View metadata, citation and similar papers at core.ac.uk brought to you by CORE RESEARCH BULLETIN 969 providedSeptember by University of 1969Missouri: MOspace UNIVERSITY OF MISSOURI-COLUMBIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION ELMER R. KIEHL, Director Method of Estimating Thyroid Hormone Secretion Rate of Rats and Factors Affecting It CHARLES W. TURNER (Publication authorized September 25, 1969) COLUMBIA, MISSOURI 2 MISSOURI AGRICULTURAL EXPERIMENT STATION ACKNOWLEDGMENT The writer wishes to express his appreciation to the Directors of the Missouri Agricultural Experiment Station and the chairmen of the Depart­ ment of Dairy Husbandry for their cooperation and financial support of the project Endocrine-genetic Interrelationship in Milk Secretion. In addi­ tion, the research reported in this bulletin has been supported by generous grants-in-aid from the United States Atomic Energy Commission since 3 1954, when iodine' ' became available for our research. This support, Con­ tract No. AT (l l-1)-301, extended until my retirement in 1967. The contract has been extended to my successor, Dr. Ralph R. Ander­ son, as No. AT(l l-1)-1758, from 1968 to date. I am also indebted to the fine group of graduate students and Post­ Doctorate Fellows who have worked on this project during the past 15 years. Credit for their contributions are indicated in the citation to the re­ views of their research. While the primary aim of this project has been the extension of our knowledge of the role of the thyroid gland and its hormones in relation to milk secretion, it has made possible, also, the training of graduate students and research fellows in the field of animal physiology and endocrinology. Many of these men now have fine positions in colleges, experiment sta­ tions, and commercial research laboratories both in the United States and abroad where their training and experience in course work and research at the University of Missouri is having an ever-widening influence upon grad­ uate education in endocrinology and the advancement of knowledge. RESEARCH BULLETIN 969 3 INDEX Introduction ..... .. ...... .. .. ... .. ..... .... ... .... .. ..... .. .. 4 The Thyroid Gland Weight of Rats ......... ... .. 20 Factors Influencing Thyroid Gland Weight .... .. ........... ... .24 Methods of Determining Thyroid Hormone Secretion Rate . ..... .. .. .. .. .. .... .. 24 Factors Influencing Thyroid Hormone Secretion Rate in the Rat ... .27 1 3 The Accumulation of I ' by the Lactating Mammary Gland ..... .... ... .. .. ... .. .. .. 33 Stimulus of Increased TSR In Lactation .... .33 Environmental Factors Influencing Thyroid Hormone Secretion Rate (TSR) . .. .... ... ....... .. 34 Endocrine Factors Influencing Thyroid Hormone Secretion Rate (TSR) ........... .. .... ... ........ ... .. 39 Effect of Various Hormones on the TSR of Normal Rats . ......... .......... ... .. ... ... ............... .. 42 Effect of Reserpine on Thyroxine Secretion Rate . .. ................ .. .... ...... .... .. .. .. .. ... 47 Biological Half-life (t\12) ofL-T4 and L-T8 In the Rat .... ........... .. ....... .. .................... 47 Relation of TSR to Hormone Content of the Thyroid Gland ............. .. ..... .. ... 50 Comparison of the Thyroid Hormone Secretion Rate of the Rat With Other Species . .. .. .. .. ... ... .. ... .. 50 Summary .... ..... ...... ............. .. .. .. .. ... ... ... .... .. 51 Method of Estimating Thyroid Hormone Secretion Rate of Rats and Factors Affecting It CHARLES W. TURNER INTRODUCTION Interest in the thyroid gland and its hormones was aroused in dairy physiologists by the observation that thyroidectomy of dairy cattle de­ pressed the rate of milk secretion and that upon replacement therapy with desiccated thyroid glands, milk yield was not only returned to normal but in many cows was increased above the preoperative yield (Graham, I 934a). It was then shown that the feeding or injection of thyroxine stimulated in­ creased lactation in many normal cows (Graham, I 934b). The fact that exogenous thyroxine stimulated increased milk and fat production in many dairy cows suggested that this was because the cow's endogenous secretion was less than optimal and upon increasing the level of thyroxine in the blood, increased milk secretion was stimulated. The role of the thyroid hormones in milk secretion was thus established. While supplemental therapy indicated the cows deficient in thyroxine secretion, the development of a method by which thyroxine secretion rate could be determined and used as a selective index in the improvement of dairy cattle was indicated. Steps in the development of such methods will be discussed. The story of the role of the thyroid glands in milk secretion has been presented (Turner, 1968). Regulation of Thyroid Hormone Secretion Rate Thyroid hormone secretion rate (TSR) is regulated by the thyroid stimulating hormone (TSH) of the anterior pituitary and it, in turn, is regu­ lated by the anterior hypothalamus, which secretes a factor that stimulates the secretion and release of TSH by the pituitary (TRF). The role of the hypothalamus in the regulation of TSH secretion has been shown recently by the effect of lesions upon the uptake and release of J131 by the thyroid, by lower protein-bound iodine (PBI) levels in the blood, and by changes in the TSH levels in the pituitary and blood. A second type of evidence is concerned with the extraction and purification of a factor which stimulates the release of TSH from the pituitary (TRF). The thyroid stimulating hormone (TSH) secreted by the pituitary has three important functions in relation to the activity of the thyroid gland: I) It stimulates the uptake (trapping) of iodine by the thyroid gland; 2) It stimulates the secretion of thyroglobulin into the lumen of the alveoli and the formation of thyroxine (L-T4) and triiodothyronine (L-T3) as an integral part of the thyroglobulin molecule; 3) It stimulates the hydrolysis of thyroglobulin and the release of L-T4 and L-T3 from the thyroid into the blood. RESEARCH BULLETIN 969 5 Hypothalamic Control of TSH Secretion The first evidence of the role of the hypothalamus in the regulation of TSH secretion was presented by Greer ( 195 l ), who produced bilateral symmetrical electrolytic lesions in the hypothalamus of rats. These anima ls failed to show the goitrogenic response to thiouracil feeding . However, the iodide-concentrating capacity of the thyroid was about normal. Bogdanove and Halmi ( 1953) repeated Greer's work using propyl­ thiouracil and noted impairment of the hyperplastic response of the thyroid but no interference of the thyroid/ serum iodide concentration ratio. Florsheim (1959) reported that rats with anterior hypothalamic lesions showed lower TSH levels after goitrogen feeding than control animals as measured by bioassays based on iodine discharge from the chick thyroids. The discharge of labeled hormone from the thyroid of lesioned and control rats was inhibited by equal doses of exogenous thyroid hormone. This was taken to indicate that the thyroid-pituitary feedback system controlling TSH release was independent of the hypothalamus. Moll et al ( 1961) produced electrolytic lesions of the hypothalamic area between the fornix columns at the level of the paraventricular nuclei which inhibited thyroid function as judged by thyroid weight, uptake and release of I 131 and response to methylthiouracil. Averill et al ( 1961) observed a significant and permanent depression of thyroid secretion effective within 6 hours, following anterior ventral hy­ pothalamic lesions in the rat. Van Beugen and van der Werff ten Bosch ( 196 l) studied the effect of lesions in the basal midline of the anterior hypothalamus on thyroid activi­ ty of rats exposed to temperatures of 24 °C and 4°C. The index of thyroid 131 function was the biological half-life (t Y2) of thyroidal-I • The t V2 values were 7.5 days at 24°C and 4 days of 4°C in rats with hypothalamic lesions. This part of the hypothalamus does not seem essential for the response of the pituitarythyroid axis to a low environmental temperature. Van der Werfften Bosch and Swanson (1963) compared the thyroid enlargement caused by 0.15 percent of propyl-thiouracil in the diet on normal and lesioned rats after 14 and 28 days. It was found that the lesions reduced the thyroid weight to about 75 percent of the intact rat on either diet. It was concluded that lesions caused a lowered steady state of the pi­ tuitary-thyroid feedback system. De Jong and Moll (1965) made lesions in midline or bilaterally in the hypothalamus of rats. Single midline lesions which destroyed the area be­ tween the fornix bundles at the level of the paraventricular nuclei (PVN) resulted in a reduced rate of thyroidal-1 131 release in rats at room tempera­ ture and blocked the thyroidal response at 4°C. Single midline lesions just inferior to the PVN caused inhibition of the response to cold but were less effective at room temperature. Lesions made anterior to the PVN caused partial blocking of the cold response but had no effect at room tempera­ ture. No effect was produced by midline lesions either superior or posterior to the PVN or bilaterally places lesions laterally to the PVN. Panda and Turner (l 967c) determined the changes in the TSH control 6 MISSOURI AGRICULTURAL EXPERIMENT STATION of the plasma and pituitary of mature female rats after placing lesions in the supraoptic area of the hypothalamus of normal and thyroidectomized animals kept at normal and low environmental temperatures. The