Chapter 20 – Endocrinology

Publisher Summary The rat is a good model for investigating the endocrine system, because the endocrine organs are of a reasonable size, being neither too small nor too big. This enables scientists to remove the organs or replace test substances easily at the laboratory level. Many different kinds of hormones have been isolated and identified in the rat, including hormones first identified in other animal species. This chapter reviews the functions of each endocrine organ, particularly their major hormones and the rat. Hypothalamic neuroendocrine cells of rats synthesize hypothalamic hormones that stimulate or inhibit the secretion of hormones from the anterior pituitary gland. These releasing/inhibiting hormones are discharged from the neuronal terminals located in the median eminence and diffuse into the capillaries where they are carried to the anterior pituitary gland via the pituitary portal system to regulate synthesis and secretion of anterior pituitary hormones. The gonads synthesize and secrete sex steroid hormones such as androgens, estrogens, and progestins. These gonadal steroids have a variety of actions, including stimulation of gamete production, development of reproductive organs, and expression of secondary sex characters and sexual behavior. Atrial natriuretic peptide (ANP) is known to act at the level of the kidney to increase sodium excretion into the urine. ANP also inhibits aldosterone release from the adrenal cortex. The thyroid gland produces two separate thyroid hormones called thyroxine/tetraiodothyronine (T4) and triiodothyronine (T3) under the control of thyroid-stimulating hormone. Parathyroid gland synthesizes and releases parathyroid hormone (PTH) from chief cells. PTH causes calcium mobilization from bones and enhances calcium reabsorption by the kidneys;these actions cause an elevation in plasma calcium concentration.