Chapter 42 Animal Hormones

1. Chapter 42 Animal Hormones Biology 102 Tri-County Technical College Pendleton, SC

2. Two Major Systems • Nervous system involved with high-speed messages • Estes and the Dutch Oven…oh yeah!!!! • Endocrine system is much slower and involves production, release, and movement of chemical messages • As always, it is the subtle interplay between the two systems that COUNTS…

3. Endocrine System Components • Most endocrine glands/tissue contain neurosecretory cells that secrete hormones • Chemical signal that communicates regulatory messages within body • Hormones reach all parts of body but only certain types of cells [target cells] are equipped to respond • Only target cells respond because ONLY they have receptors for that hormone

4. Target Cell Visual

5. Insect Development Hormones • In insects/crustaceans, molting triggered by ecdysone • Secreted (insects) by prothoracic glands just behind head • Favors development of adult characteristics • i.e., from caterpillar to butterfly • Ecdysone production in insects controlled by brain hormone (BH) • Balanced by juvenile hormone (JH) secreted by corpora allata (pair of small glands just behind brain) • Promotes retention of larval characteristics

6. General Chemical Classes • Peptide hormones: oxytocin, ADH, calcitonin, PTH, and thymosin • Protein hormones: GH, prolactin, insulin, and glucagon • Glycoprotein hormones: FSH, LH, TSH • Amine hormones: T3 and T4, epinephrine and norepinephrine, and melatonin • Steroid hormones: glucocorticoids, mineralocorticoids, androgens, estrogens, and pregesterone

7. Two General Classes • Hormones can be divided into two general classes: steroid and nonsteroid • Nonsteroid hormones such as amines, peptides, and proteins usually combine with receptors in target cell membrane • First messenger/second messenger • Signal transduction pathway: converts extracellular chemical signal to specific intracellular response

8. Classes, cont. • Steroid hormones are lipid-soluble and easily diffuse into cells • Once inside target cell, steroid hormone MAY combine with specific protein molecules (the receptor[s]) or they may exert their influence directly • Typically, steroid hormone bind with their specific protein receptor

9. Stirring the Steroids… • Steroids easily cross plasma membrane • Binding of steroid (signal molecule) with specific receptor (usually in nucleus) initiates signal transduction process • Signal initiates cascade of events in which proteins interact with other proteins until final response(s) are achieved • In many cases, signal-receptor complex binds to DNA to modify gene expression

10. Target Cell Specificity • Each chemical signal has specific shape recognized by that signal’s target cells • Signal’s action begins when it binds to specific receptor • Receptor protein may be in PM of target cell or inside the cell • Diversity of responses of target cells depends on nature of target cell and the affinity of receptor molecules on or within cell • Cells are unresponsive to signal if they lack appropriate receptors

11. Hypothalamus • Hypothalamus is region of lower brain • Hormone releasing cells are 2 sets of neurosecretory cells whose secretions are stored in posterior pituitary (antidiuretic hormone/oxytocin) • Hormones released into capillaries in region at base of hypothalamus • Capillaries drain into portal vessels (short blood vessels that subdivide into second capillary bed within anterior pituitary

12. Hypothalamus, cont. • Hypothalamic hormones have direct access to gland they control (releasing/release-inhibiting hormones)

13. Pituitary: Location/Function • Pituitary located at base of hypothalamus • Has 2 discrete parts that develop from 2 separate regions of embryo and have very different functions • Anterior pituitary (adenohypophysis) consists of endocrine cells that synthesize and secrete several hormones directly into blood

14. Pituitary, cont. • Posterior pituitary (neurohypophysis) is extension of the brain • Stores and secretes 2 hormones made by hypothalamus • Oxytocin: contraction of uterus and mammary gland cells; regulated by nervous system • Antidiuretic hormone (ADH): promotes water retention by kidneys; regulated by water/salt balance

15. ADH/Homeostasis/Neg. FB • Osmoreceptors in hypothalamus monitor blood osmolarity • Plasma osmolarity >s; osmoreceptors shrink slightly and transmit nerve impulse to certain hypothalamic neurosecretory cells • These cells respond by releasing ADH into general circulation from their tips in posterior pituitary • Target cells for ADH are cells lining collecting ducts of nephrons in the kidneys

16. ADH, cont. • ADH binds to receptors on target-cells and activates signal-transduction pathway that increases water permeability of collecting ducts • Water retention is >ed as water exist collecting ducts and enters nearby capillaries • Osmoreceptors also simulate thirst drive

17. ADH III • As more dilute blood (lower osmolarity) arrives at brain, hypothalamus responds by reducing ADH secretion and lowering thirst sensation • This prevents overcompensation by stopping hormone secretion and quenching thirst • This negative feedback scheme includes hormonal action and behavioral response

18. Tropic Hormones • Have other endocrine glands as their targets • Four of hormones secreted by anterior pituitary gland are tropic hormones • Thyroid-stimulating hormone (TSH)) stimulates thyroid gland • Adrenocorticotropic hormone (ACTH) stimulates adrenal cortex to secrete glucocorticoids

19. Tropic Hormones, cont. • Follicle-stimulating hormone (FSH) stimulates production of ova and sperm • Luteinizing hormone (LH) stimulates ovaries and testes • Anterior pituitary also secretes other hormones with profound effects on body: Growth hormone (GH), Prolactin; Melanocyte-stimulating hormone; Endorphins and enkephalins

20. Control of Anterior Pituitary • Neurosecretory cells in hypothalamus exert control over anterior pituitary by secreting 2 kinds of hormones into blood • Releasing hormones: anterior pituitary secretes its hormones • Inhibiting hormones: make A.P. stop secreting hormones • Is subtle blend of R and I hormones that control timing and amount of hormonal release by A.P.

21. Thyroid Gland • In humans/other mammals, thyroid gland consists of 2 lobes located on ventral side of trachea • Produces 2 very similar hormones from tyrosine • Triiodothyronine (T3) and Tetraiodothyronine (T4)/thyroxine • In mammals, T3 is more active form

22. Thyroid, cont. • Thyroid important in human development • Deficiency (cretinism) results in markedly retarded skeletal growth/poor mental development • Thyroid hormones important for normal functioning of bone-forming cells and for branching nerve cells during embryonic development of the brain • In adults, help maintain normal BP, heart rate, muscle tone, digestion, and reproductive functions

23. Thyroid III • Hyperthyroidism (> amounts) produces high body temperature, profuse sweating, weight loss, irritability, and high BP • Hypothyroidism (< amounts) produces cretinism in infants and symptoms of weight gain, lethargy, and intolerance to cold in adults

24. Thyroid IV • Deficiency of iodine in diet can lead to shortage of thyroid hormones and cause enlargement of thyroid called a goiter • Thyroid also produces calcitonin which lowers calcium levels in blood as part of calcium homeostasis • Stimulates osteoblasts to use circulating calcium to deposit new bone

25. Thyroid Hormones Loop • HypothalamusTRH (thyroid releasing hormone) [some texts call thyrotropin-releasing hormone or TRH] to anterior pituitaryTSH (thyroid stimulating hormone)induces thyroid to manufacture and release T3 and T4 • Increasing levels of TSH, T3, and T4 signal hypothalamus to reduce secretion of TRH

26. Parathyroid Glands • Four parathyroid glands embedded in surface of thyroid • Function in homeostasis of calcium ions • Parathyroids secrete parathyroid hormone (PTH) • Raises blood levels of calcium [has opposite effect of thyroid hormone calcitonin

27. Parathyroids, cont. • Stimulates Ca2+ reabsorption in kidneys and induces osteoclasts to decompose mineralized matrix of bone and release Ca2+ into blood • Vitamin D (synthesized in skin) essential to PTH function • Lack of PTH causes blood levels of calcium to drop dramatically • Leads to convulsive contractions of skeletal muscles • If unchecked, tetany follows which is FATAL!!!

28. Pancreas • Pancreas has both endocrine and exocrine functions • Islets of Langerhans are cluster of endocrine tissue in the pancreas which secretes 2 hormones directly into circulatory system • Alpha (α) cells secrete peptide hormone called glucagon • Beta (β) cells secrete the hormone insulin

29. Hormones of Pancreas • Glucagon and insulin work together (antagonistic) to regulate [ ] of glucose in blood • In humans, blood glucose levels must remain about 90 mg/100 mL for proper body functioning • At glucose levels above set point, insulin secreted and lowers blood glucose [ ] by stimulating body cells to take up glucose from blood

30. Pancreas III • Insulin also slows glycogen breakdown in liver and inhibits conversion of AAs/fatty acids to sugar • Blood glucose levels drop below set point, glucagon is secreted and increases blood glucose [ ] by stimulating liver to increase hydrolysis of glycogen, convert AAs/Fas to glucose, and slowly release glucose into blood

31. Adrenal Medulla • Adrenal medulla synthesizes/secretes catecholamines (epinephrine and norepinephrine) • Secreted in times of stress • Nerve cells excited by stressful stimuli release neurotransmitter acetylcholine in medulla • Acetylcholine combines with cell receptors and stimulates release of epinephrine • Norepinephrine released independently of epinephrine

32. Adrenal Medulla, cont. • Have rapid/dramatic effect on several targets • Glucose mobilized in skeletal muscle cells • Fatty acid released from fat cells • Rate and stroke of heartbeat increased • Delivery of blood changed • Oxygen delivery to body cells increased

33. Adrenal Cortex • Adrenal cortex synthesizes and secretes corticosteroids • Stressful stimuli cause hypothalamus to secrete releasing hormone that stimulates release of ACTH from anterior pituitary • ACTH stimulates release of corticosteriods from adrenal cortex • In humans, 2 primary types are glucocorticoids (cortisol) and mineralocorticoids (aldosterone)

34. Adrenal Cortex, cont. • Glucocorticoids promote glucose synthesis from noncarbohydrate substances such as proteins • Also have immunosuppressive effects and are used to treat inflammation • Mineralocorticoids affect salt and water balance • Aldosteone stimulates kidneys to reabsorb sodium ioins and water from filtrate

35. Adrenal Cortex III • Glucocorticoids/mineralocorticoids important to maintaining body homeostasis during extended periods of stress • Short-term stress responses triggers release of epinephrine and norepinephrine (fight or flight) • Long-term stress responses trigger the release of mineralocorticoids and glucocorticoids

36. Gonadal Hormones • In humans, gonads are ovaries and testes • Hormones are androgens, estrogens, and progestins • Primary androgen is testosterone • Primary estrogen is estradiol • Progestins include progesterone

37. Gonadal Hormones, cont. • Gonadotropins from anterior pituitary (FSH and LH) control synthesis of both androgens and estrogens • FSH and LH controlled by gonadotropin-releasing hormone (GnRH) from hypothalamus

38. Wrapping it up…so to speak • Pineal gland is small mass of tissue near center of mammalian brain • It secretes melatonin which regulates functions related to light and to seasons marked by changes in day length (biorhythms) • Pineal contains light sensitive cells or has nervous connections from the eyes (depends on species) • Some studies indicate it has role in fertility

39. Wrapping, cont. • Thymus is located just posterior to sternum • Produces thymosin • Promotes development and maturation of lymphocytes • Important in immunity • Larger and most active in younger years, by adulthood, has atrophied and is mostly just gone….as is this chapter…