The Endocrine System

1. The Endocrine System • hormones are chemical regulators produced in one part of the body that act on cells in another part •  secreted by glands directly into the blood • hormones are classified according to their activation site: • some are non-target hormones( i.e. growth hormone (GH) or somatropin (ST) act on long bone development; insulin regulates blood sugar; epinephrine (adrenaline) reacts to stress • others affect target sites (i.e. parathyroid hormone (PTH) regulates Ca+ levels by activating kidney, intestine, liver; gastrin stimulates stomach cells)

2. Chemical Control Systems • the endocrine system is designed to maintain control of tissues and organs for a long duration (i.e. growth and sex hormones) while the nervous system reacts more quickly •  coordination of two control systems allows the continuation of function • few hormonal reponses work independently, most causing secretion of others to help compensate  some glands produce several hormones (i.e. thyroid/parathyroid) that affect different organs Steroid Hormones derived from cholesterol (lipid) and therefore fat-soluble (see Fig. 4, P. 374, “Action of Steroid Hormones” animation on CD) • diffuse from capillaries •  interstitial fluid  target cells  receptor molecules •  hormone-receptor complex  nucleus  complimentary chromatin  gene activated  ribosomes in cytoplasm  protein synthesis

3. Protein Hormones • receptors at specific sites on cell membrane (see Fig. 5, P. 375, “Mechanism for peptide hormone action” animation on CD) • hormone-receptor complex activates G-protein • activates adenylyl cyclase • ATP converted to cyclic adenosine monophosphate (cyclic AMP) which activates enzymes to carry out normal functions in the cytoplasm • (i.e. THS attaches to thyroid gland receptor  cyclic AMP activates enzymes that produce thyroxine, a hormone that regulates metabolism)

4. Pituitary Gland • “master gland” because it controls other endocrine glands • produces and stores hormones, stimulated by nerves routed through a stalk connected to the hypothalamus (see “Interaction between pituitary and hypothalamus” animation) • posterior lobe stores and secretes ADH and oxytocin produced in the hypothalamus, and travel via specialized nerve cells • anterior lobe produces it’s own hormones • hypothalamus secretes hormones from nerve cells for regulation of secretion at specific sites  target tissues • hypothalamus-releasing factors also inhibit secretion of hormones from anterior lobe •  dopamine inhibits secretion of prolactin (PRL)  milk production • somatostatin inhibits secretion of somatotropin  growth of long bones • Summary: Importance of the Endocrine System (P. 377)

5. 8.2 - Hormones That Affect Blood Sugar • (see Fig. 1, P. 378, “Glucose control by isulin and glucagon” animation) • beta cells in the islets of Langerhans on the pancreas secrete insulin in response to high blood sugar muscle and liver calls become permeable to glucose  taken up by muscle cells, converted to glycogen for storage in liver • glucagon is secreted by alpha cells in the islets of Langerhans in response to low blood sugar  liver converts glycogen to glucose  released into blood

6. diabetes is caused by an insufficient production or use of insulin • hyperglycemia draws water from body •  impermeability of cells to glucose results in low energy levels •  metabolism of fat for E releases acetone which changes blood pH • Type I results from destruction of beta cells  requires insulin injections • Type II results from reduced production or utilization  diet, exercise, sulfonamides • gestational diabetes increases risk of diabetes in mother and child • Canadians Banting (Alliston) and Best tied off pancreatic ducts of dogs which killed cells producing digestive enzymes but left islets of Langerhans intact  extracted hormone relieved symptoms of diabetes • Lab Exercise 8.2.1 – Effects of Hormones on Blood Sugars

7. Adrenal Glands(see Fig. 5, P. 382, “Glucose regulation by cortisol” animation) • adrenal medulla is stimulated by sympathetic nerves when an organism is under stress (short-term stress response) • release of epinephrine and norepinephrine conversion of glycogen into glucose, increased heart rate, breathing rate, cell metabolism, blood vessel and pupil dilation

8. adrenal cortex releases hormones that regulates a long-term stress response: • hypothalamus sends a releasing hormone to the anterior lobeof pituitaryadrenocorticotropic hormone (ACTH) stimulates adrenal cortex • glucocorticoids affect blood glucose levels • i.e. cortisol: • increases amino acid levels, which are converted into glucose or used to repair cells • converts fats to energy-rich fatty acids, conserving glucose • inhibits glucose uptake • mineralocorticoids include alderosterone which increases Na+ retention and water reabsorption by the kidney to maintain body fluid levels N.B adrenal cortex also releases small amounts of sex hormones

9. 8.3 - Hormones That Affect Metabolism • Thyroid Gland • ventral to larynx, secretes thyroxine (T4) and triiodothyronine (T3) for regulation of metabolism, and growth and differentiation of tissue •  increased T4 results in faster oxidation of sugars and other nutrients (some is lost as heat, the rest is converted to ATP which is consumed during activity) •  lower levels of T4 result in higher amounts of sugar which is converted to glycogen (excess is converted to fat) • hypothyroidism results in myxedema, characterized by muscle weakness, cold intolerance, and dry skin and hair (see Fig. 2, P. 384) • T4 level is controlled by negative feedback •  receptors in hypothalamus detect lower metabolism rate  nerve cells secrete thyroid-releasing hormone (TRH)  pituitary secretes thyroid-stimulating hormone (TSH)  carried in blood to thyroid  secretes thyroxine (T4) •  higher thyroxine levels inhibits TRH secretion which shuts off thyroxine pathway

10. iodine is an important part of both T4 and T3 inadequate dietary iodine reduces secretion of thyroxine, but TSH is still secreted  relentless influence of TSH results in continual development of thyroid cells, enlargement of gland (goiter) •  salt is iodized to prevent goiters

11. Parathyroid Glands • low Ca+ levels in blood result in release of parathyroid hormone (PTH) kidneys and intestines retain Ca+, bones release it  absorbed into blood • PTH is inhibited by high levels of Ca+ in blood • high PTH results in break down of bones, accumulation of Ca+ in kidneys and blood • PTH also helps activate vitamins  deficiency causes rickets

12. Anterior Pituitary • growth hormone (somatotropin) affects most cells in body, but especially cartilage and bone cells • increase number (hyperplasia) and size (hypertrophy) of cells • increase in size is accomplished by promoting protein synthesis while inhibiting protein decomposition • GH also stimulates the liver to secrete insulin-like growth factors which stimulate cell division in growth plates, causing the skeleton to elongate • GH converts fats to fatty acids which is used by muscles for energy, leaves glucose for organs like the brain which cannot utilize fatty acids for energy • utilization of fat stores and promotion of protein synthesis, changes body form away from adipose “baby fat” to protein and muscle • Section 8.3 Questions – P. 387, #1-8