Endocrinology

Endocrinology


Endocrine System

• A Communication System Via Chemicals –

  1.  Nervous system: electrical communication

• Slower to respond but longer lasting than nervous system

• Maintains homeostasis via hormones

  1. – Control and regulate cell and organ activity – Act on target cells

• During exercise

  1. endocrine system – Regulates substrate metabolism

  2. endocrine system Regulates fluid & electrolyte balance

Endocrine glands—ductless glands that secrete hormones directly into blood

  1. Secretion regulated by negative feedback (thus help with homeostasis!)

3 Characteristics
– Travel long distances, from cells that secrete them to target cells/organs 

– Short half-life
– Low concentration in blood

  • Chemical messengers; travel in blood placing them in direct contact with all cells

  • Travel In The Blood To Their Specific Target Organs

  • Receptors are specific to hormones such that only the correct hormone will “fit” the correct receptor

  • Each cell has 2,000 to 10,000 specific receptors

  • # of receptors can increase (upregulation) or decrease (downregulation)

  • Upregulation -  increase number of receptors = sensitization

  • Downregulation: decrease number of receptors = desensitization

  • 2 types of hormones:steroid and nonsteroid

Steroid Hormones

• Formed from cholesterol

• Lipid soluble – diffuse through membranes

• Receptors found inside cell, in cytoplasm or nucleus

• Secreted by four major glands:

  1. Adrenal cortex (cortisol, aldosterone) – 

  2. Ovaries (estrogen, progesterone)

  3. Testes (testosterone)

  4. Placenta (estrogen, progesterone)

Nonsteroid Hormones

• non steroid hormones - Not lipid soluble, unable to cross membranes

– Receptors on cell membrane

– 2nd messengers carry message into cell

• Cyclic adenosine monophosphate (cAMP), Cyclic guanosine monophosphate (cGMP), Inositol triphosphate (IP3), diacylglycerol (DAG)

• Twogroups

  1. – Protein or peptide

Most are this kind

 From pancreas, hypothalamus, pituitary gland

  1. – Amino acid derived

• Thyroid hormones (Triiodothyronine (T3), Thyroxine (T4)) • Adrenal medulla (epinephrine and norepinephrine)

Hormones

• Hormones- From Third Classof(pseudo)hormones

• hormones - Are derived from arachidonic acid

• Act as local hormones (immediate area)

– Mediate inflammatory response (assoc w/ swelling, vasodilation) – Sensitize free nerve endings (assoc w/ pain)

Key Points

  • Hormones Secreted Into Blood,circulate target cells,bind to receptors specific to hormone

  • Steroid Hormones Pass Through Cell Membrane To Bind A/receptors

  • Nonsteroid Hormones Bind Receptors Cell Membrane,activates 2nd messenger system

  • Secretion Controlled By Negative Feedback

  • Can Alter Number Of Receptors

Hormonal Regulation of Metabolism During Exercise

• Major glands responsible for metabolic regulation:

  1. – Anterior pituitary gland 

  2. – Thyroid gland

  3. – Adrenal gland

  4. – Pancreas

• Affect carbohydrate and fat metabolism during exercise

Anterior pituitary gland

  1. anterior pituitary gland – Releases growth hormone (GH) – anabolic hormone• Stimulates muscle hypertrophy, fat metabolism

  2. anterior pituitary gland - Releases thyroid stimulating hormone (TSH), stimulates release of T3 and T4

• Thyroid gland

  1. thyroid gland - Secrete triiodothyronine (T3) and thyroxin (T4)

• Leads to increased metabolic rate of all tissue, glucose uptake, FFA mobilization

  1. - Adrenal gland (medulla)

  2. – Releases Epi and NE

  3. – With exercise, SNS triggers release

• Increases glycogenolysis, FFA release into blood

• Adrenal gland (cortex)

  1. Releases cortisol

• Increases FFA mobilization, protein catabolism

Pancreas – releases insulin and glucagon

• Insulin – lowers blood glucose

  1. insulin – Moves glucose into cells

  2. insulin – Enhances synthesis of glycogen, fat

• Glucagon – raises blood glucose

  1. – Opposite of insulin

  2. – Promotes glucose release (glycogenolysis, gluconeogenesis)

Glucose Regulation During Exercise

Blood glucose levels are a balance between uptake by active muscles and release by liver

  • –  Glucagon: promotes liver glycogen breakdown and glucose formation from amino acids

  • –  Epinephrine (Epi): promotes glycogenolysis (glycogen glucose)

  • –  Norepinephrine (NE): promotes glycogenolysis

  • –  Cortisol: promotes protein catabolism

Amount of glucose released by liver depends on exercise

intensity and duration

• As intensity increases:

  1. intensity increases – catecholamine release increases (SNS)

  2. intensity increases – glycogenolysis rate increases (liver, muscles) – muscle glycogen used before liver glycogen

• As duration increases – Use more liver glycogen

  1. duration increases – Muscles take up more glucose

  2. duration increases – decrease glycogen stores necessitates increase in glucagon level

Insulin enables glucose uptake in muscle

  1. during exercise – insulin concentrations decrease

  2. during exercise – cellular insulin sensitivity increases

  3. during exercise – results in more glucose taken up into cells with less insulin

• Exercise may enhance insulin binding to receptors on muscle fiber, reducing need for high concentrations of insulin to transport glucose

Regulation of Fat Metabolism During Exercise

• FFA mobilization and fat metabolism critical to endurance exercise

• Use Of Fat Protects Glycogen Stores

Lipolysis is hormonally controlled during exercise by: 

  • Decreased insulin, Epi, NE, Cortisol, Growth hormone

Hormonal Control of Metabolism During Exercise

Key Points

  • Plasma glucose ↑ by combined actions of glucagon, Epi, NE, & cortisol

  • Insulin helps glucose enter the cell,but declines during prolonged exercise

  • When CHO reserves low,fat oxidation,lipolysis increased
    – Facilitated by ↓ insulin and ↑ Epi, NE, cortisol, and growth hormone

Fluid and Electrolyte Balance During Exercise

  • During exercise, plasma volume

  • PVdeclinesdueto – Sweating
    – Pressure (higher pressure in vessels than outside forces fluid out)

  • Fluid balance critical for optimal metabolic, cardiovascular, & thermoregulatory function

  • Hormones that correct fluid imbalances released from – Posterior pituitary gland
    – Adrenal cortex
    – Kidneys

Hormones regulate electrolyte balance (especially Na+)

• Posterior pituitary gland

• Releases Antidiuretic hormone (ADH)

• Release triggered by high osmolality brought about by low PV (osmolality = concentration of dissolved particles – proteins, ions)

• Increases water absorption at kidneys, minimizing water loss 

• To dilute plasma back to normal

Fluid and Electrolyte Balance During Exercise

• Adrenal cortex

– Secretes aldosterone (a mineralocorticoid hormone)

  • Secretion is stimulated by: – ↓ plasma Na+
    – ↓ blood volume
    – ↓ blood pressure
    – ↑ plasma potassium concentration

  • Promotes renal reabsorption of sodium, causing body to retain Na+

  • Retention of Na+ leads to water retention

Kidneys

– Target tissue for ADH & aldosterone 

– Secrete renin, erythropoietin (EPO)

• Kidneys influence maintenance of PV & BP through release of renin

• Renin initiates the renin-angiotensin-aldosterone mechanism – Renin converts angiotensinogen angiotensin I

– ACE converts angiotensin = Iangiotensin II

– Angiotensin II stimulates aldosterone release

Key Points

• Primary hormones in fluid balance are ADH & aldosterone • ADH in response to ↑ plasma osmolality, low BV

• ADH acts on kidney, promoting water conservation

• WhenPV,BP↓,kidneys release renin

– Angiotensin II ↑ peripheral resistance, ↑ blood pressure

– Angiotensin II triggers release of aldosterone promoting sodium reabsorption in kidney