Endocrine System Functions and Hormonal Responses in Exercise

Three major functions of the endocrine system during exercise

Cardiac regulation, Mobilizes energy substrates for metabolism, Thermoregulation

Aspects regulated by the endocrine system during exercise

Metabolism, cardiac/smooth muscle, Osmolarity of blood

Factors determining the rate of hormone secretion

Magnitude of input, Stimulatory vs. inhibitory factors, Involvement of feedback loops

Main purpose of hormone responses to exercise

Respond to stress, provide fuel for muscle, prevent dehydration

Physiological systems involved in hormone responses to exercise

Cardiopulmonary, Metabolism, Thermoregulation

Effect of exercise intensity and duration on hormone response

The magnitude of response depends on the intensity and duration of the exercise bout.

Change in hormone response with exercise training

As someone becomes more trained, exercise is less stressful on the body, leading to a decreased magnitude of hormone release (attenuated hormone response). Hormones in trained vs. untrained individuals follow the same trends, but the magnitude of release is different.

Three main types of hormones based on mechanism of action/secretion

Amine, Peptide, Steroid

Amine/Peptide hormones

Water soluble; Examples: Catecholamines, glucagon, insulin, ADH, growth hormone; Derived from amino acids (amine) or proteins (peptide); Released from endocrine gland, go into plasma, bind to a receptor, then activate a second messenger system to elicit a response.

Steroid hormones

Lipid soluble; Circulate bound to carrier proteins; Can cross the cell membrane; Examples: Cortisol, Aldosterone; Bind to specific receptors inside target cells, causing direct gene activation (altering gene expression to form new proteins).

Hormones involved in cardiopulmonary responses during exercise

Catecholamines (nutrient regulation), Cortisol (vasodilation to redirect blood flow, heart rate response, cardiac contractility).

Catecholamines

Epinephrine/Norepinephrine

Source of Catecholamines

The adrenal medulla

Stimulus for Catecholamine release

Increased SNS (Sympathetic Nervous System) activity

Roles of Catecholamines in cardiopulmonary responses

Increase HR (heart rate), increase cardiac contractility, increase blood flow to skeletal muscles, heart, liver, and adipose tissue; decrease blood flow to skin and other visceral organs; dilate airways.

End results of Catecholamine action during exercise

Increased cardiac output, increased BP (blood pressure), redistributed blood flow, increased energy substrate mobilization, increased ventilation.

Cortisol

The adrenal cortex

Cortisol release control

Under the control of the HPA (Hypothalamic-Pituitary-Adrenal) axis.

Cortisol role in cardiopulmonary responses

Makes blood vessels more sensitive to hormones that cause vasoconstriction.

Cortisol's cardiopulmonary action result

Decrease in BP.

Hormones in metabolic responses during exercise

Catecholamines, Cortisol, Growth hormone (permissive to catecholamines, takes longer), Glucagon, Insulin.

Catecholamines roles in metabolic responses

Increase substrate mobilization/metabolism; Increase glycogenolysis and lipolysis (for energy); Increase glycolysis (to produce ATP quickly at the beginning of exercise).

Catecholamine release stimulation

Decreased blood glucose levels.

Epinephrine and norepinephrine production relationship

Epinephrine cannot be made without norepinephrine first.

Cortisol roles in metabolic responses

Increased substrate mobilization and metabolism; Increase lipolysis, gluconeogenesis, and proteolysis (to minimize loss of glucose during exercise).

Cortisol release stimulation

Decreased blood glucose levels.

Hormones with intensity and duration prerequisites

Cortisol and Growth hormone.

Primary hormones for glucose utilization in initial exercise phase

Catecholamines, THEN some cortisol.

Growth hormone release location

The anterior pituitary gland.

Growth hormone roles in metabolic responses

Substrate mobilization/metabolism; Increase glycogenolysis, lipolysis, and gluconeogenesis.

Growth hormone release stimulation

Decreased blood glucose, increased SNS activity.

Growth hormone release timing during exercise

Later, around the 25-30 minute mark, and needs moderate-high intensity.

Growth hormone and Cortisol action speed

Slow-acting hormones.

Permissive function of Growth hormone

Helps other hormones to do their job (e.g., permissive to catecholamines).

Glucagon release location

The pancreas.

Glucagon roles in metabolic responses

Substrate mobilization/metabolism; Increase glycogenolysis, lipolysis, and gluconeogenesis.

Glucagon release stimulation

Decreased blood glucose levels, increased SNS activity.

Decreased blood glucose levels

Increased SNS activity.

Glucagon

Fast-acting hormone; responds quickly.

Insulin release location

The pancreas.

Main role of Insulin

Opposite of glucagon.

Specific roles of Insulin

Increase glycogenesis, lipogenesis, and uptake of amino acids; Decrease lipolysis, glycogenolysis, and gluconeogenesis.

Stimulus for Insulin release

Increased blood glucose levels.

Plasma insulin levels during exercise

Plasma insulin levels decrease.

SNS activity effect on insulin during exercise

Responds to increased SNS activity; Prevents rapid uptake of plasma glucose; Favors mobilization of liver glucose and lipid FFA.

Fast-acting hormones for blood glucose homeostasis during exercise

Catecholamines, Glucagon, Insulin.

Maintenance of plasma glucose during exercise

By increasing liver glucose mobilization, increasing levels of plasma FFA, decreasing glucose uptake, and increasing gluconeogenesis.

Permissive and slow-acting hormones for blood glucose homeostasis

Cortisol and Growth hormone.

Role of permissive and slow-acting hormones

They support the actions of other hormones, along with their direct roles.

Hormones involved in thermoregulatory responses during exercise

ADH (vasopressin), Angiotensin II, Aldosterone (part of the Renin-Angiotensin-Aldosterone System/RAAS).

Another name for ADH

Vasopressin.

ADH release location

The posterior pituitary.

Roles of ADH in thermoregulation/fluid balance

Maintain plasma volume and BP; Decrease urine production by causing the kidneys to reabsorb water; Constriction of blood vessels.

Stimulus for ADH release

Changes in plasma osmolality (specifically high osmolality) and low plasma volume from sweating.

Sweating effect on plasma osmolarity

Sweating causes the loss of plasma (water). If this water is not replenished, the ratio of solute to solvent changes, leading to a lower concentration of solvent and an increase in the osmolarity of the blood.

System for Angiotensin II and Aldosterone

The Renin-Angiotensin-Aldosterone System (RAAS).

Major stimulus for RAAS activation

Reduction in blood pressure, followed by an increase in plasma osmolarity.

Roles of Angiotensin II

Maintain BP by constricting vessels; Stimulate the release of aldosterone from the adrenal cortex.

Roles of Aldosterone

Maintain plasma Na+ (sodium) and K+ (potassium); Regulate blood pressure by reabsorbing water from the kidneys.

Stimuli for release of Angiotensin II and Aldosterone

Changes in osmolarity, low plasma volume, low BP.