Chapter 18: The Endocrine System

1. Chapter 18: The Endocrine System What is the importance of intercellular communication?

2. Endocrine System • Regulates long-term processes: • growth • development • reproduction

3. Endocrine System • Uses chemical messengers to relay information and instructions between cells

4. What are the modes of intercellular communication used by the endocrine and nervous systems?

5. Direct Communication • Exchange of ions and molecules between adjacent cells across gap junctions • Occurs between 2 cells of same type • Highly specialized and relatively rare

6. Paracrine Communication • Uses chemical signals to transfer information from cell to cell within single tissue • Most common form of intercellular communication

7. Endocrine Communication • Endocrine cells release chemicals (hormones) into bloodstream • Alters metabolic activities of many tissues and organs simultaneously

8. Target Cells • Are specific cells that possess receptors needed to bind and “read” hormonal messages

9. What is the functional significance of the differences between the two systems?

10. Endocrine System • Is unable to handle split-second responses

11. Nervous System • Handles crisis management

12. Synaptic Communication • Releases neurotransmitter at a synapse that is very close to target cells • Ideal for crisis management

13. Mechanisms of Intercellular Communication Table 18–1

14. Endocrine and Nervous Systems • Are similarly organized: • rely on release of chemicals • share many chemical messengers • are regulated primarily by negative feedback • share a common goal: to preserve homeostasis

15. How do the cellular components of the endocrine system compare with those of other tissues and systems?

16. Endocrine System • Includes all endocrine cells and body tissues that produce hormones or paracrine factors

17. Endocrine Cells • Glandular secretory cells that release their secretions into extracellular fluid

18. Exocrine Cells • Secrete their products onto epithelial surfaces

19. Endocrine System Figure 18–1

20. What are the major structural classes of hormones?

21. Hormones • Can be divided into 3 groups: • amino acid derivatives • peptide hormones • lipid derivatives

22. Classes of Hormones Figure 18–2

23. Amino Acid Derivatives • Small molecules structurally related to amino acids • Synthesized from the amino acids tyrosine and tryptophan

24. Tyrosine Derivatives • Thyroid hormones • Compounds: • epinephrine (E) • norepinephrine (NE) • dopamine, also called catecholamines

25. Tryptophan Derivative • Melatonin: • produced by pineal gland

26. Peptide Hormones • Chains of amino acids • Synthesized as prohormones: • inactive molecules converted to active hormones before or after secretion

27. 2 Groups of Peptide Hormones • Group 1: • glycoproteins: • more than 200 amino acids long, with carbohydrate side chains: • thyroid-stimulating hormone(TSH) • luteinizing hormone(LH) • follicle-stimulating hormone(FSH)

28. 2 Groups of Peptide Hormones • Group 2: • all hormones secreted by: • hypothalamus • heart • thymus • digestive tract • pancreas • posterior lobe of pituitary gland • anterior lobe of pituitary gland

29. 2 Classes of Lipid Derivatives • Eicosanoids: • derived from arachidonic acid • Steroid hormones: • derived from cholesterol

30. Eicosanoids • Are small molecules with five-carbon ring at one end • Are important paracrine factors • Coordinate cellular activities • Affect enzymatic processes in extracellular fluids

31. Leukotrienes • Are eicosanoids released by activated white blood cells, or leukocytes • Important in coordinating tissue responses to injury or disease

32. Prostaglandins • A second group of eicosanoids produced in most tissues of body • Are involved in coordinating local cellular activities • Sometimes converted to thromboxanes and prostacyclins

33. Steroid Hormones • Are lipids structurally similar to cholesterol • Released by: • reproductive organs (androgens by testes, estrogens, and progestins by ovaries) • adrenal glands (corticosteroids) • kidneys (calcitriol)

34. Steroid Hormones • Remain in circulation longer than peptide hormones • Are absorbed gradually by liver • Are converted to soluble form • Are excreted in bile or urine

35. Hormones • Circulate freely or bound to transport proteins

36. Free Hormones • Remain functional for less than 1 hour: • diffuse out of bloodstream: • bind to receptors on target cells • are absorbed: • broken down by cells of liver or kidney • are broken down by enzymes: • in plasma or interstitial fluids

37. Thyroid and Steroid Hormones • Remain in circulation much longer • Enter bloodstream: • more than 99% become attached to special transport proteins

38. Bloodstream • Contains substantial reserve of bound hormones

39. What are the general mechanisms of hormonal action?

40. Hormone Receptor • Is a protein molecule to which a particular molecule binds strongly • Responds to several different hormones

41. Cells • Different tissues have different combinations of receptors • Presence or absence of specific receptor determines hormonal sensitivity

42. Catecholamines andPeptide Hormones • Are not lipid soluble • Unable to penetrate cell membrane • Bind to receptor proteins at outer surface of cell membrane (extracellular receptors)

43. Eicosanoids • Are lipid soluble • Diffuse across membrane to reach receptor proteins on inner surface of membrane (intracellular receptors)

44. Hormone • Binds to receptors in cell membrane • Cannot have direct effect on activities inside target cell • Uses intracellular intermediary to exert effects

45. Intracellular Intermediaries • First messenger: • leads to second messenger • may act as enzyme activator, inhibitor, or cofactor • results in change in rates of metabolic reactions

46. Important Second Messengers • Cyclic-AMP (cAMP): • derivative of ATP • Cyclic-GMP (cGMP): • derivative of GTP • Calcium ions

47. Amplification • Is the binding of a small number of hormone molecules to membrane receptors • Leads to thousands of second messengers in cell • Magnifies effect of hormone on target cell

48. Receptor Cascade • A single hormone promotes release of more than 1 type of second messenger

49. Down-regulation • Presence of a hormone triggers decrease in number of hormone receptors • When levels of particular hormone are high, cells become less sensitive

50. Up-regulation • Absence of a hormone triggers increase in number of hormone receptors • When levels of particular hormone are low, cells become more sensitive