The Endocrine System: Chapter 17

• Chapter 17 is about The Endocrine System
• Includes Assigned Reading for 17.1 to 17.11

Neural and Endocrine Signaling

• Endocrine system: chemical signaling, hormones, travels long or short distances, has a fast or slow response time, and targets an internal environment
• Nervous system: chemical/electrical signals, neurotransmitters, travels always short distances, has an always fast response time, which targets internal and external environments

Structures of the Endocrine System

• Endocrine glands and cells are located throughout the body for the important role of homeostasis

Endocrine Glands vs. Exocrine Glands

• Focuses on how the endocrine and exocrine glands differ in structure and function

Circulating Hormones vs. Local Hormones

• Circulating hormones travel throughout the body via the bloodstream to target cells
• Local hormones act on nearby cells (paracrine) or on the same cell that secreted them (autocrine)

General Functions of the Endocrine System

• Maintaining homeostasis of blood composition and volume
• Regulating growth, development, & metabolism
• Controlling reproductive activities
• Controlling digestive processes

Endocrine Glands and Their Major Hormones

• Pituitary gland (anterior): growth hormone (GH), protein; which promotes growth of body tissues
• Pituitary gland (anterior): prolactin (PRL), peptide; which promotes milk production
• Pituitary gland (anterior): thyroid-stimulating hormone (TSH), glycoprotein; which stimulates thyroid hormone release
• Pituitary gland (anterior): adrenocorticotropic hormone (ACTH), peptide; which stimulates hormone release by adrenal cortex
• Pituitary gland (anterior): follicle-stimulating hormone (FSH), glycoprotein; which stimulates gamete production
• Pituitary gland (anterior): luteinizing hormone (LH), glycoprotein; which stimulates androgen production by gonads
• Pituitary gland (posterior): antidiuretic hormone (ADH), peptide; which stimulates water reabsorption by kidneys
• Pituitary gland (posterior): oxytocin, peptide; which stimulates uterine contractions during childbirth
• Thyroid: thyroxine (T4), triiodothyronine (T3), amine; which stimulates basal metabolic rate
• Thyroid: calcitonin, peptide; which reduces blood Ca2+ levels
• Parathyroid: parathyroid hormone (PTH), peptide; which increases blood Ca2+ levels
• Adrenal cortex: aldosterone, steroid; which increases blood Na+ levels
• Adrenal cortex: cortisol, corticosterone, cortisone, steroid; which increases blood glucose levels
• Adrenal medulla: epinephrine, norepinephrine, amine; which stimulate fight-or-flight response
• Pineal: melatonin, amine; which regulates sleep cycles
• Pancreas: insulin, protein; which reduces blood glucose levels
• Pancreas: glucagon, protein; which increases blood glucose levels
• Testes: testosterone, steroid; which stimulates development of sex characteristics
• Ovaries: estrogens and progesterone, steroid; which develop sex characteristics and prepare the body for childbirth

Types of Hormones

• Two major groups based on their chemical structure include lipid-soluble (steroid hormones) and water-soluble (amine, peptide, and protein hormones)
• Amine hormones: amino acids are modified
• Peptide hormones: short chains of linked amino acids
• Protein Hormones: long chains of linked amino acids
• Steroid Hormones: Derived from the lipid cholesterol

Pathways of Hormone Action

• Lipid-soluble hormones bind to intracellular hormone receptors
• Lipid-soluble hormone diffuses through plasma membrane
• Hormone and receptors bind in cytoplasm forming a receptor-hormone complex
• Receptor-hormone complex enters the nucleus and triggers gene transcription
• Transcribed mRNA is translated into proteins that alter cell activity

Mechanism of Hormone Action

• Water-soluble hormones bind to cell membrane hormone receptors
• Water-soluble hormones are membrane insoluble and bind to membrane receptors
• The binding activates a G protein
• The activated G protein activates adenylyl cyclase
• Adenylyl cyclase catalyzes the conversion of ATP to cAMP, the secondary messenger
• cAMP activates protein kinases.
• Protein kinases phosphorylate proteins in the cytoplasm which activates these proteins, allowing them to alter cell activity

Factors Affecting Target Cell Response

• Blood levels of the hormone
• Relative number of receptors on the target cell
• Upregulation: increase receptor number
• Downregulation: decrease receptor number
• Influence exerted by other hormones
• Permissive effect: First hormone allows action of second
• Synergistic effect: two hormones with similar effects amplify cell's response
• Antagonistic effects: hormones with opposing effects

Regulation of Hormone Secretion

• Hormone release is regulated by reflexes to stimuli
• Types of stimuli: hormonal, humoral, nervous
• Feedback loops govern the initiation and maintenance of most hormone secretion in response to various stimuli

Negative Feedback Loop

• Most hormone regulation is achieved via negative feedback
• Adrenal glucocorticoids release is stimulated by hormones from the hypothalamus and pituitary gland

Hypothalamus

• Master endocrine gland (along w/ pituitary) is in a specific location Functions:
  • Direct release of HM (ADH and Oxytocin)
  • Indirect control through release of regulatory hormones
  • Direct control by nervous system: Adrenal medulla releases Epinephrine & Norepinephrine

Anatomy of Pituitary Gland

• Has infundibulum and 2 lobes
  • Anterior pituitary (Adenohypophysis) makes up approximately 75%
  • Posterior pituitary (Neurohypophysis) makes up approximately 25%

Pituitary Hormones

• Anterior pituitary gland: growth hormone (GH), protein, promotes growth of body tissues
• Anterior pituitary gland: prolactin (PRL), peptide, promotes milk production from mammary glands
• Anterior pituitary gland: thyroid-stimulating hormone (TSH), glycoprotein, stimulates thyroid hormone release from thyroid
• Anterior pituitary gland: adrenocorticotropic hormone (ACTH), peptide, stimulates hormone release by adrenal cortex
• Anterior pituitary gland: follicle-stimulating hormone (FSH), glycoprotein, stimulates gamete production in gonads
• Anterior pituitary gland: luteinizing hormone (LH), glycoprotein, stimulates androgen production by gonads
• Posterior pituitary gland: antidiuretic hormone (ADH), peptide, stimulates water reabsorption by kidneys
• Posterior pituitary gland: oxytocin, peptide, stimulates uterine contractions during childbirth
• Intermediate zone: melanocyte-stimulating hormone, peptide, stimulates melanin formation in melanocytes

Posterior Pituitary

• Neurosecretory cells in the hypothalamus release oxytocin (OT) or ADH in the posterior lobe
• The hormones are stored then released into the blood via capillary plexus

Posterior Pituitary Hormones

• Oxytocin (OT) has two target tissues that are involved in neuroendocrine reflexes: uterus and breast/mammary glands
• Antidiuretic Hormone (ADH) is aka Vasopressin
• ADH conserves body H₂O, and is regulated by the osmotic pressure of blood
• Alcohol inhibits ADH release

Anterior Pituitary

• The anterior pituitary manufactures seven hormones
• The hypothalamus produces separate hormones that stimulate or inhibit hormone production in the anterior pituitary
• Hormones from the hypothalamus reach the anterior pituitary via the hypophyseal portal system

Connection between Hypothalamus and Anterior Pituitary

• Hypothalamus hormonally stimulates anterior pituitary to release its hormones
• Regulatory hormones of the hypothalamus are releasing and inhibiting hormones Releasing hormones:
  • Increase anterior pituitary secretion
  • Include: thyrotropin-releasing hormone (TRH), prolactin-releasing hormone (PRH), gonadotropin-releasing hormone (GnRH), corticotropin-releasing hormone (CRH), and growth hormone-releasing hormone (GHRH) Inhibiting hormones:
  • Decrease secretion of anterior pituitary hormones
  • Include: prolactin-inhibiting hormone (PIH) and growth hormone-inhibiting hormone (GHIH)

Major Pituitary Hormones

• Major pituitary hormones and their target organs

Hormonal Regulation of Growth

• Growth hormone (GH) directly accelerates the rate of protein synthesis in skeletal muscle and bones
• Insulin-like growth factor 1 (IGF-1) is activated by growth hormone and indirectly supports the formation of new proteins in muscle cells and bone.

Growth Hormone Disorders

• Hyposecretion of hGH in childhood leads to Hypopituitary dwarfism
• Hypersecretion of hGH in childhood leads to giantism; and in adults, it leads to acromegaly

Thyroid Gland

• Largest endocrine gland that is located either side of trachea
• 2 lobes are connected by isthmus Functions:
  • Secretes hormones for growth & proper metabolism Consists of follicles
  • Follicular cells: Thyroglobulin, Triiodothyronine (T3) & Tetraiodothyronine/Thyroxine(T4)
  • Parafollicular cells (aka C cells): Calcitonin

Thyroid Hormone Synthesis

• Thyroglobulin (Tg) synthesis
• Uptake and concentration of iodide (I¯)
• Oxidation of iodide (I¯) to iodine (I)
• Iodination of tyrosine residues on Tg (catalyzed by peroxidase) and the formation of MIT & DIT
  • I+Tg → MIT (Monoiodotyrosine)
  • (2 x I) + Tg → DIT (Diiodotyrosine)
• Coupling of MIT & DIT to form T3 and T4
  • MIT + DIT T3 (Triiodothyronine)
  • DIT + DIT → T4 (Tetraiodothyronine/ Thyroxine)
• Release of T3 and T4 into circulation

Function and Regulation of Thyroid Hormones

• Increases basal metabolic rate (BMR)
• Helps maintain normal body temperature
• Stimulates protein synthesis
• Increases the use of glucose and fatty acids for ATP production
• Upregulates beta (β) receptors that attach to catecholamines
• Works with hGH and insulin to accelerate body growth
• Regulation occurs through TSH from the Anterior Pituitary

Thyroid Gland Disorders

• Hypothyroidism can lead to cold intolerance • Autoimmune related disorders

Parathyroid Glands

• 4 glands are embedded in posterior surfaces of lateral lobes of the thyroid
• 2 types of cells: chief cells (principal cells) and oxyphil cells

Hormonal Regulation of Calcium in Blood

• Thyroid glands release Calcitonin when calcium is too high
• Parathyroid glands release PTH when calcium is too low

Ca2+ Homeostasis

• Blood calcium concentration normally: 9-11 mg/100 ml
• Calcitonin promotes osteoblast activity
• Parathyroid hormone (PTH) inhibits osteoblast activity
• Parathyroid hormone (PTH) stimulates osteoclast activity, to release Ca2+ from bone
• Release of PTH is also regulated by a vitamin D hormone, calcitriol

Adrenal Glands (Suprarenal)

• Paired, pyramidal shaped on top of both kidneys
• Retroperitoneal and has 2 regions: medulla (inside) and cortex (outside)
• Cortex has 3 layers: zona glomerulosa, zona fasciculata, and zona reticularis
• Mineralcorticoids (regulate mineral balance): Examples include Aldosterone
• Glucocorticoids (regulate glucose metabolism): Examples include Cortisol, Corticosterone, Cortisone
• Androgens (stimulate masculinization): Examples include Dehydroepiandrosterone
• Stress hormones (stimulate sympathetic ANS): Examples include Epinephrine, Norepinephrine

Adrenal Gland Disorders

• Hyperaldosteronism or Conn Syndrome: adrenal adenoma 80%
• Sx: HTN, hypokalemia
• Tx: removal of adenoma (Corrects HTN & hypokalemia)
• Cushing's Disease & Addison's Disease
  • Hyperadrenocorticism or Cushing's causes excess ACTH usually by pituitary tumor from chronic glucocorticoid hormones
  • Hypocorticism or Addison's chronic shortage from genetic or autoimmune glucocorticoids/mineralocorticoids

Pancreas

• Location of pancreas
• Acinar cells (exocrine 99%) and islet cells The Pancreatice endocrine makes alpha (a) and Beta (b) cells

Pancreatice Exocrine Functions

• Acinar cells are secreting digestive enzymes
• Transported to pancreatic cut to small intestine

Regulation of Blood Glucose Levels

• Blood glucose concentration is tightly maintained between 70 mg/dL and 110 mg/dL
• If blood glucose concentration rises, insulin is released and stimulates body cells to remove glucose from the blood
• If blood glucose concentration drops, glucagon is released and stimulates body cells to release glucose into the blood

Clinical Application: Diabetes Mellitus Types

• Type 1 diabetes (IDDM), pancreatic deficiency makes little or no insulin which is an autoimmune reaction that shows in childhood
• Type 2 diabetes (IIDM) insulin deficiency or resistance linked to diet, obesity, which is more common than type 1 and shows in adulthood
• Gestational Diabetes • Glucose during pregnancy increases chance of later developing type 2 diabetes • Hyperglycemia can cause several problems: ➢ Excessive thirst ➢ Frequent urination ➢ Hunger can't be stilled ➢ Fatigue

What is diabetes insipidus?

• Excess thirst and peeing for hormonal control

Ovaries and Testes

• Gonads make gametes
• Ovaries make relaxin, estrogen, progesterone, inhibin
• Testes make testosterone/inhibin

Pineal Gland

• Attached to roof of the 3rd ventricle of brain • Secretes melatonin • Regulates biological clock • Released during darkness • Linked to seasonal affective disorder (SAD), Jet lag

Organs with Secondary Endocrine Functions and Their Major Hormones

• Heart: Atrial natriuretic peptide (ANP) which reduces blood volume, blood pressure, and Na+ concentration
• Gastrointestinal tract: Gastrin, secretin, and cholecystokinin which aids digestion of food and buffering of stomach acids
• Gastrointestinal tract: Glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide 1 (GLP-1) which stimulates beta cells of the pancreas to release
• Kidneys: Renin that stimulates release of aldosterone
• Kidneys: Aids in the absorption of Ca2+
• Kidneys: Erythropoietin and Triggers the formation of red blood cells in the bone marrow
• Liver: Stimulates bodily growth

Aging and the Endocrine System

• Changes in hormone levels
• Histological changes and increasing fibrous connective tissue
• Decreased GH, T3/T4, ovaries, increased glucose PTH levels
• Thymus atrophies replaced with adipose
• Les cortisol adrenal and reduced glucose tolerance

This lesson covers the endocrine system, including neural and endocrine signaling, structures, and the differences between endocrine and exocrine glands. It further explores circulating versus local hormones and the general functions of the endocrine system. Assigned reading covers sections 17.1 to 17.11.