Chapter 6
Endocrine System Responses
and Adaptations
Copyright © 2012 American College of Sports Medicine
Endocrine System: Overview
• General Functions
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Helps body maintain normal function
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Prepares body for exercise
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Mediates several adaptations
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Is involved in every system
• Mechanism
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Releases chemical messenger (hormone)
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Transports hormone to target tissue
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Elicits chain of events leading to desired function
Copyright © 2012 American College of Sports Medicine
Hormones Come in 3 Forms
1.
Steroid: Synthesized from cholesterol. Androgens like testosterone,
estrogens and glucocorticoids like cortisol. Anabolic steroids are
another type.
2.
Peptide: Proteins: Small chains (peptides) of less than 20 amino
acids and large are polypetides. Several hormones are proteins like
insulin, insulin-like growth factor (IGF-1). Peptides are prone to
degradation, so pharmaceuticals (insulin & GH) are not consumed
orally and need direct routes into circulation (Injection).
3.
Amine Hormone: Derived from AA’s and may be classified as protein
hormones. Catecholamines—epinephrine, norepinephrine and
dopamine.
Copyright © 2012 American College of Sports Medicine
Hormone Actions
• Autocrine Actions
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Hormone acts on cells that produced it
• Paracrine Actions
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Hormone acts on cells adjacent to the ones that produced it
• Endocrine Actions
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Hormone enters general circulation & travels systematically to
specific target tissues
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Role of Releasing Hormone
• Releasing Hormones
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Hormones that cause the release of other hormones
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Hypothalamus
• Segment of brain that acts as an endocrine gland
• Provides link between nervous & endocrine systems
• Synthesizes & releases neurohormones to the pituitary gland
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Role of Releasing Hormone (cont’d)
• Hormones Released by the Hypothalamus
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Corticotropin-releasing hormone (CRH)
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Gonadotropin-releasing hormone (GnRH)
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Growth hormone-releasing hormone
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Growth hormone-inhibiting hormone
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Thyrotropin-releasing hormone (TRH)
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Prolactin-inhibiting hormone (PIH)
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Role of Releasing Hormone (cont’d)
• Hormones Released by the Pituitary in Response to the
Release of Hormones from the Hypothalamus
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Growth hormone (GH)
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Follicle-stimulating hormone (FSH)
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Luteinizing hormone (LH)
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Adrenocorticotropic hormone (ACTH)
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Thyroid-stimulating hormone (TSH)
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Prolactin
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Beta-endorphins
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Types of Hormones
• Steroid Hormones
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Made of three 6-carbon rings & one 5-carbon ring
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Synthesized from cholesterol via synthetic pathways
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Actions:
• Released into circulation
• Arrive at target tissue
• Diffuse through cell membrane
• Bind to specific receptor within cell
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Types of Hormones (cont’d)
• Common Steroid Hormones
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Androgens: testosterone (TE)
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Glucocorticoids: cortisone
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Progesterone
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Prostaglandins
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Anabolic steroids: commonly used/abused by athletes
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Types of Hormones (cont’d)
• Peptide Hormones
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Proteins of various sizes
• Peptides: small chains
• Polypeptides: large chains
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Direct product of:
• mRNA translation
• Cleavage from larger parent molecules
• Other postsynthesis modifications
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Circulate & bind to specific receptors on cell membranes of target
tissues
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Types of Hormones (cont’d)
• Common Peptide Hormones
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Insulin
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Insulin-like growth factor-I (IGF-I)
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Superfamily of GH molecules
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Peptides are prone to degradation so pharmaceuticals (insulin
and GH) are not consumed orally and need direct routes into
circulation (injection).
Copyright © 2012 American College of Sports Medicine
Types of Hormones (cont’d)
• Amine Hormones
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Have amine (NH2) group at end of molecule
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Derived from amino acids
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Classified as protein hormones
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Synthesized from:
• Tyrosine
• Phenylalanine
• Tryptophan
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Must bind to surface-bound receptor on target tissue
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Can act as neurotransmitters in autonomic nervous system
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Types of Hormones (cont’d)
• Common Amine Hormones (catecholamines)
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Epinephrine
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Norepinephrine
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Dopamine
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Major Endocrine Glands in the Human
Body
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Hormone Activity: Production, Release,
and Transportation
• Production & Release
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Peptides & amines are synthesized in advance & stored in
vesicles until needed
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Steroids are synthesized from cholesterol & released (not stored)
• Transportation
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Released into circulation & transported to target tissues
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Half-life: time it takes for half of hormone to be degraded
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Transport (binding) proteins
• Protect hormone from metabolism
• Deliver hormone to its receptor
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Hormone Activity: Production, Release,
and Transportation (cont’d)
• What Determines Hormonal Concentrations in the Blood?
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Amount of hormone released
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Pattern of release (pulsatility)
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Rate of metabolism
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Quantity of transport proteins
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Time of day (circadian patterns)
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Plasma volume shifts (during exercise)
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Hormone Activity: Production, Release,
and Transportation (cont’d)
• Negative Feedback Control
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Elevates a hormone when it is low
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Reduces a hormone when it is elevated
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Example: athlete using anabolic steroids—
• Reductions in testosterone production lead to
• Testicular shrinkage due to
• Negative feedback inhibition of endogenous TE production
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Receptor Interaction
• Receptor Specificity
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Lock-and-key principle: only one hormone will unlock or
activate each receptor (homone-key/receptor-lock)
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Cross-reactivity: allows more than one hormone or molecule to
activate a receptor
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Figure 3.3
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Receptor Interaction (cont’d)
• Steroid and Thyroid Hormones
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Steroids, because they are lipophilic:
• Diffuse through target cell membranes
• Bind to receptors in cytoplasm or nucleus
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Receptors are bound to heat-shock proteins in cytoplasm prior to
hormone binding
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Hormones and Exercise
• Exercise Presents a Potent Stimulus for Hormonal
Adaptations
• Resistance Training (RT) Acute Program Variables
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Intensity
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Volume
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Rest intervals
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Exercise selection & sequence
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Repetition velocity
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Frequency
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Hormones and Exercise (cont’d)
• Aerobic, Plyometric, Sprint, & Agility Training Variables
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Intensity
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Volume & duration
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Modality
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Work:rest ratio
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Frequency
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Hormones and Exercise (cont’d)
• Other Variables Affecting Hormonal Responses
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Genetic predisposition
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Gender
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Fitness level
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Nutritional intake
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Potential for adaptation
Copyright © 2012 American College of Sports Medicine
Hormones and Exercise (cont’d)
• Classifications of Hormonal Responses and Adaptations
1. Acute responses during exercise
2. Chronic changes in resting concentrations
3. Chronic changes in acute response to exercise
4. Receptor changes
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Hormones and Exercise (cont’d)
• Testosterone
• Elevated during & immediately following exercise
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Acute TE response weaker in women
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Changes in resting TE during RT are inconsistent
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Regulated by luteinizing hormone (LH)
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Bound to sex hormone-binding globulin (SHBG)
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Precursors: dehydroepiandrosterone (DHEA), androstendione, &
androstenediol
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Androgen, also called androgenic hormone or testoid, is any natural or
synthetic compound, usually a steroid hormone, that stimulates or
controls the development and maintenance of male characteristics
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In a woman's body, one of the main purposes of androgens is to be
converted into the female hormones called estrogens.
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Hormones and Exercise (cont’d)
• Growth Hormone (GH) Superfamily (anterior pituitary-brain)
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Secreted by anterior pituitary
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Released in pulsatile manner
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Under control of GHRH & inhibited by somatostatin
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Exercise is a potent stimulus for GH secretion
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No apparent change in resting GH with consistent exercise
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50% of GH binds to GH-specific binding proteins (GHBPs) that
extend its half-life & enhance its effects
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Hormones and Exercise (cont’d)
• Insulin-Like Growth Factors (IGFs) (liver)
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Structurally related to insulin & mediate many actions of GH
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Small polypeptide hormones secreted by liver in response to GH
stimulation
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Increase proliferation & differentiation of satellite cells & protein
synthesis
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Enhance muscle hypertrophy
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IGF-1 response to exercise is unclear
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No consistent pattern of change in IGF-1 during exercise
Copyright © 2012 American College of Sports Medicine
Hormones and Exercise (cont’d)
• Insulin (pancreas)
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Secreted from Islets of Langerhans (B cells) in pancreas
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Secreted in response to glucose intake
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Increases muscle protein synthesis when amino acid is adequate
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Aerobic & anaerobic training:
• Improves insulin sensitivity
• Reduces insulin resistance
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Hormones and Exercise (cont’d)
• Glucagon (pancreas)
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Protein hormone consisting of 29 amino acids
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Synthesized in A cells in Islets of Langerhans of pancreas
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Inhibited by glucose levels
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Stimulates breakdown of glycogen
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Increases energy availability
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Elevated during exercise as energy demands increase
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Hormones and Exercise (cont’d)
• Cortisol (adrenal cortex; perimeter of adrenal gland top of kidney)
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Catabolic glucocorticoid released from adrenal cortex in response
to stress under the control of CRH & ACTH
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Stimulates lipolysis in adipose cells
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Increases protein degradation in muscle cells
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Decreases protein synthesis in muscle cells
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Elevated during aerobic exercise, with greater response
occurring with greater intensity
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Increased during RT
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Resting concentrations reflect a long-term stress & are transient
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Hormones and Exercise (cont’d)
• Catecholamines (adrenal medulla; adrenal gland-top kidney)
• Included among catecholamines
are epinephrine (adrenaline), norepinephrine (noradrenaline), anddopamine,
all of which are produced from phenylalanine and tyrosine. Release of
the hormones epinephrineand norepinephrine from the adrenal medulla of
the adrenal glands is part of the fight-or-flight response.[3
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Secreted by adrenal medulla
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Secreted in response to stress (physical, heat, hypoxia, hypoglycemia)
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Increased during both aerobic & anaerobic exercise
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Magnitude of increase depends on:
• Muscle mass involved
• Posture
• Intensity
• Duration
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Hormones and Exercise (cont’d)
• β-Endorphins (anterior pituitary-brain)
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31-amino acid peptide cleaved in anterior pituitary from parent
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Act as neurotransmitters in nervous system & as analgesics
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Increase relaxation
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Enhance immune function
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Increased with exercise, with response depending on intensity &
duration
Copyright © 2012 American College of Sports Medicine
Hormones and Exercise (cont’d)
• Thyroid Hormones (thyroid gland-neck)
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Thyroxine (T4) & triiodothyronine (T3) released into circulation
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Travel mostly bound to transport proteins
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Increase basal metabolic rate
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Increase protein synthesis
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Augment actions of catecholamines
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Response to exercise not clear
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Hormones and Exercise (cont’d)
• Fluid-Regulatory Hormones
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Fluid homeostasis is critical to exercise performance
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Elevations in AVP, atrial peptide, renin, aldosterone, &
angiotensin II during exercise
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Magnitude of increase dependent on intensity, duration, fitness
level, & hydration status
Copyright © 2012 American College of Sports Medicine
Hormones and Exercise (cont’d)
• Leptin (adipose tissue)
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Product of ob gene in adipose tissue
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Relays satiety signal to hypothalamus to regulate energy balance
& appetite
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Higher levels in obese individuals (4 times greater) & women
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Crosses blood-brain barrier to act with receptors
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Concentrations influenced by insulin, glucocorticoids,
catecholamines, thyroid hormones, TE, GH, & stimulants
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Not affected by exercise, independent of % body fat
Copyright © 2012 American College of Sports Medicine
• Leptin and ghrelin are two hormones that have been recognized to have a
major influence on energy balance. Leptin is a mediator of long-term
regulation of energy balance, suppressing food intake and thereby inducing
weight loss. Ghrelin on the other hand is a fast-acting hormone, seemingly
playing a role in meal initiation. As a growing number of people suffer from
obesity, understanding the mechanisms by which various hormones and
neurotransmitters have influence on energy balance has been a subject of
intensive research. In obese subjects the circulating level of the anorexigenic
hormone leptin is increased, whereas surprisingly, the level of the orexigenic
hormone ghrelin is decreased. It is now established that obese patients are
leptin-resistant. However, the manner in which both the leptin and ghrelin
systems contribute to the development or maintenance of obesity is as yet not
clear.
Copyright © 2012 American College of Sports Medicine
Hormones and Exercise (cont’d)
• Estrogens (Ovaries)
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Include estradiol, estriol, & estrone
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Perform many functions such as promoting secondary female sex
characteristics, reducing bone resorption, stimulating growth plate
closure, enhancing metabolism, retaining sodium and water, and
reducing muscle damage.
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Have long half-lives
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Synthesized & secreted primarily by ovaries in women, under control of
LH & FSH
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Produced from conversion of androgens in men
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Response to exercise unclear
Copyright © 2012 American College of Sports Medicine