Chapter 6 Endocrine System Responses and Adaptations

1. Chapter 6Endocrine System Responses and Adaptations

2. Endocrine System: Overview • General Functions • Helps body maintain normal function • Prepares body for exercise • Mediates several adaptations • Is involved in every system • Mechanism • Releases chemical messenger (hormone) • Transports hormone to target tissue • Elicits chain of events leading to desired function

3. Hormone Actions • Autocrine Actions • Hormone acts on cells that produced it • Paracrine Actions • Hormone acts on cells adjacent to the ones that produced it • Endocrine Actions • Hormone enters general circulation & travels systematically to specific target tissues

4. Role of Releasing Hormone • Releasing Hormones • Hormones that cause the release of other hormones • Hypothalamus • Segment of brain that acts as an endocrine gland • Provides link between nervous & endocrine systems • Synthesizes & releases neurohormones to the pituitary gland

5. Role of Releasing Hormone (cont’d) • Hormones Released by the Hypothalamus • Corticotropin-releasing hormone (CRH) • Gonadotropin-releasing hormone (GnRH) • Growth hormone-releasing hormone • Growth hormone-inhibiting hormone • Thyrotropin-releasing hormone (TRH) • Prolactin-inhibiting hormone (PIH)

6. Role of Releasing Hormone (cont’d) • Hormones Released by the Pituitary in Response to the Release of Hormones from the Hypothalamus • Growth hormone (GH) • Follicle-stimulating hormone (FSH) • Luteinizing hormone (LH) • Adrenocorticotropic hormone (ACTH) • Thyroid-stimulating hormone (TSH) • Prolactin • Beta-endorphins

7. Types of Hormones • Steroid Hormones • Made of three 6-carbon rings & one 5-carbon ring • Synthesized from cholesterol via synthetic pathways • Actions: • Released into circulation • Arrive at target tissue • Diffuse through cell membrane • Bind to specific receptor within cell

8. Types of Hormones (cont’d) • Common Steroid Hormones • Androgens: testosterone (TE) • Glucocorticoids: cortisone • Progesterone • Prostaglandins • Anabolic steroids: commonly used/abused by athletes

9. Types of Hormones (cont’d) • Peptide Hormones • Proteins of various sizes • Peptides: small chains • Polypeptides: large chains • Direct product of: • mRNA translation • Cleavage from larger parent molecules • Other postsynthesis modifications • Circulate & bind to specific receptors on cell membranes of target tissues

10. Types of Hormones (cont’d) • Common Peptide Hormones • Insulin • Insulin-like growth factor-I (IGF-I) • Superfamily of GH molecules

11. Types of Hormones (cont’d) • Amine Hormones • Have amine (NH2) group at end of molecule • Derived from amino acids • Classified as protein hormones • Synthesized from: • Tyrosine • Phenylalanine • Tryptophan • Must bind to surface-bound receptor on target tissue • Can act as neurotransmitters in autonomic nervous system

12. Types of Hormones (cont’d) • Common Amine Hormones (catecholamines) • Epinephrine • Norepinephrine • Dopamine

13. Major Endocrine Glands in the Human Body

14. Hormone Activity: Production, Release, and Transportation • Production & Release • Peptides & amines are synthesized in advance & stored in vesicles until needed • Steroids are synthesized from cholesterol & released (not stored) • Transportation • Released into circulation & transported to target tissues • Half-life: time it takes for half of hormone to be degraded • Transport (binding) proteins • Protect hormone from metabolism • Deliver hormone to its receptor

15. Hormone Activity: Production, Release, and Transportation (cont’d) • What Determines Hormonal Concentrations in the Blood? • Amount of hormone released • Pattern of release (pulsatility) • Rate of metabolism • Quantity of transport proteins • Time of day (circadian patterns) • Plasma volume shifts (during exercise)

16. Hormone Activity: Production, Release, and Transportation (cont’d) • Negative Feedback Control • Elevates a hormone when it is low • Reduces a hormone when it is elevated • Example: athlete using anabolic steroids— • Reductions in testosterone production lead to • Testicular shrinkage due to • Negative feedback inhibition of endogenous TE production

17. Receptor Interaction • Receptor Specificity • Lock-and-key principle: only one hormone will unlock or activate each receptor • Cross-reactivity: allows more than one hormone or molecule to activate a receptor

18. Receptor Interaction (cont’d) • Steroid and Thyroid Hormones • Steroids, because they are lipophilic: • Diffuse through target cell membranes • Bind to receptors in cytoplasm or nucleus • Receptors are bound to heat-shock proteins in cytoplasm prior to hormone binding

19. Hormones and Exercise • Exercise Presents a Potent Stimulus for Hormonal Adaptations • Resistance Training (RT) Acute Program Variables • Intensity • Volume • Rest intervals • Exercise selection & sequence • Repetition velocity • Frequency

20. Hormones and Exercise (cont’d) • Aerobic, Plyometric, Sprint, & Agility Training Variables • Intensity • Volume & duration • Modality • Work:rest ratio • Frequency

21. Hormones and Exercise (cont’d) • Other Variables Affecting Hormonal Responses • Genetic predisposition • Gender • Fitness level • Nutritional intake • Potential for adaptation

22. Hormones and Exercise (cont’d) • Classifications of Hormonal Responses and Adaptations • Acute responses during exercise • Chronic changes in resting concentrations • Chronic changes in acute response to exercise • Receptor changes

23. Hormones and Exercise (cont’d) • Testosterone (testes) • Elevated during & immediately following exercise • Acute TE response weaker in women • Changes in resting TE during RT are inconsistent • Regulated by luteinizing hormone (LH) • Bound to sex hormone-binding globulin (SHBG) • Precursors: dehydroepiandrosterone (DHEA), androstendione, & androstenediol

24. Hormones and Exercise (cont’d) • Growth Hormone (GH) Superfamily (anterior pituitary-brain) • Secreted by anterior pituitary • Released in pulsatile manner • Under control of GHRH & inhibited by somatostatin • Exercise is a potent stimulus for GH secretion • No apparent change in resting GH with consistent exercise • 50% of GH binds to GH-specific binding proteins (GHBPs) that extend its half-life & enhance its effects

25. Hormones and Exercise (cont’d) • Insulin-Like Growth Factors (IGFs) (liver) • Structurally related to insulin & mediate many actions of GH • Small polypeptide hormones secreted by liver in response to GH stimulation • Increase proliferation & differentiation of satellite cells & protein synthesis • Enhance muscle hypertrophy • IGF-1 response to exercise is unclear • No consistent pattern of change in IGF-1 during exercise

26. Hormones and Exercise (cont’d) • Insulin (pancreas) • Secreted from Islets of Langerhans (B cells) in pancreas • Secreted in response to glucose intake • Increases muscle protein synthesis when amino acid is adequate • Aerobic & anaerobic training: • Improves insulin sensitivity • Reduces insulin resistance

27. Hormones and Exercise (cont’d) • Glucagon (pancreas) • Protein hormone consisting of 29 amino acids • Synthesized in A cells in Islets of Langerhans of pancreas • Inhibited by glucose levels • Stimulates breakdown of glycogen • Increases energy availability • Elevated during exercise as energy demands increase

28. Hormones and Exercise (cont’d) • Cortisol (adrenal cortex; perimeter of adrenal gland top of kidney) • Catabolic glucocorticoid released from adrenal cortex in response to stress under the control of CRH & ACTH • Stimulates lipolysis in adipose cells • Increases protein degradation in muscle cells • Decreases protein synthesis in muscle cells • Elevated during aerobic exercise, with greater response occurring with greater intensity • Increased during RT • Resting concentrations reflect a long-term stress & are transient

29. Hormones and Exercise (cont’d) • Catecholamines (adrenal medulla; adrenal gland-top kidney) • Secreted by adrenal medulla • Secreted in response to stress (physical, heat, hypoxia, hypoglycemia) • Increased during both aerobic & anaerobic exercise • Magnitude of increase depends on: • Muscle mass involved • Posture • Intensity • Duration

30. Hormones and Exercise (cont’d) • β-Endorphins (anterior pituitary-brain) • 31-amino acid peptide cleaved in anterior pituitary from parent • Act as neurotransmitters in nervous system & as analgesics • Increase relaxation • Enhance immune function • Increased with exercise, with response depending on intensity & duration

31. Hormones and Exercise (cont’d) • Thyroid Hormones (thyroid gland-neck) • Thyroxine (T4) & triiodothyronine (T3) released into circulation • Travel mostly bound to transport proteins • Increase basal metabolic rate • Increase protein synthesis • Augment actions of catecholamines • Response to exercise not clear

32. Hormones and Exercise (cont’d) • Fluid-Regulatory Hormones • Fluid homeostasis is critical to exercise performance • Elevations in AVP, atrial peptide, renin, aldosterone, & angiotensin II during exercise • Magnitude of increase dependent on intensity, duration, fitness level, & hydration status

33. Hormones and Exercise (cont’d) • Leptin (adipose tissue) • Product of ob gene in adipose tissue • Relays satiety signal to hypothalamus to regulate energy balance & appetite • Higher levels in obese individuals (4 times greater) & women • Crosses blood-brain barrier to act with receptors • Concentrations influenced by insulin, glucocorticoids, catecholamines, thyroid hormones, TE, GH, & stimulants • Not affected by exercise, independent of % body fat

34. Hormones and Exercise (cont’d) • Estrogens (Ovaries) • Include estradiol, estriol, & estrone • Have long half-lives • Synthesized & secreted primarily by ovaries in women, under control of LH & FSH • Produced from conversion of androgens in men • Response to exercise unclear