Hormonal Control During Exercise

Control Systems of Bodily Functions

• Comparison of Regulatory Systems:

  • Nervous System:

  • Characterized by an immediate physiological response.

  • Produces localized, short-term effects.

  • Endocrine System:

  • Responds with a slower onset of action.

  • Produces longer-lasting, more generalized systemic effects.

Endocrine vs. Exocrine Glands

• Endocrine Glands:

  • Ductless glands that secrete chemical messengers (hormones) directly into the blood plasma.

• Exocrine Glands:

  • Secretory glands that release products through specialized ducts onto epithelial surfaces or into target organs (e.g., sweat glands).

• Pancreas Dual Functionality:

  • Functions as an exocrine gland by secreting digestive enzymes into the digestive tract.

  • Functions as an endocrine gland by secreting hormones (insulin and glucagon) directly into the bloodstream.

• Major Endocrine Organs:

  • Hypothalamus

  • Pituitary gland

  • Thyroid and parathyroid glands

  • Thymus gland

  • Adrenal glands

  • Pancreas

  • Ovaries

  • Testes

Chemical Nature and Mechanisms of Action of Hormones

• General Hormone Characteristics:

  • Hormones act as chemical messengers traveling in the blood, establishing direct contact with all cells in the body.

  • Non-endocrine tissues can also release hormones (e.g., nerve endings).

  • Receptors are specific to hormones such that only the correct hormone fits into its designated receptor.

  • Each target cell contains between $2{,}000$ and $10{,}000$ specific receptors.

• Steroid Hormones:

  • Chemical Nature: Lipid-soluble molecules whose chemical structures are derived from or similar to cholesterol.

  • Diffusion Ability: Diffuse easily through lipophilic cell membranes.

  • Receptor Localization: Receptors are located within the cell (in the cytoplasm or nucleus).

  • Secretion Sites:

  • Adrenal cortex (e.g., cortisol)

  • Ovaries (e.g., estrogen)

  • Testes (e.g., testosterone)

  • Placenta (e.g., estrogen)

  • Mechanism of Action (Direct Gene Activation):

  1. The steroid hormone enters the target cell by diffusing directly across the phospholipid cell membrane.

  2. The hormone binds to a specific receptor located in the cytoplasm or inside the nucleus.

  3. The formed hormone-receptor complex activates the cell's DNA, initiating transcription to form mRNA.

  4. The mRNA leaves the nucleus and enters the cytoplasm.

  5. The mRNA directs protein synthesis in the cytoplasm.

• Nonsteroid Hormones:

  • Chemical Nature: Nonlipid-soluble molecules derived from amino acids or proteins.

  • Diffusion Ability: Cannot pass directly through the cell membrane.

  • Receptor Localization: Binds to specific target receptors located on the cell membrane.

  • Classification:

  • Amino acid derivatives (e.g., epinephrine, norepinephrine).

  • Protein or peptide hormones (e.g., insulin).

  • Mechanism of Action (Second Messenger Signal Cascade):

  1. Nonsteroid hormones cannot pass through the cell membrane.

  2. The hormone binds to a specific receptor on the outer cell membrane surface.

  3. The hormone-receptor complex activates adenylate cyclase within the cell membrane.

  4. Adenylate cyclase catalyzes the conversion of adenosine triphosphate ($\text{ATP}$) into cyclic adenosine monophosphate ($\text{cAMP}$).

  5. $\text{cAMP}$ acts as a second messenger, activating inactive protein kinase enzymes that lead to cellular changes, active substrates, and specific hormonal effects.

• Prostaglandins:

  • A third class of pseudo-hormones derived from arachidonic acid (a fatty acid component of cell membranes).

  • Function as local hormones acting in the immediate tissue area.

  • Mediate the inflammatory response (inducing swelling and vasodilation).

  • Sensitize nociceptor free nerve endings, modulating pain transmission.

Regulation of Hormone Secretion and Cell Sensitivity

• Plasma Concentration Fluctuation:

  • Plasma levels of specific hormones fluctuate dynamically throughout the day.

• Negative Feedback Systems:

  • Hormone secretion induces a physiological change, and that resulting change directly inhibits further secretion of the hormone (analogous to a thermostat).

  • Example: High plasma glucose levels stimulate insulin release. Insulin lowers blood glucose levels. Reduced plasma glucose levels subsequently inhibit further insulin release.

• Regulation of Cell Surface Receptors:

  • Down-regulation: A decrease in the number of cell receptors. Less hormone can bind to the cell, leaving higher concentrations of the hormone remaining in the blood plasma and reducing cellular sensitivity.

  • Up-regulation: An increase in the number of cell receptors. More hormone can bind to the cell, lowering blood plasma concentrations of the hormone and increasing cellular sensitivity.

Anatomical Overview of Endocrine Glands and Their Secretions

• The Pituitary Gland:

  • Functions as a relay center between the nervous system and the endocrine system.

  • Strongly influenced by exercise; physical activity increases the release of all six anterior pituitary hormones.

  • Anterior Lobe:

  • Tropic Hormones (4): Regulate the functioning of other endocrine glands:

    • Adrenocorticotropin ($\text{ACTH}$) – regulates the adrenal cortex.

    • Thyrotropin ($\text{TSH}$) – regulates the thyroid gland.

    • Follicle-stimulating hormone ($\text{FSH}$) – regulates the gonads.

    • Luteinizing hormone ($\text{LH}$) – regulates the gonads.

  • Non-tropic Hormones (2):

    • Prolactin.

    • Growth Hormone ($\text{GH}$): Promotes muscle growth and tissue hypertrophy by facilitating cellular amino acid transport; directly stimulates lipolysis; plasma concentrations elevate during aerobic exercise in direct proportion to exercise intensity.

  • Posterior Lobe:

  • Antidiuretic hormone ($\text{ADH}$ / vasopressin).

  • Oxytocin.

  • Intermediate Lobe:

  • Non-functional in humans.

• Thyroid Gland:

  • Triiodothyronine ($T_3$) and Thyroxine ($T_4$):

  • Increase cellular protein and enzyme synthesis.

  • Increase the total size and number of mitochondria in target cells.

  • Promote rapid cellular glucose uptake.

  • Enhance pathways of glycolysis and gluconeogenesis.

  • Increase free fatty acid ($\text{FFA}$) availability for cellular oxidation.

  • Can increase overall basal metabolic rate by 60\n\%\text{--}100\%.

  • Calcitonin:

  • Involved in calcium metabolism; not a primary metabolic regulator in adults.

• Adrenal Glands:

• Adrenal Medulla:

  • Secretes catecholamines: Epinephrine and Norepinephrine.

  • Stimulated by the sympathetic nervous system to prepare the body for immediate physical action.

  • Increases metabolic rate, glycogenolysis, and release of glucose and $\text{FFAs}$ into blood plasma.

  • Increases heart rate, force of cardiac contraction, blood pressure, and respiration rate.

  • Facilitates blood redistribution to active skeletal muscles via specific vasodilation and vasoconstriction.

  • Norepinephrine rises quicker and remains elevated longer than epinephrine across increasing exercise intensities ($50\%$, $75\%$, and $100\%\,\dot{V}\text{O}_2\text{max}$) and extended duration ($0\text{ to }180\,\text{min}$).

• Adrenal Cortex:

  • Mineralocorticoids: Maintain electrolyte balance in extracellular fluids. Includes Aldosterone (promotes $\text{Na}^+$ retention, leading to osmotic $\text{H}_2\text{O}$ retention).

  • Glucocorticoids: Maintain consistent plasma glucose levels between meals. Includes Cortisol (key stress hormone).

  • Gonadocorticoids: Sex hormones secreted in lesser quantities than primary gonadal secretions; includes androgens, estrogens, and progesterones.

  • Pancreas:

• Insulin:

  • Secreted during hyperglycemia (elevated plasma glucose).

  • Promotes glucose uptake by tissues (especially muscle and connective tissue).

  • Promotes glycogenesis and inhibits gluconeogenesis.

• Glucagon:

  • Secreted during hypoglycemia (below normal plasma glucose).

  • Increases hepatic glycogenolysis and enhances gluconeogenesis.

• Exercise and Training Response:

  • Glucagon: In untrained individuals, plasma glucagon increases sharply during continuous exercise ($125\,\text{pg/ml}\text{ to }>200\,\text{pg/ml}$). In trained individuals, plasma glucagon remains significantly lower ($100\,\text{pg/ml}\text{ to }135\,\text{pg/ml}$).

* *Insulin*: Plasma insulin decreases over time during exercise. Trained individuals maintain higher overall plasma insulin during prolonged exercise compared to untrained subjects.

* *Glucose*: Untrained individuals exhibit a rapid decline in plasma glucose over $180\,\text{min}$ of exercise ($5.2\,\text{mmol/L}\text{ to }3.0\,\text{mmol/L}$). Trained individuals maintain stable blood glucose levels ($\sim 5.0\text{--}6.0\,\text{mmol/L}$).

• Reproductive Hormones:

  • Androgens (Testosterone):

  • Responsible for male secondary sex characteristics.

  • Major anabolic hormone that stimulates skeletal muscle hypertrophy.

  • Estrogens:

  • Responsible for female secondary sex characteristics and influence muscle function.

  • Progesterone:

  • Prepares the uterus and female reproductive system for pregnancy.

• Kidneys:

  • Secrete Erythropoietin ($\text{EPO}$), which regulates red blood cell (erythrocyte) production by stimulating bone marrow cells.

  • Critical for aerobic training adaptations and altitude acclimatization due to the oxygen-carrying capacity of red blood cells.

Hormonal Regulation of Carbohydrate and Fat Metabolism During Exercise

• Glucose Metabolism Requirements:

  • Adequate blood glucose supply during exercise demands balanced hepatic glucose release and muscle glucose uptake.

  • Hormones that increase circulating blood glucose levels:

  • Glucagon

  • Epinephrine

  • Norepinephrine

  • Cortisol

  • Growth Hormone ($\text{GH}$) increases $\text{FFA}$ mobilization while decreasing cellular glucose uptake.

  • Thyroid hormones ($T_3$, $T_4$) enhance overall glucose catabolism and fat metabolism.

• Effects of Exercise Intensity and Duration:

  • Exercise Intensity:

  • Higher intensity increases catecholamine secretion.

  • Increases the rate of glycogenolysis in both liver and skeletal muscle.

  • Muscle glycogen stores are broken down and used prior to liver glycogen.

  • Exercise Duration:

  • Prolonged duration increases reliance on liver glycogen.

  • Increased muscle glucose uptake drives greater hepatic glucose release.

  • As muscle/liver glycogen stores decrease, glucagon levels progressively rise.

• Insulin Dynamics During Prolonged Effort:

  • Plasma insulin concentrations gradually decline throughout prolonged exercise.

  • Cellular insulin sensitivity increases substantially during physical activity, enabling muscle fibers to take up adequate glucose while requiring less circulating insulin.

• Fat Metabolism Regulation:

  • $\text{FFA}$ mobilization and fat metabolism are critical for endurance performance following carbohydrate depletion.

  • Lipolysis breaks down stored triglycerides in adipose tissue into free fatty acids and glycerol:     $\text{Triglycerides} \rightarrow \text{FFAs} + \text{Glycerol}$

  • Released $\text{FFAs}$ travel through blood to active muscles; the rate of triglyceride breakdown often dictates the rate of cellular fat metabolism.

  • Lipolysis is stimulated by:

  • Decreased insulin concentrations

  • Epinephrine

  • Norepinephrine

  • Cortisol

  • Growth Hormone ($\text{GH}$)

  • These hormones activate the enzyme lipase.

  • Cortisol levels peak early and return toward baseline during extended exercise, at which point catecholamines and $\text{GH}$ sustain $\text{FFA}$ mobilization.

Hormonal Regulation of Fluid and Electrolyte Balance

• Primary Fluid Balance Hormones:

  • Aldosterone:

  • Secreted by the adrenal cortex in response to decreased blood pressure or reduced plasma volume.

  • Promotes $\text{Na}^+$ reabsorption in the renal tubules, causing osmotic $\text{H}_2\text{O}$ retention to expand plasma volume.

  • Antidiuretic Hormone ($\text{ADH}$ / Vasopressin):

  • Secreted by the posterior pituitary in response to increased blood osmolarity (detected by hypothalamic osmoreceptors during sweating/hemoconcentration).

  • Promotes $\text{H}_2\text{O}$ conservation by increasing permeability of renal tubules and collecting ducts, decreasing urine volume.

• The Renin-Angiotensin-Aldosterone System (RAAS):

  1. Prolonged exercise without fluid intake leads to dehydration.

  2. Dehydration reduces blood pressure, which is sensed by juxtaglomerular cells in the kidneys.

  3. The kidneys secrete the enzyme renin into the blood.

  4. Renin converts plasma angiotensinogen (released by the liver) into angiotensin I.

  5. Angiotensin-converting enzyme (ACE) in the pulmonary vasculature converts angiotensin I to angiotensin II.

  6. Angiotensin II stimulates aldosterone secretion from the adrenal cortex and induces systemic arterial vasoconstriction, raising blood pressure.

  7. Aldosterone increases renal $\text{Na}^+$ reabsorption.

  8. Water follows $Na^+$, reducing urine output and restoring blood pressure/plasma volume.

• Plasma Volume Dynamics and Adaptations:

  • Following an initial drop at exercise onset, plasma volume stabilizes due to:

  1. Actions of aldosterone and $\text{ADH}$.

  2. Fluid returning from active muscle tissues into the blood.

  3. Increased metabolic water production from cellular oxidation inside working muscles.

• Hemoconcentration vs. Hemodilution:

  • Hemoconcentration: An increase in the concentration of blood particles/solutes due to fluid plasma loss.

  • Hemodilution: A dilution of blood constituents due to increased plasma fluid volume. Endurance-trained athletes typically exhibit resting hemodilution.

  • Repeated exercise in hot environments expands baseline plasma volume over consecutive days in parallel with aldosterone-induced $\text{Na}^+$ retention.

Hormonal Control of Appetite and Hunger

• Neuroendocrine Regulation of Appetite:

  • Cholecystokinin ($\text{CCK}$): Released from the small intestine when food enters a full stomach; signals via the vagus nerve to suppress appetite.

  • Ghrelin: Secreted by the stomach and pancreas when empty; acts on central brain receptors to stimulate hunger.

  • Leptin: Secreted by adipose tissue cells; acts on the brain to suppress hunger and signal energy satiety.