Hormonal Control During Exercise Study Notes

Chapter 4: Hormonal Control During Exercise

Overview of Chapter 4

  • Focus on key areas related to hormonal regulation during physical exercise.

    • Endocrine System

    • Types of Hormones

    • Hormone Receptors & Actions

    • Endocrine Glands and Their Hormones

    • Hormonal Regulation of Metabolism During Exercise

    • Hormonal Regulation of Fluid and Electrolytes During Exercise

The Endocrine System

  • Definition: A communication system in the body that uses hormones for signaling.

    • Comparison to Nervous System:

    • Nervous System: Electrical communication.

    • Endocrine System: Chemical communication.

    • Characteristics:

    • Slower to respond but effects last longer than the nervous system.

    • Maintains homeostasis through hormones.

  • Functions:

    • Regulates and controls cell and organ activity.

    • Constantly monitors the internal environment.

    • Maintains homeostasis during exercise, controlling substrate metabolism and regulating fluid and electrolyte balance.

Major Components of the Endocrine System

  • Glands:

    • Hypothalamus

    • Pituitary gland

    • Thyroid gland

    • Parathyroid glands

    • Thymus gland

    • Adrenal glands

    • Pancreas

    • Kidneys

    • Ovaries (in females)

    • Testes (in males)

Types of Hormones

Steroid Hormones

  • Nature:

    • Derived from cholesterol.

    • Lipid soluble, can diffuse through membranes.

  • Source:

    • Secreted by major glands such as:

    • Adrenal cortex (producing cortisol or cortisone, aldosterone).

    • Ovaries (producing estrogen, progesterone).

    • Testes (producing testosterone).

    • Placenta (producing estrogen, progesterone).

Nonsteroid Hormones

  • Nature:

    • Not lipid soluble, cannot cross cell membranes.

  • Types:

    • Protein/Peptide Hormones: Majority of non-steroid hormones, derived from pancreas, hypothalamus, pituitary gland.

    • Amino Acid-Derived Hormones: Include thyroid hormones (T3, T4), and adrenal medulla hormones (epinephrine, norepinephrine).

Hormone Classification

  • Amine Hormone:

    • Modified amino acids.

    • Example: Norepinephrine (contains a benzene ring).

  • Peptide Hormone:

    • Short chains of linked amino acids.

    • Example: Oxytocin.

  • Protein Hormone:

    • Long chains of linked amino acids.

    • Examples: Human Growth Hormone, Testosterone, Progesterone.

Hormone Secretion

  • Secretion Pattern:

    • Hormones are released in bursts (pulsatile).

    • Plasma concentrations fluctuate over time (minutes to weeks).

  • Regulation:

    • Controlled by negative feedback loops:

    • High levels of a downstream factor decrease hormone secretion.

    • Low levels of a downstream factor increase hormone secretion.

Hormone Activity

  • Plasma Concentration:

    • Not a reliable indicator of hormone activity.

  • Cell Sensitivity:

    • Cells can change their sensitivity to hormones.

    • Number of Receptors: Can increase or decrease on cell surfaces:

    • Downregulation: Decrease in receptors during high plasma concentration leads to desensitization.

    • Upregulation: Increase in receptors during high plasma concentration leads to sensitization.

Hormone Receptors

  • Characteristics:

    • Hormones exert effects through specific receptors.

    • No receptor means no hormonal effect.

    • Typical cells contain 2,000 to 10,000 hormone receptors.

    • When a hormone binds to its receptor, it forms a hormone-receptor complex.

Actions of Hormones

Steroid Hormones

  • Mechanism:

    • Lipid soluble, can cross cell membranes.

    • Act mainly within the cytoplasm or nucleus.

    • Hormone-receptor complex binds to DNA, leading to direct gene activation and regulating mRNA synthesis and protein synthesis.

Nonsteroid Hormones

  • Mechanism:

    • Not lipid soluble, cannot enter the cell.

    • Bind to receptors on the cell membrane and activate second messengers.

    • Common second messengers include:

    • Cyclic adenosine monophosphate (cAMP).

    • Cyclic guanine monophosphate (cGMP).

    • Inositol triphosphate (IP3), diacylglycerol (DAG).

Prostaglandins

  • Classification: Retrieved as a third class of (pseudo) hormones.

  • Source: Derived from arachidonic acid.

  • Function: Act as local hormones, mediating inflammatory responses, including swelling and vasodilation, and sensitizing nociceptor free nerve endings (involved in pain perception).

Hormonal Regulation of Metabolism During Exercise

  • Key Endocrine Glands: Major glands responsible for metabolic regulation during exercise include:

    • Anterior Pituitary Gland

    • Thyroid Gland

    • Adrenal Gland

    • Pancreas

  • Hormonal Effects: The hormones released from these glands affect the metabolism of carbohydrates and fats during exercise.

Anterior Pituitary Gland

  • Characteristics:

    • Attached to the inferior part of the hypothalamus.

    • Composed of three lobes: anterior, intermediate, posterior.

    • Secretes hormones in response to hypothalamic hormones (both releasing factors and inhibiting factors).

  • Response to Exercise:

    • Exercise increases the secretion of all anterior pituitary hormones.

  • Neurosecretory Cells:

    • Produce Antidiuretic Hormone (ADH) and Oxytocin.

    • These hormones travel down axons to axon endings, getting secreted as needed into the bloodstream.

Hormones Release Examples

  • Growth Hormone (GH):

    • Potent anabolic hormone; crucial for building tissues and organs.

    • Promotes muscle growth (hypertrophy) and stimulates fat metabolism while decreasing carbohydrate utilization.

    • GH release is proportional to exercise intensity.

  • Thyroid Stimulating Hormone (TSH):

    • Released by the anterior pituitary, stimulates T3 and T4 release from the thyroid.

    • Exercise increases TSH release, resulting in a temporary increase in T4, followed by stabilized T3 levels during prolonged exercise.

Thyroid Gland Hormones Effects

  • Secretion:

    • Produces T3 (triiodothyronine) and T4 (thyroxine).

    • Increases basal metabolic rate in all tissues and enhances various metabolic processes such as:

    • Protein synthesis,

    • Number/size of mitochondria,

    • Glucose uptake by cells,

    • Rate of glycolysis and gluconeogenesis,

    • Mobilization of free fatty acids (FFAs).

Adrenal Medulla Regulation

  • Response:

    • Increases release of catecholamines (epinephrine 80%, norepinephrine 20%) during exercise.

    • Increased catecholamines lead to increased heart rate, contractile force, and blood pressure, as well as enhanced glycogenolysis and FFA availability for energy during exercise.

Adrenal Cortex Hormones

  • Release:

    • Produces corticosteroids, including glucocorticoids and mineralocorticoids.

  • Primary Glucocorticoid: Cortisol (cortisone), aids in gluconeogenesis, preserves glucose, stimulates FFA mobilization, and has anti-inflammatory effects.

Pancreas Hormones

  • Insulin:

    • Lowers blood glucose levels, an anabolic hormone that counters hyperglycemia by promoting glucose transport into cells and facilitating the synthesis of glycogen, protein, and fat.

  • Glucagon:

    • Raises blood glucose levels by promoting glycogenolysis and gluconeogenesis, countering hypoglycemia.

Carbohydrate Metabolism Regulation During Exercise

  • Key Hormones Involved:

    • Glucagon, Epinephrine, Norepinephrine, Cortisol

  • Process: Glucose release by the liver and uptake by muscle is essential, with hormone levels influencing circulating glucose.

  • Exercise Intensity Effect:

    • High intensity increases catecholamine release, thus enhancing glycogenolysis rates in liver and muscles. Muscle glycogen is utilized first, while more liver glycogen is consumed as exercise duration increases.

Glucose Mobilization Issues

  • Insulin Role During Exercise:

    • Insulin concentrations drop, but cellular insulin sensitivity increases, allowing for enhanced glucose uptake into cells with less insulin needed.

Fat Metabolism Regulation During Exercise

  • Importance:

    • Essential for endurance performance, especially when glycogen is depleted, hormones facilitate fat breakdown (lipolysis).

  • Triglycerides:

    • Stored in adipose tissue, converted to FFAs and glycerol and utilized for energy in muscles.

  • Factors That Stimulate Lipolysis:

    • Decreased insulin, increased epinephrine, norepinephrine, cortisol, and growth hormone (GH).

Hormonal Regulation of Fluid and Electrolytes During Exercise

  • Effects of Exercise on Plasma Volume:

    • Decreases plasma volume due to sweating, leading to strain on heart and drop in blood pressure.

  • Hormonal Responses:

    • Posterior pituitary, adrenal cortex, and kidneys regulate fluid imbalances through various hormonal actions.

Posterior Pituitary and ADH

  • Functions:

    • Secretes Antidiuretic Hormone (ADH), produced in the hypothalamus, promoting water reabsorption in kidneys, reducing urine output and assisting in fluid retention during exercise.

Adrenal Cortex and Aldosterone

  • Role of Aldosterone:

    • Enhances sodium retention in kidneys, leading to water retention and subsequently increasing blood pressure.

  • Stimuli for Release:

    • Low plasma sodium, low blood volume, increased plasma potassium levels.

Kidney Functions in Regulation

  • Erythropoietin (EPO):

    • Stimulated by low blood oxygen levels, promoting red blood cell production necessary for training adaptations and altitude adjustments.

  • Renin Release Mechanism:

    • Triggered by low blood volume/blood pressure or sympathetic nervous system impulses. Renin converts angiotensinogen to angiotensin I; conversion to angiotensin II occurs in the lungs, stimulating aldosterone release and increasing blood pressure.

Osmolality and Fluid Regulation

  • Definition:

    • Measure of concentration of dissolved particles in body fluids; normal value approximately 300 mOsm/kg.

  • Mechanism:

    • Changes in osmolality influence fluid movement; if osmolality increases, water is drawn in, and if it decreases, water moves out.

Post-Exercise Hormonal Effects

  • Sustained Effects: Effects of ADH and aldosterone persist for 12 to 48 hours post-exercise, aiding in water and sodium retention to counterbalance fluid loss during workouts.

  • Rehydration Challenges: Prolonged retention of sodium and water can lead to elevated sodium concentrations after exercise, necessitating rehydration strategies.