Endocrine System: Glands and Hormones

• The endocrine system consists of scattered glands that produce and release hormones into the blood
• Controls the body with slow and prolonged effects, unlike the nervous system's fast, short-lived effects

Gland Types

• Exocrine glands produce non-hormonal secretions through ducts
• Endocrine glands are ductless and secrete hormones directly into body fluids
• Mixed glands perform both endocrine and exocrine functions

Hormones

• Chemical messengers secreted by cells to affect the metabolic function of other cells

Structural Types of Hormones

• Amino acid based hormones are proteins, water-soluble, and bind to cell membrane receptors
• Steroid-based hormones are built from cholesterol, lipid-soluble, secreted by gonads and adrenal cortex
  • They attach to carrier proteins for transport in blood, and must bind to intracellular receptors
• Eicosanoids are lipid-based hormones that act locally, such as leukotrienes and prostaglandins which mediate inflammation

Mechanism of Action

• Target cells have hormone-specific receptors that respond to the hormone's presence
• Hormone effects on target cells include altered cell membrane permeability, synthesis of proteins/enzymes, induced cellular secretion, and stimulated mitosis/cell division

Process for Activating G protein

• Water-soluble hormones bind to plasma membrane receptors, relying on intracellular 2nd messengers
  • Hormone is the '1st messenger' that activates the receptor
  • Receptor activates the G protein, which slides along the membrane

The Cyclic AMP Mechanism

• G protein activates adenylate cyclase, an effector enzyme
• Adenylate cyclase makes cyclic AMP (cAMP), a 2nd messenger, from ATP
• cAMP activates protein kinases, causing target cell responses

The PIP-Calcium Mechanism

• Hormone binding acts as the 1st messenger, activating the G protein
• G protein activates the phospholipase C enzyme
• Phospholipase splits PIP₂ protein into diacylglycerol (DAG) and inositol triphosphate (IP3), the 2nd messengers
• DAG activates protein kinases, and IP3 triggers the release of calcium ions, resulting in the cellular response

Direct Gene Activation

• Steroid based hormones and thyroid hormones are lipid-soluble
  • They cross target cell plasma membranes to bind to intracellular receptors

Target Cell Regulation

• Target cells affect the amount of hormone binding by regulating their receptor numbers

• Up-regulation increases receptors for more hormone binding

• Down-regulation decreases receptors to limit hormone binding, often in response to overstimulation

• Half-life is the time it takes for a given hormone quantity to decrease to half its concentration

  • Lipid-soluble hormones have longer half-lives

Hormone Interactions

• Permissiveness is when the action of one hormone allows the effects of another
• Synergism is when two hormones magnify each other's effects
• Antagonism is when two hormones counteract each other's effects

Control of Hormone Release

• Humoral stimuli are changes in body fluids (ions, nutrients)
• Neural stimuli are nervous stimulation
• Hormonal stimuli are hormones from other endocrine glands
• Tropic hormones secreted by one endocrine gland trigger secretion by other endocrine glands
• Negative feedback, rising hormone levels cause a stop in secretion of that hormone

The Pituitary Gland

• Hangs from the hypothalamus via the infundibulum
  • All pituitary secretions are initiated by the hypothalamus
• The posterior pituitary (neurohypophysis) is nervous tissue that stores and releases hormones made in the hypothalamus
• The anterior pituitary (adenohypophysis) is true glandular tissue that produces and releases its own hormones

Anterior Pituitary Hormones

• Growth Hormone (GH) stimulates cell division, especially in bones and skeletal muscles
  • Hypersecretion causes gigantism, and hyposecretion causes pituitary dwarfism
  • Regulated by GHRH and GHIH
• Thyroid Stimulating Hormone (TSH or thyrotropin) is a tropic hormone that stimulates the thyroid gland
  • Regulated by TRH
• Adrenocorticotropic hormone (ACTH) is a tropic hormone that stimulates the adrenal cortex
  • Regulated by CRH
• Follicle Stimulating Hormone (FSH) stimulates gamete production in the gonads
• Luteinizing Hormone (LH) stimulates hormone release from the gonads
  • FSH and LH are gonadotropins regulated by GnRH
• Prolactin (PRL) stimulates milk production in the breasts
• Regulated by PRH and PIH

Posterior Pituitary Hormones

• Oxytocin stimulates contraction of uterine muscles during childbirth
• Antidiuretic Hormone (ADH) inhibits urination and prevents changes in water balance by stimulating water reabsorption in kidney tubules

The Thyroid Gland

• Bilobed gland in the anterior neck on the trachea
• Follicles containing colloid, built by follicular cells
  • Parafollicular cells sit outside the follicles
• Thyroid Hormone (TH) is a mix of thyroxine (T4) and triiodothyronine (T3), named for iodine atoms
  • Controls metabolic rate, regulates tissue growth and development, and maintains blood pressure
• Lack of iodine causes colloid buildup, enlarging the thyroid, leading to a goiter
• Calcitonin is a protein hormone made by parafollicular cells
  • Released when calcium levels are high, triggering bone deposit via osteoblasts

The Parathyroid Glands

• Usually 4, located on the posterior side of the thyroid gland
• Parathyroid Hormone (PTH) regulates calcium levels in the blood

PTH Effects

• Secreted when blood Ca2+ levels drop
• Triggers osteoclasts to resorb bone, releasing calcium to the blood
• Triggers increased absorption of dietary calcium in the small intestine
• Triggers increased calcium reabsorption in the kidneys, preventing urination of calcium

The Adrenal Glands

• Paired glands located above each kidney, divided into an outer cortex and inner medulla
  • All cortical hormones are steroid-based, built from cholesterol
• The adrenal cortex has 3 zones secreting corticosteroids
  • Zona glomerulosa secretes mineralocorticoids, regulating Na+ and K+ levels in the blood and blood pressure, such as aldosterone
  • Zona Fasciculata secretes glucocorticoids, regulating energy metabolism and glucose blood levels, such as cortisol
  • Zona Reticularis secretes gonadocorticoids, helping to trigger the onset of puberty
• Addison's disease is hyposecretion of corticosteroids, causing weight loss, fatigue, and electrolyte imbalance
• Cushing's syndrome is hypersecretion of corticosteroids, causing weight gain, hypertension, and electrolyte imbalance
• The adrenal medulla is the inner layer of the adrenal gland
• Secretes epinephrine and norepinephrine when stimulated by the sympathetic division of the ANS
  • These hormones trigger the physiological changes of the "fight or flight" response

The Pancreas

• Located beneath the stomach
  • Mixed gland
  • Acinar cells have exocrine (digestive) function
  • Pancreatic islets secrete hormones
• Glucagon is secreted by alpha cells, to elevate blood glucose levels
  • Insulin is secreted by beta cells, to lower blood glucose levels
• When blood glucose is low, pancreas alpha cells release glucagon
  • Glucose is released from the liver through glycogen breakdown
  • Resulting in elevated blood glucose
• When blood glucose is high, pancreas beta cells release insulin
  • Glucose plus other nutrients are absorbed by tissues
  • Glucose absorbed by the liver is stored as glycogen
    • Glycogen breakdown is inhibited
  • Resulting in decreased blood glucose
• In Diabetes Mellitus, insulin deficiency causes hunger, thirst, and urination
  • Type 1 (insulin dependent) has onset at younger ages, due to autoimmune destruction of beta cells
  • Type 2 (non-insulin dependent) has later onset, occurs in overweight individuals with loss of tissue cell insulin sensitivity

Other Endocrine Glands

• The gonads are reproductive organs that produce steroidal sex hormones
  • Ovaries produce estrogen and progesterone, triggers changes at puberty and regulate the menstrual cycle
  • Testes produce testosterone, triggers changes at puberty and regulates sperm production
• The pineal gland is in the epithalamus of the brain, secretes melatonin to regulate sleep-wake cycle by inducing drowsiness
• The thymus sits above the heart and diminishes with age. It produces group of hormones that activate T-lymphocytes