Hormonal Control During Exercise

Chapter 4: Hormonal Control During Exercise

Overview

• The chapter will cover the following topics:
  • The endocrine system
  • Endocrine glands and their hormones
  • Hormonal regulation of metabolism during exercise
  • Hormonal regulation of fluid and electrolytes during exercise
  • Hormonal regulation of caloric intake
  • Growth hormone and testosterone

Endocrine System

• Communication system of the body.
  • Nervous system: electrical communication
  • Endocrine system: chemical communication (hormones)
• Slower to respond but longer lasting than the nervous system.
• Maintains homeostasis via hormones.
  • Controls and regulates cell and organ activity.
  • Acts on target cells.
• Constantly monitors internal environment.
• Coordinates integration of physiological systems during rest and exercise.
• Maintains homeostasis during exercise.
  • Controls substrate metabolism.
  • Regulates fluid and electrolyte balance.

Endocrine Glands

• Major endocrine glands include:
  • Hypothalamus
  • Pituitary gland
  • Thyroid gland
  • Parathyroid glands
  • Thymus gland
  • Adrenal glands
  • Pancreas
  • Kidneys
  • Adipose tissue
  • Ovaries (in females)
  • Testes (in males)

Hormones: Steroid

• 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

• Not lipid soluble, unable to cross membranes.
• Two groups
  • Protein or peptide
    • Most nonsteroid hormones
    • From pancreas, hypothalamus, pituitary gland
  • Amino acid derived
    • Thyroid hormones (T3, T4)
    • Adrenal medulla hormones (epinephrine, norepinephrine)

Hormone Secretion

• Secreted in brief bursts (pulsatile).
  • Plasma concentrations fluctuate over minutes/hours.
  • Concentrations can also fluctuate over days/weeks.
  • Regulation/triggers of hormone bursts:
• Regulated by negative feedback.
  • Hormone release causes change in body.
  • Large downstream change reduces secretion.
  • Small downstream change increases secretion.

Hormone Secretion and Plasma Concentration

• Plasma concentration can be a 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 affects only 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, directs gene activation.
  • Regulates mRNA synthesis and protein synthesis.

Nonsteroid Hormone Actions

• Not lipid soluble: unable to 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)

Hormones: Prostaglandins

• Form third class of (pseudo)hormones.
• Are derived from arachidonic acid.
• Act as local hormones (autocrines).
  • Exert effects in immediate area where they are produced.
  • Mediate inflammatory response (swelling, vasodilation).
  • Sensitize free nerve endings (pain).

Endocrine Glands and Their Hormones

• Each of the body’s several endocrine glands 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 affect metabolism of carbohydrate and fat during exercise.

Endocrine Regulation of Metabolism: Anterior Pituitary Gland

• Pituitary gland is attached to inferior hypothalamus.
• Three lobes: anterior, intermediate, posterior.
• Secretes hormones in response to hypothalamic hormone factors:
  • Both releasing factors and inhibiting factors.
  • Exercise increases secretion of all anterior pituitary hormones.
• Releases growth hormone (GH).
  • Is a 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 rates of all tissues,
  • protein synthesis,
  • number and size of mitochondria,
  • glucose uptake by cells,
  • rates of glycolysis and gluconeogenesis, and
  • FFA mobilization.
• Anterior pituitary releases thyrotropin.
  • Also called thyroid-stimulating hormone (TSH).
  • Travels to thyroid, stimulates T3 and T4 release.
• Exercise increases TSH release.
  • Short term: T4 increases (delayed release).
  • Prolonged exercise: T4 is constant, T3 decreases.

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
  • Blood flow redistribution to skeletal muscle
  • Increased glucose, FFA release

Endocrine Regulation of Metabolism: Adrenal Cortex

• Releases corticosteroids.
  • Glucocorticoids
  • Mineralocorticoids
  • Gonadocorticoids
• Major glucocorticoid is cortisol.
  • Stimulates gluconeogenesis.
  • Increases FFA mobilization, protein catabolism.
  • Acts as an anti-inflammatory, depresses anti- immune reactions.
  • Stimulates protein catabolism.

Endocrine Regulation of Metabolism: Pancreas

• Insulin: Lowers blood glucose.
  • Counters hyperglycemia, opposes glucagon.
  • Facilitates glucose transport into cells.
  • Enhances synthesis of glycogen, protein, fat.
  • Inhibits gluconeogenesis.
• Glucagon: Raises blood glucose.
  • Counters hypoglycemia, opposes insulin.
  • Promotes glycogenolysis, gluconeogenesis.

Regulation of Carbohydrate Metabolism During Exercise

• Glucose must be available to tissues.
• Glycogenolysis (glycogen glucose)
• Gluconeogenesis (FFAs, protein glucose)
• Adequate glucose during exercise requires:
  • glucose release by liver and
  • glucose uptake by muscles.
• Some hormones increase circulating glucose.
  • Glucagon
  • Epinephrine
  • Norepinephrine
  • Cortisol
• Circulating glucose during exercise is 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 and duration.
• As exercise intensity increases,
  • catecholamine release increases,
  • glycogenolysis rate increases (liver, muscles), and
  • muscle glycogen is used before liver glycogen.
• As exercise duration increases,
  • more liver glycogen is used;
  • muscle glucose uptake liver glucose release;
  • glycogen stores , glucagon levels .
• Glucose mobilization is only half the story.
• Insulin enables glucose uptake in muscle.
• During exercise
  • Insulin concentrations decrease;
  • Cellular insulin sensitivity increases.
  • More glucose is taken up into cells, and less insulin is used.

CNS–Endocrine System Interaction

• CNS regulates carbohydrate metabolism through hormones (insulin) and nutrients (glucose, fatty acids, amino acids).
  • Brain is sensitive to glucose; helps control insulin release.
  • Leptin and GLP-1, hormones released by adipose tissue and the gut, respectively; act through the CNS to decrease glucose production.
• Glucose: only substrate for brain metabolism

Regulation of Fat Metabolism During Exercise

• FFA mobilization and fat metabolism: critical to endurance exercise
  • Glycogen depleted, fat energy substrates needed
  • Response: fat breakdown (lipolysis) accelerated
• Triglycerides FFAs + glycerol
  • Fat stored as triglycerides in adipose tissue
  • Broken down into FFAs, transported to muscle
  • Rate of triglyceride breakdown into FFAs: possible determinant of rate of cellular fat metabolism
• Lipolysis is stimulated by
  • (decreased) insulin,
  • epinephrine,
  • norepinephrine,
  • cortisol, and
  • GH.
• They stimulate lipolysis via lipase.

Muscle as an Endocrine Organ

• Skeletal muscle fibers release cytokines and myokines
  • Myostatin
  • Interleukins
  • BDNF
  • Irisin
  • Lactate
• Cross talk between exercising muscle and other tissues

Hormonal Regulation of Fluid and Electrolytes During Exercise

• During exercise, plasma volume , causing
  • hydrostatic pressure, tissue osmotic pressure;
  • plasma water content via sweating; and
  • 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.
  • Is produced in hypothalamus, travels to posterior pituitary.
  • Is secreted upon neural signal from hypothalamus.
• Only ADH is involved with exercise:
  • water reabsorption at kidneys
  • Less water in urine, antidiuresis
• Osmolality
  • Measure of concentration of dissolved particles (e.g., proteins, ions) in body fluid compartments
  • Normal value = ~$300$ mOsm/kg
• Osmolality and osmosis
  • If compartment osmolality , water is drawn in.
  • If compartment osmolality , water is drawn out.
• Increased plasma osmolality serves as stimulus for ADH release.
  • plasma volume = hemoconcentration = osmolality
  • osmolality stimulates osmoreceptors in hypothalamus
• ADH is released, increasing water retention by kidneys.
• Minimizes water loss, severe dehydration.

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
• Stimuli for aldosterone release
  • Plasma $Na^+$
  • Blood volume, blood pressure
  • Plasma $K^+$
• Also, indirect stimulation by blood volume, blood pressure in kidneys

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.
• ADH, aldosterone effects persist for 12 to 48 h after exercise.
• Prolonged $Na^+$ retention leads to abnormally high [$Na^+$] after exercise.
  • Water follows $Na^+$.
  • Prolonged rehydration effects.

Hormonal Regulation of Fluid and Electrolytes: Kidneys

• Kidneys
  • Target tissues for ADH, aldosterone.
  • Secrete erythropoietin (EPO), renin.
• 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.
• EPO
  • Released in response to low blood O2 in kidneys.
  • Stimulates red blood cell production.
  • Critical for adapting to training, altitude.

Hormonal Regulation of Calorie Intake

• Hypothalamus: the brain’s appetite control center
  • Satiety center in ventromedial nucleus
  • Hunger center in lateral hypothalamus
• GI tract releases hormones that affect hunger signals.
  • Cholecystokinin (CCK): stimulated when stomach is full; decreases appetite.
  • Glucagon-like peptide 1 (GLP-1): released in small intestine; decreases appetite.
  • Peptide YY (PYY): released in small intestine; decreases appetite.
  • Ghrelin: Increases appetite.
• Adipose: an endocrine organ
  • Leptin is released from adipose stores and reduces hunger.
  • Leptin and ghrelin act in opposing ways.
  • Obese people have higher leptin but are often resistant to its effects.
• Exercise affects hunger and satiety hormones.
  • Acute, vigorous exercise increases PYY and GLP-1 and reduces ghrelin, reducing hunger.
  • Exercise training does not affect ghrelin except in energy deficit.

Growth Hormone and Testosterone

• Elevated during resistance exercise
• Testosterone
  • Limited effect on hypertrophy
  • Not associated with postexercise protein synthesis
• GH
  • Not anabolic in actual muscle tissue
  • May alter lipolysis