Thyroxine Secretion Rates During Food Restriction in Rats.

Abstract

Until recently studies of changes in thyroid gland function as a result of various degrees of food restriction were based upon morphological evidence ( 1 ) , studies of altered 1131 uptake ( 2 ) , and upon thyrotropin content of blood(3). With availability of a direct method of estimating thyroxine secretion rate (TSR)(4) it is possible to obtain quantitative data on influence of various degrees of food restriction on TSR.In studies of factors influencing TSR it is important to be able to differentiate the effect of experimental procedure per se and restriction of food consumption which may be associated with experimental conditions or endocrine gland removal. Reduction in TSR in mice associated with degrees of feed restriction has been reported(5). The present paper reports upon effect of feed restriction upon TSR of rats.Materials and methods. Mature female Sprague-Dawley-Rolfsmeyer rats weighing approximately 250 g were maintained at temperature of 78 L -1°F with artificial lighting provided during daylight hours. Control TSR values were determined while animals were on ad Zibitum diet. TSR determinations were repeated beginning 3 and 17 days after initiation of food restriction in each animal. The end-point of each determination required up to 12 days. Animals in lower TSR range reached end-point earlier than in higher range and were restricted for a shorter period. Each animal received 2 pc carrier free 1131 and was allowed 48 hrs for uptake. Counts were made at 48-hr intervals and thyroxine (T4) dosage was begun at 0.25 pg/lOO g BW and without blockage of thyroidal 1131 release (95% previous dose) increased by 0.25 pg at every counting interval. Purina Lab Chow in granular form was weighed out daily according to individual initial weights of animals. An 8, *Contribution from Mo. Agr. Exp. Sta. Journal t This investigation was supported in part by a Series No. 2427. Approved by Director. grant from U. S. Atomic Energy Commission.17, 22 and 48% reduction from the normal mean value of 5.26 g/100 g BW was given to each animal(6). Body weights were taken on alternate days.Results. Reduction of food intake depressed TSR depending on degree of food restriction in almost every case (Table I). During the first interval (3-14 days), an 8% reduction in food intake had no effect on TSR as shown by a non-significant increase in TSR (t = 0.274). Reductions in food intake of 17% resulted in an 8% reduction of TSR as compared to control value. A 2276 food reduction depressed TSR by 13%. These reductions are non-significant. However, a reduction of 48% resulted in a 2870 depression of TSR which was significant (t = 2.3).During second interval of food restriction ( 17 to 2 7 days) , more profound reductions in TSR were apparent. A 23% reduction in TSR occurred with an 8% reduction of food intake during this period. When food intake was reduced 1770, TSR was depressed by 15%. Statistically, these reductions in TSR are not significant. A 22% reduction of food results in a significant (t = 2.445) 42% depression in TSR while a ...

Responses of Thyroid Activity to Feed Restriction in the Goat

“…Feed restriction in both series of experiments resulted in significantly decreased thyroxine secretion rates per animal. Similar results have been reported for feed restriction in neonatal rats (Shambaugh and Wilber 1974), adult rats (Grossie and Turner 1962) and cattle (Post 1965). Whether the decreased rate of secretion from the thyroid observed during feed restriction can be considered due to a decrease in TSH secretion, or a lack of response of the thyroid to circulating TSH, or both, is unclear.…”

“…Studies on the effects of feed restriction, malnutrition or starvation on thyroid function in mammals have lead to the general conclusion of a depression in secretion (see Milner 1972); Measurements of thyroid secretion rate in rats and cattle with restriction offeed showedsignificant decreases (Grossie and Turner 1962;Post 1965). The cause of this decrease is not clearly established, since measurements of circulating TSH in rats and humans on inadequate diets which depress thyroid secretion do not necessarily show reduced TSH concentrations (Srebnik et al 1963;Florsheim et al 1970;Chopra and Smith 1975).…”

Effect of various protein levels in the diet on mammary gland growth in rats

“…Since the growth of the mammary gland depends on the metabolic hormones, it has been assumed that changes in the level of secretion of these hormones may also influence mammary gland growth. Thus, most researchers have been engaged in assessing the levels of the growth (Meites & Fiel, 1965) and thyroid hormones (Grossie & Turner, 1962) by decreasing the food allowance or applying a low-protein diet (Leathern, 1958). Many studies have shown the deleterious effect of a low-protein diet on the production of milk and its protein content (Armstrong, 1968;Thomas, 1971), but little attention has been given to mammary gland growth in relation to the protein level in the diet.…”

Effect of thyroxine injection on bone growth in malnourished rats at different growth stages.

“…However, the mechanism inducing this difference between infant and older rats is obscure. Many workers have reported that malnutrition decreases thyroid functions in rats (3)(4)(5). The retardation of bone growth might be due to reduced thyroxine secretion because thyroid hormones are necessary for bone growth.…”