolism during lactation in the rat

Metabolism of nonesterified fatty acid (palmitate, 1.1 mM) and triacylglycerol (TAG; triolein, 0.4 mM in the form of both rat chylomicrons and very low density lipoproteins) was studied in isolated perfused working hearts from fed nulliparous, lactating, and weaned rats. Hearts from virgin rats oxidized palmitate readily, but optimal cardiac mechanical performance occurred during perfusion with chylomicrons. In hearts from lactating dams, there was a significant increase in palmitate oxidation and a marked decrease in TAG oxidation from both chylomicrons and very low density lipoproteins compared with hearts from nulliparous animals. There was a concomitant decrease in lipoprotein lipase activity in hearts from lactating animals, and TAG in the absence of palmitate could not support optimal cardiac mechanical function. After litter removal, the changes in fatty acid and TAG metabolism observed in lactation returned to nulliparous values within 96 h. These results suggest that, during lactation, both exogenous and endogenous TAGs are directed away from heart and toward the lactating mammary gland; the heart, therefore, has to rely to a greater extent on nonesterified fatty acid for energy provision under these conditions.

Triiodothyronine-induced hyperthyroidism and reproduction in female rats

The effect of L-triiodothyronine (L-T3)-induced hyperthyroidism on reproduction in female rats was studied. Treatment groups used were normal control, 1, 2, 4, 8, and 16 micrograms L-T3/100 g body weight (b.w.)/day. Subcutaneous injections of L-T3 began the first day of estrus until day 1 or day 19 of lactation. All rats given 1, 2, or 4 micrograms L-T3 had regular estrous cycles and normal number of pregnancies. Levels of L-T3 at 8 or 16 micrograms results in abnormal estrous cycles. Only one pregnancy occurred in the 8 micrograms group and no rats became pregnant in the 16 micrograms group. While groups on 1 or 2 micrograms L-T3 had normal parturition, groups on 4 micrograms or more showed signs of dystocia. Only the 1 microgram L-T3 group lactated to day 20, but at 67% of normal control. All levels above 1 micrograms showed loss of pups by day 2 of lactation due to lack of milk production, loss of maternal behavior, and cannibalism. Mammary gland wet and dried fat-free tissue (DFFT) weights were decreased in all treated groups except 1 microgram L-T3. The wet and DFFT weights of thyroids and uteri on days 2 or 20 of lactation were not altered by L-T3 treatment, but levels above 1 microgram increased both wet and DFFT weights of adrenal glands and ovaries. The data indicate that the no observed adverse effect level (NOAEL) of L-T3 for female rat reproduction would be below 1 microgram/100 g b.w./day. L-T3 treatment at 1 microgram/100 b.w./day or above is detrimental. These effects may be mediated through a hormonal imbalance due to the accelerated metabolic rate in reproductive tissues associated with hyperthyroidism.