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SUMMARY
Rates of thyroid secretion (TSR) were determined in female Sprague-Dawley-Rolfsmeyer and male Holtzman rats at varying temperatures. When exposed to cold, TSR reached the highest value within the first 20 days after exposure, with a gradual increase up to 90 days. When rats reared in the cold were exposed to warm temperatures, TSR declined slightly at first, but 35–40 days were required to produce a significant decrease.
Treatment before and after exposure to cold with l-thyroxine (T4), corticosterone and T4 plus corticosterone significantly increased survival at 4·5°. Corticosterone plus T4 was most effective, T4 alone was next best and corticosterone alone had the poorest effect.
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In a previous study, it was shown that thyroid hormone secretion rate (TSR) of female rats aged 55 days was reduced 22·8% by the s.c. injection of 50 μg. melatonin/day, by 14·8% when 75 μg./day was injected at 85 days, and by 9·1% when 100 μg./day was administered at 115 days (Narang, Singh & Turner, 1967). Since the rats were increasing in body weight during the period, the decrease in TSR with age may have been due to the effects of a lower dose of melatonin/g. body weight (Narang & Turner, 1966; Kumaresan & Turner, 1967). The object of the present experiment was to determine the effect of graded levels of melatonin upon the TSR of groups of rats of the same age and body weight and to determine whether melatonin has an effect on TSR of mature animals.
The TSR of 62 Sprague—Dawley—Rolfsmeyer female rats were estimated at 33
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SUMMARY
One hundred and twenty albino female rats (Sprague-Dawley-Rolfsmeyer) were divided into five equal groups. Rates of thyroxine secretion (TSR) and food consumption were determined during the control period, and 10 and 25 days after initiation of dietary treatment. Animals in each group served as their own controls for the following modifications of their diet: (1) protein-free diet, (2) 5% protein (casein) diet, (3) 10% protein diet, (4) 15% protein diet, and (5) 20% protein diet. Purina lab chow (23·4% protein) and the 20% casein diet served as control diets. The TSR, the body weight and amount of food consumed were depressed significantly in the group fed on a protein-free diet for 10 and 25 days. The group fed 5% protein diet had a non-significant decrease in TSR as compared with the controls. Similarly, TSR was not reduced by 10, 15 or 20% protein diets. Food consumption decreased significantly in the groups fed a 5, 10 and 15% protein diet, but not in the group on 20% casein. Body weight decreased significantly in the groups on a protein-free diet and on a 5% protein diet.
It would appear from these results that protein content of the diet does not become a limiting factor for TSR until it is lower than 5%. It is suggested that the calorie intake plays a more important role in regard to TSR than a low protein content of the food.
