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Hormonal Control During Exercise CHAPTER 4 Overview • Endocrine system • Hormones (types, receptors, actions) • Endocrine glands and their hormones • Hormonal regulation of metabolism during exercise • Hormonal regulation of fluid and electrolytes during exercise The Endocrine System • A communication system – Nervous system = electrical communication – Endocrine system = chemical communication • Slower responding, longer lasting than nervous system • Maintains homeostasis via hormones – Chemicals that control and regulate cell/organ activity – Act on target cells • Constantly monitors internal environment The Endocrine System • Coordinates integration of physiological systems during rest and exercise • Maintains homeostasis during exercise – Controls substrate metabolism – Regulates fluid, electrolyte balance Figure 4.1 Hormones: Steroid Hormones • Derived from cholesterol • Lipid soluble, diffuse through membranes • Secreted by four major glands – – – – Adrenal cortex (cortisol, aldosterone) Ovaries (estrogen, progesterone) Testes (testosterone) Placenta (estrogen, progesterone) Hormones: Nonsteroid Hormones • Not lipid soluble, cannot cross membranes • Divided into two groups – Protein/peptide hormones • Most nonsteroid hormones • From pancreas, hypothalamus, pituitary gland, – Amino acid-derived hormones • Thyroid hormones (T3, T4) • Adrenal medulla hormones (epinephrine, norepinephrine) Hormone Secretion • Secreted in bursts (pulsatile) – Plasma concentrations fluctuate over minutes/hours – Concentrations also fluctuate over days/weeks – What triggers or regulates hormone bursts? • Secretion regulated by negative feedback – – – – Hormone release causes change in body High level of downstream change secretion Low level of downstream change secretion Example: home thermostat Hormone Activity • Plasma concentration can be poor indicator of hormone activity – Cells change sensitivity to hormones – Number of receptors on cell surface can change • Downregulation: number of receptors during high plasma concentration = desensitization • Upregulation: number of receptors during high plasma concentration = sensitization Hormone Receptors • Hormones limit scope of their effects by using hormone-specific receptors • No receptor on cell surface = no hormone effect – Hormone only affects tissues with specific receptor – Hormone exerts effects after binding with receptor – Typical cell has 2,000 to 10,000 receptors • Hormone binds to receptor: hormone– receptor complex Steroid Hormone Actions • Lipid soluble (can cross cell membranes) • Steroid hormone receptors found inside cell, in cytoplasm or nucleus • Hormone–receptor complex enters nucleus – Binds to DNA, direct gene activation – Regulates mRNA synthesis, protein synthesis Figure 4.2 Nonsteroid Hormone Actions • Not lipid soluble (cannot cross cell membrane) • Receptors on cell membrane second messengers – Carry out hormone effects – Intensify strength of hormone signal • Common second messengers – Cyclic adenosine monophosphate (cAMP) – Cyclic guanine monophosphate (cGMP) – Inositol triphosphate (IP3), diacylglycerol (DAG) Figure 4.3 Hormones: Prostaglandins • Third class of (pseudo)hormones • Derived from arachidonic acid • Act as local hormones, immediate area – Inflammatory response (swelling, vasodilation) – Sensitize nociceptor free nerve endings (pain) Endocrine Glands and Their Hormones • Several endocrine glands in body; each may produce more than one hormone • Hormones regulate physiological variables during exercise Hormonal Regulation of Metabolism During Exercise • Major endocrine glands responsible for metabolic regulation – – – – Anterior pituitary gland Thyroid gland Adrenal gland Pancreas • Hormones released by these glands affect metabolism of carbohydrate and fat during exercise Endocrine Regulation of Metabolism: Anterior Pituitary Gland • Pituitary gland attached to inferior hypothalamus • Three lobes: anterior, intermediate, posterior • Secretes hormones in response to hypothalamic hormone factors – Releasing factors, inhibiting factors – Exercise secretion of all anterior pituitary hormones Endocrine Regulation of Metabolism: Anterior Pituitary Gland • Releases growth hormone (GH) – – – – Potent anabolic hormone Builds tissues, organs Promotes muscle growth (hypertrophy) Stimulates fat metabolism • GH release proportional to exercise intensity Endocrine Regulation of Metabolism: Thyroid Gland • Secretes triiodothyronine (T3), thyroxine (T4) • T3 and T4 lead to increases in – – – – – – Metabolic rate of all tissues Protein synthesis Number and size of mitochondria Glucose uptake by cells Rate of glycolysis, gluconeogenesis FFA mobilization Endocrine Regulation of Metabolism: Thyroid Gland • Anterior pituitary releases thyrotropin – Also called thyroid-stimulating hormone (TSH) – Travels to thyroid, stimulates T3 and T4 • Exercise increases TSH release – Short term: T4 (delayed release) – Prolonged exercise: T4 constant, T3 Endocrine Regulation of Metabolism: Adrenal Medulla • Releases catecholamines (fight or flight) – Epinephrine 80%, norepinephrine 20% – Exercise sympathetic nervous system epinephrine and norepinephrine • Catecholamine release increases – Heart rate, contractile force, blood pressure – Glycogenolysis, FFA – Blood flow to skeletal muscle Endocrine Regulation of Metabolism: Adrenal Cortex • Releases corticosteroids – Glucocorticoids – Also, mineralocorticoids, gonadocorticoids • Major glucocorticoid: cortisol – Gluconeogenesis – FFA mobilization, protein catabolism – Anti-inflammatory, anti-immune Endocrine Regulation of Metabolism: Pancreas • Insulin: lowers blood glucose – Counters hyperglycemia, opposes glucagon – Glucose transport into cells – Synthesis of glycogen, protein, fat – Inhibits gluconeogenesis • Glucagon: raises blood glucose – Counters hypoglycemia, opposes insulin – Glycogenolysis, gluconeogenesis Regulation of Carbohydrate Metabolism During Exercise • Glucose must be available to tissues • Glycogenolysis (glycogen glucose) • Gluconeogenesis (FFAs, protein glucose) Regulation of Carbohydrate Metabolism During Exercise • Adequate glucose during exercise requires – Glucose release by liver – Glucose uptake by muscles • Hormones that circulating glucose – – – – Glucagon Epinephrine Norepinephrine Cortisol Regulation of Carbohydrate Metabolism During Exercise • Circulating glucose during exercise also affected by – GH: FFA mobilization, cellular glucose uptake – T3, T4: glucose catabolism and fat metabolism • Amount of glucose released from liver depends on exercise intensity, duration Regulation of Carbohydrate Metabolism During Exercise • As exercise intensity increases – Catecholamine release – Glycogenolysis rate (liver, muscles) – Muscle glycogen used before liver glycogen • As exercise duration increases – More liver glycogen utilized – Muscle glucose uptake liver glucose release – As glycogen stores , glucagon levels Figure 4.4 Regulation of Carbohydrate Metabolism During Exercise • Glucose mobilization only half the story • Insulin: enables glucose uptake in muscle • During exercise – Insulin concentrations – Cellular insulin sensitivity – More glucose uptake into cells, use less insulin Figure 4.5 Regulation of Fat Metabolism During Exercise • FFA mobilization and fat metabolism critical to endurance exercise performance – Glycogen depleted, need fat energy substrates – In response, hormones accelerate fat breakdown (lipolysis) • Triglycerides FFAs + glycerol – Fat stored as triglycerides in adipose tissue – Broken down into FFAs, transported to muscle – Rate of triglyceride breakdown into FFAs may determine rate of cellular fat metabolism Regulation of Fat Metabolism During Exercise • Lipolysis stimulated by – – – – – (Decreased) insulin Epinephrine Norepinephrine Cortisol GH • Stimulate lipolysis via lipase Hormonal Regulation of Fluid and Electrolytes During Exercise • During exercise, plasma volume , causing – Hydrostatic pressure, tissue osmotic pressure – Plasma water content via sweating – Heart strain, blood pressure • Hormones correct fluid imbalances – Posterior pituitary gland – Adrenal cortex – Kidneys Hormonal Regulation of Fluid and Electrolytes: Posterior Pituitary • Posterior pituitary – Secretes antidiuretic hormone (ADH), oxytocin – Produced in hypothalamus, travels to posterior pituitary – Secreted upon neural signal from hypothalamus • Only ADH involved with exercise – Water reabsorption at kidneys – Less water in urine, antidiuresis Hormonal Regulation of Fluid and Electrolytes: Posterior Pituitary • Stimuli for ADH release – Plasma volume = hemoconcentration = osmolality – Osmolality stimulates osmoreceptors in hypothalamus • ADH released, increasing water retention by kidneys • Minimizes water loss, severe dehydration Figure 4.6 Hormonal Regulation of Fluid and Electrolytes: Adrenal Cortex • Adrenal cortex – Secretes mineralocorticoids – Major mineralocorticoid: aldosterone • Aldosterone effects – Na+ retention by kidneys – Na+ retention water retention via osmosis – Na+ retention K+ excretion Hormonal Regulation of Fluid and Electrolytes: Adrenal Cortex • Stimuli for aldosterone release – Plasma Na+ – Blood volume, blood pressure – Plasma K+ • Also indirectly stimulated by blood volume, blood pressure in kidneys Hormonal Regulation of Fluid and Electrolytes: Kidneys • Kidneys – Target tissue for ADH, aldosterone – Secrete erythropoietin (EPO), renin • EPO – Low blood O2 in kidneys EPO release – Stimulates red blood cell production – Critical for adaptation to training, altitude Hormonal Regulation of Fluid and Electrolytes: Kidneys • Stimulus for renin (enzyme) release – Blood volume, blood pressure – Sympathetic nervous system impulses • Renin-angiotensin-aldosterone mechanism – Renin: converts angiotensinogen angiotensin I – ACE: converts angiotensin I angiotensin II – Angiotensin II stimulates aldosterone release Figure 4.7 Figure 4.8 Figure 4.9 Hormonal Regulation of Fluid and Electrolytes: Osmolality • Osmolality – Measure of concentration of dissolved particles (proteins, ions, etc.) in body fluid compartments – Normal value: ~300 mOsm/kg • Osmolality and osmosis – If compartment osmolality , water drawn in – If compartment osmolality , water drawn out Hormonal Regulation of Fluid and Electrolytes: Osmolality • Aldosterone and osmosis – Na+ retention osmolality – Osmolality water retention – Where Na+ moves, water follows • Osmotic water movement minimizes loss of plasma volume, maintains blood pressure Hormonal Regulation of Fluid and Electrolytes: Osmolality • ADH, aldosterone effects persist for 12 to 48 h after exercise • Prolonged Na+ retention abnormally high [Na+] after exercise – Water follows Na+ – Prolonged rehydration effects