Thyroid peroxidase activity, hepatic glucose-6-phosphate dehydrogenase activity and corticosterone level in plasma and tissues of rats fed different dietary fats

Abstract

Thyroid and adrenal gland activity were evaluated together with liver lipogenic activity in rats fed diets containing different fats. After one- week of a fat- free regimen, male Wistar rats weighing 100-120 g (at the beginning of the experiment) were divided into four groups fed diets with different fats: L - lard, S - sunflower oil, P - palm oil, R - rape seed oil. The activity of thyroid peroxidase (TPO), the key enzyme in thyroid hormone biosynthesis, corticosterone (Cs) concentration in plasma, the adrenal glands, aorta, and heart, plasma total cholesterol concentration and glucose-6-phosphate dehydrogenase (G-6-PD) activity in the liver, as an index of lipogenic activity, were determined after fat-free feeding and after one and three weeks of fat-supplying diets. TPO activity increased after one week on fat-containing diets fed to groups S and P; after three weeks it was higher than after the fat-free period in groups P, S and L and directly related to palmitic acid intake and plasma total cholesterol concentration. Various patterns of changes in Cs concentration in plasma and tissues were observed suggesting an influence of dietary fat composition on hormone distribution. After three weeks of feeding fat, Cs concentration in plasma was lower than after the fat-free regimen and did not differ among dietary groups, while in tissues it was dependent on the type of dietary fat. Tissue Cs was higher in the adrenals in groups P and R, in the aorta in group R, and in the heart in groups L and S. After three weeks of feeding fat the Cs concentration in the aorta of groups S and L, and in the adrenals in group S was lower than after the fat- free regimen, whereas in the heart it was higher in all groups. The decrease in hepatic G-6-PD activity after introducing diets with fats depended on the kind of dietary fat; it decreased to lower levels in groups S and R than in groups L and P. The results obtained indicate that dietary fat composition can affect thyroid activity, hepatic lipogenesis and corticosterone secretion and/or distribution in different manners. This suggests that, among others, metabolic effects of dietary fat can result from changes in relations between thyroid and adrenal gland activities.