Endocrine System: Hormones and Organs

Endocrine System Overview

• Composed of a host organ, chemical messengers (hormones), and target organ
• Endocrine glands release substances into extracellular spaces

Major Endocrine Organs

• Pituitary
• Thyroid
• Parathyroid
• Adrenal
• Pineal
• Thymus
• Pancreas
• Gonads
• Hypothalamus
• Adipose tissues

Hormones

• Produced by specific host glands
• Function as messengers that circulate in the blood and affect tissues
• Two categories exist, steroid-derived, and amine and polypeptide (synthesized from amino acids)

Types of Hormones

Hormone Influence on Target Cells

• Alters reactions by modifying the rate of intracellular protein synthesis
• Impacts enzyme activity
• Modifies plasma membrane transport and induces secretory activity

Hormone-Receptor Binding

• Activation depends on hormone concentration in blood
• Activation also depends on the number of target cell receptors, as well as, the sensitivity or strength of hormone to receptor union

Cyclic AMP (cAMP)

• Cyclic 3’5’ - adenosine monophosphate, acts as a second messenger
• Activates a specific protein kinase and target enzyme to alter reactions
• Required for a hormone to bind with its specific receptor
• Three factors influence the reaction set in motion by cyclic AMP
• Type of target cell affects reaction, as well as enzymes in cell
• Specific hormones can act a first messenger

Hormone Effects on Enzymes

• Hormones increase enzyme production
• Hormones combine with enzymes to alter shape and effectiveness
• Activates inactive enzyme forms

Factors That Determine Hormone Levels

• Hormone secretion meets the demands of changing bodily conditions
• Plasma level of a hormone is impacted by quantity that synthesized in gland
• Plasma level is impacted by catabolism rate or secretion into blood
• Transport proteins and plasma volume also impact hormone levels

Patterns of Hormone Release

• Can be at regular intervals during a 24-h cycle
• Can occur in secretory cycles which span several weeks (others are daily)
• Patterns of release and amplitude of frequency of discharge offers insight into hormone dynamics

Pituitary Gland Secretions and Targets

• Hypothalamus is connected to the pituitary gland.
• Anterior pituitary Hormones.
  • Lactogen (prolactin) targets Breast
  • Gonadotropic hormone (FSH, LH) target Ovaries and Testes
  • ACTH (corticotropin) targets Adrenal Cortex
  • Thyrotropin targets Thyroid
  • Growth hormone (somatotropin) targets Many organs
  • Enkephalins target diverse organs and tissues.

Growth Hormone (GH)

• Promotes cell division and proliferation throughout the body
• Facilitates protein synthesis by increasing amino acid transport across plasma membrane
• Stimulates RNA formation – Activates cellular ribosomes to increase protein synthesis
• Slows carbohydrate breakdown
• Initiates mobilization and use of fat for energy

Physical Activity & GH

• Exercise augments GH's action on target tissues
• Benefits: muscle, bone, connective tissue growth, and remodeling
• Optimizes fuel mixture during physical activity
• Net metabolic effect:Preserves plasma glucose concentration for the CNS and muscle functions
• GH concentration increases similarly during exhaustion in trained and sedentary individuals
• Sedentary individuals maintain higher GH levels following exercise

Growth Hormone’s Metabolic Actions

• Indirect actions promote anabolism, where direct actions are anti-insulin

Insulin-Like Growth Factors (IGFs)

• IGFs mediate many of GH’s effects
• In response to GH stimulation, liver cells synthesize IGF-I and IGF-II
• IGFs travel in blood attached to binding proteins waiting to interact with receptors
• Influenced by binding proteins and nutritional and insulin levels

Thyrotropin

• Controls thyroid gland secretion
• TSH maintains growth and development of thyroid gland
• TSH increases growth cell metabolism
• Important to regulate metabolic rate post-exercise

Adrenocorticotropic Hormone (ACTH)

• Functions as part of hypothalamic-pituitary-adrenal axis
• Regulates adrenal cortex output
• Enhances FFA mobilization from adipose tissue
• Stimulates gluconeogenesis and protein catabolism
• Corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) mediate ACTH release
• Intense training favors AVP, prolonged training favors CRH

Gonadotropic Hormones

• Stimulate male/female sex organs growth and the hormones they secrete at a faster rate
• Follicle-Stimulating Hormone (FSH):
• Initiates follicle growth in the ovaries and stimulates estrogen secretion
• Promotes sperm development in males
• Luteinizing Hormone (LH):
• Causes estrogen secretion and follicle rupture
• Stimulates testes to secrete testosterone

Posterior Pituitary Hormones

• Antidiuretic hormone (ADH; vasopressin) inhibits water excretion by the kidneys
• Physical activity provides a potent stimulus for ADH secretion

Thyroid Hormones

• Thyroxine (T4) secretion raises metabolism except in the brain, spleen, tests, uterus, and thyroid gland
• Triiodothyronine (T3) facilitates neural reflex activity
• T4 and T3 regulate tissue growth and development, skeletal and nervous system formation, and reproduction

Thyroid Hormone Effects

• Hyperthyroidism: increased metabolism and protein catabolism
• Hypothyroidism: intolerance to the cold, decreased reflexes, and fatigue

Parathyroid Hormone

• Parathyroid hormone (PTH) controls blood calcium balance
• Decreased calcium triggers secretion, increases inhibit release.
• Effects include releasing Calcium into blood from bone and phosphate.
• PTH enhances calcium reabsorption and decreases retention phosphate by kidney
• PTH increases absorption Calcium into intestinal mucosa

Adrenal Hormones

• The adrenal gland has two parts
• Medulla releases epinephrine and norpepinephrine, stimulate glucogenesis and lipolysis
• Cortex release mineralocorticoids, glucocoritcoids, and androgens

Mineralocorticoids

• The primary function of mineralocorticoids is to regulate mineral balance and extracellular fluids
• Aldosterone is the most important mineralocorticoid to do this
• Controls total sodium concentration and extracellular fluid volume
• Stimulates sodium reabsorption in distal kidney tube

Renin-Angiotensin Mechanism

• Increased sympathetic constriction lowers kidney blood flow
• Reduced flow stimulates renin
• Renin stimulates hormones angiotensin II/III to constrict arterial tone and retain sodium, excrete potassium

Glucocorticoids

• Cortisol is the major glucocorticoid that raises protein and FFA breakdown

Gonadal Hormones

• Testes and ovaries produce hormones
• Testosterone is most important androgen. It stimulates sperm production,
• Ovaries make estrogen for ovulation, pregnancy and lactation

Pancreatic Hormones

• The Pancreas Is made up of Acini and lslets of Langerhans
• Is mostly made of:
  • α-cells that secrete the hormone glucagon
  • Beta cells that secrete Insulin and amylin

Insulin

• Regulates glucose entry into tissues like muscle and adipose, EXCEPT the brain.
• High Insulin levels lower blood glucose; also triggers enzymes activity aiding protein synthesis by increasing amino acid transport and ribosomes RNA levels.

Glucose Transporters

• Muscle contains GLUTs-1, GLUTs-4 for exercise and after meals

Glucagon

• Produced by pancreatic α-cells
• Stimulates liver glycogenolysis and gluconeogenesis
• Increases lipid catabolism
• Plasma glucose regulates pancreatic output

Exercise Training and Endurance Function

• Less dramatic rise during exercise
• Females will retain higher GH resting levels that disappear over long exercises

ACTH, Reproductive Harmones and Exercise

• ACTH increases with training
• Regular exercise depress reproductive response
• Male athletes have 60%-80% testosterone levels of others

Gender Differences in Reproductive

• High intensity is linked menstrual dysfunction. No differences from trained/untrained

Potassium Pituitary Hormones and Exercise

• Endurance increases ATH in all, ADH has no change regardless of exercise

Thyroid Harmones and Exercise

• The result reflect hormones.

Adrenal Harmones and Excerise

• Plasma cortisol is lower in trained. Enlarge of adrenal glands increases both high cortiols putput,

Exercise Sympathetic Nervous System

• Sympathetic is lower when trained but greater with catecholnine

Pancratic exercise

• The exercise increase in blood less with the exercise