Endocrine Hormones

• Endocrinology is the scientific study of the endocrine system, focusing on communication within living organisms through hormones.
• Hormones are chemical messengers produced by the endocrine system.

Endocrine vs. Exocrine Glands

• Endocrine glands are ductless and secrete hormones directly into the bloodstream.
• Endocrine gland hormones reach distant tissues.
• Exocrine glands have ducts and secrete products onto outer surfaces or into the lumen of the gastrointestinal tract.
• Exocrine gland hormones cannot reach distant tissues.

Control Systems of the Body

• The body's control systems include the nervous and endocrine systems, both aimed at maintaining homeostasis.
• The nervous system provides fast responses via nerve stimulation and neurotransmitters.
• The endocrine system offers slower, more delayed responses through gland stimulation and hormone secretion.

General Endocrinology Topics

• The topics include hormone definition, classification, properties, regulation of effects, hormone cycle, biological half-life, and regulation of secretion.

Definition of Hormones

• Hormones are chemical messengers released either by endocrine glands for general circulation via the bloodstream or by specialized tissues/cells for local action.
• Hormones regulate the functions of many organs in minute amounts.

Classification of Hormones

• Hormones can be classified by their site of release and action, chemical nature, and solubility.

Classification by Site of Release and Action

• General (endocrine) hormones are secreted by endocrine glands, enter the blood directly, and influence distant target tissues in minute quantities.
• Local (tissue) hormones are secreted by specialized tissues or cells, act locally in minute amounts.
• Autocrine hormones regulate the activity of the same cells that produce them.
• Paracrine hormones diffuse through interstitial fluid to influence adjacent cells.
• Neurocrine hormones are secreted from nerve cells and regulate the function of adjacent nerve/muscle cells via synapses.
• Solinocrine hormones are secreted directly into the lumen of gastrointestinal, respiratory, and reproductive tracts.

Classification by Chemical Nature

• Hormones can be classified as proteins, polypeptides, amino acid derivatives, eicosanoids, or steroids.
• Proteins consist of more than 100 amino acids, are water-soluble, exemplified anterior pituitary hormones like PRL, TSH, GH, FSH, and LH.
• Polypeptides contain less than 100 amino acids, are water-soluble, examples include hypothalamic RF, posterior pituitary hormones like oxytocin/ADH, calcitonin, parathormone, and insulin.
• Amino acid derivatives are derived from tyrosine (dopamine, epinephrine, norepinephrine, thyroid hormones), histidine (histamine), or tryptophan (serotonin, melatonin).
• Eicosanoids are derived from unsaturated fatty acids (arachidonic acid) and include prostaglandins, prostacyclins, leukotrienes, and thromboxane.
• Steroids are lipid-soluble, derived from cholesterol, and include androgens, estrogens, progesterone, and corticosteroids.

Classification by Solubility

• Fat-soluble hormones include steroids and thyroid hormones (T3 & T4).
• Water-soluble hormones include all other types, such as fatty acid derivatives.

Properties of Hormones

• Hormones are secreted in minute amounts, and their synthesis/release is controlled by the body's needs.
• Hormones affect specific target cells with high affinity specific receptors to capture low concentrations from extracellular fluid.
• Water-soluble hormones are stored in the cytoplasm of producing cells in granules and vesicles.
• Thyroid hormones (fat-soluble) are stored outside producing cells inside follicles.
• Steroid hormones (fat-soluble) are not stored, but are synthesized according to the body's needs.

Hormone Release Patterns

• Some hormones are released at a nearly constant level, like thyroid hormones, prolactin (in non-lactating), & parathormone.
• Some hormones are released in a pulsatile manner, like GH, GnRH, LH, and FSH.
• Some hormones are released based on a circadian rhythm, like glucocorticoids and ACTH.

Factors Regulating Hormone Effects

• The factors include hormone concentration, presence of target tissue/active receptors, and hormone interactions.

Hormone Concentration Effects

• Effects of some hormones are directly proportional to concentration (e.g., insulin).
• Effects of others are inversely proportional to concentration (e.g., estrogen).
• Physiologically low estrogen levels stimulate FSH, while physiologically high levels inhibit FSH and stimulate LH, abnormally high levels inhibit both.

Target Tissue and Receptor Presence

• Gonadotropic hormones do not affect castrated animals or those with inactive/blocked receptors.
• Insulin fails to decrease blood glucose without functional receptors.

Hormone Interactions

• Synergistic effects occur when two or more hormones work together to produce a particular result.
  • Additive effect: Epinephrine and norepinephrine increase heart rate separately; together, they have a greater effect.
  • Complementary effect: FSH and testosterone act synergistically, each separately stimulating spermatogenesis during puberty.
• Permissive effects enhance a target organ's responsiveness to a second hormone or increase its activity, e.g., estrogen exposure induces progesterone receptor formation.
• Antagonistic effects occur when one hormone's actions counteract another's, such as lactation being inhibited during pregnancy by high estrogen concentrations that inhibit prolactin secretion, or the actions of insulin and glucagon.

Other Factors Regulating Effect of Hormones

• Specific antibodies against hormones, resulting from protein/polypeptide injections, antagonize hormone action (e.g., insulin in diabetics).
• Vitamins are essential:
  • Vitamin C for adrenal gland function.
  • Vitamin E for gonadal hormone action.
  • Vitamin B for thyroid gland function.
  • Vitamin D for parathyroid gland function.

Hormone Cycle

• The hormone cycle spans from its synthesis in the endocrine gland to utilization by target tissue and excretion.

Steps in the Hormone Cycle

• Hormone synthesis occurs where protein/peptide hormones are synthesized in the rER, steroid hormones by mitochondria/sER, and thyroid hormones in the colloid of thyroid follicles.
• Storage: Hormones are stored.
• Release: Hypothalamic releasing hormones, pituitary tropic hormones and hormone metabolites get released.
• Transport occurs in bloodstream (free for water-soluble, bound to proteins for fat-soluble).
• Utilization occurs when hormones bind to receptors and perform functions, followed by inactivation and excretion from the body.

Hormone Utilization

• Hormones inactivate in the liver and kidneys and are excreted via bile and urine.
• When a hormone binds to its receptors and performs its function, it's considered hormone utilization.
• Termination of action usually requires dissociation from the receptor.

Biological Half-Life

• Biological half-life is the time it takes for half the hormone amount to get cleared from circulation.
• A shorter half-life indicates increased hormone utilization, and vice versa.
• Frequency of exogenous hormone treatment relies on half life of the hormone.

Hormone Action

• Hormones affect target cells by binding to protein or polypeptide receptors in the hypothalamus, pituitary, parathyroid, and pancreatic.
• Cell membrane receptors activate secondary messengers like cAMP, Ca-calmodulin, or DAG & IP3.
• A secondary messenger changes the activity of already present enzymes.
• Steroid hormones are adrenocortical, bind to gonadal receptors in the cytoplasm and cause synthesis of new proteins.
• Hormones are thyroid and adrenal medullary, derived from amino acids, bind to nuclear receptors and cause synthesis of new proteins.

Regulation of Hormone Secretion

• Regulation can be genetic, environmental, or involve feedback mechanisms.

Genetic Control

• Genetic structure governs endocrine gland activity, where errors may cause hypo or hyperfunction in hormone secretion.

Environmental Factors

• High Temperatures can inhibit TRH, TSH, T3 & T4
• Emotion stimulates adrenaline, glucocorticoids, ACTH, PRL, and GH release.
• The food Intake increases gut hormones.
• Prolonged exercise increases GH and PRL while decreasing sex hormones.
• Sleep increases GH and PRL.

Feedback Control

• Synthesis and secretion of most hormones are under feedback control.
• Hormonal secretion from an endocrine gland gets regulated by the circulating hormone.
• Regulation can be direct, or mediated by another hormone, a metabolite, physical factor.
• Regulation can result in more (positive feedback) or less hormone (negative feedback).

Negative Feedback

• Negative mechanisms act to lower hormone levels in the blood when they rise above normal, maintaining homeostasis.
• Endocrine glands operate under mechanisms, acting like a thermostat and controlled by hormone/metabolite levels in the blood.
• An example involves the hypothalamus-pituitary-thyroid where TRH stimulates TSH from the pituitary gland to produce more T3/T4. Higher concentrations cause secretion of TRH and TSH reduced.
• Glucocorticoids impact ACTH secretion via negative mechanisms.

Feedback Control, Metabolites

• Some hormones are subject to feedback control via metabolites/physical factors, ionized calcium negatively controls PTH secretion.
• Glucose negatively controls glucagon, but positively controls insulin, and extracellular fluid negatively controls aldosterone.

Positive Feedback

• Increasing estradiol levels during the estrous cycle exert positive feedback on the pituitary to increase the pulsatile release of LH.
• The Increased LH levels lead to further increased estradiol secretion, resulting in surge of LH.

No Feedback Regulation

• Placental hormones (equine chorionic gonadotropin, progesterone, estrogens) lack classic mechanisms, and hormones produced by ectopic glands are free of control.

Positive Feedback, Hormonal Regulation

• This feed back mechanism although less common, sees a hormones increases its own level and inhibits its own release.
• Furthermore the hormone levels can be used to regulate the level of another hormone.

Examples:

• Preovulatory release of LH relates to estrogen Production by the follicle.
• FSH and Estrogen where high levels of FSH is stimulates the granulosa cells to increase estrogen release

Types of Endocrine Gland Stimuli

• Hormonal, Neural and Humoral

Endocrine Gland Stimuli, Humoral

• The parathyroid gland secretes parathyroid hormone (PTH) in response to low Ca2+ concentrations in the blood.

Endocrine Gland Stimuli, Neural

• The adrenal medulla cells secrete epinephrine and norepinephrine in response to preganglionic sympathetic fiber action potentials.

Endocrine Gland Stimuli, Hormonal

• The anterior pituitary secretes hormones to stimulate other endocrine glands due to hormones from the hypothalamus.

List of Endocrine Glands

• The list includes pineal, hypothalamus, pituitary, parathyroid, thyroid, adrenal gland, ovary, placenta, pancreas, and testis (male).

Hypothalamus

• This neuroendocrine gland secretes releasing and inhibiting hormones.
• The Releasing and Inhibiting hormones are a cluster of hormones that stimulates or Inhibits the release of tropic hormones from the pituitary gland.
• It also secretes Oxytocin and Vasopressin (antidiuretic hormone, ADH).

Anatomy of the Hypothalamus

• Below the thalamus and above the pituitary gland, the hypothalamus is dividied into various nuclei.
• The hypothalamus stalk connects Hypothalamus to the pituitary gland.

Hypothalamus-Pituitary Connections

• There is the hypothalamo-hypophyseal portal system and the hypothalamo-hypophyseal tract.

Hypothalamo-Hypophyseal Portal System

• This system is for the anterior pituarity gland whre releasing and inhibiting hormones pass to be stored in median eminence.

Hypothalamo-Hypophyseal Tract

• The posterior pituitary hormones such as antidiuretic hormone, and oxytocin are directly transported along the axons.

Releasing and Inhibiting Hormones

• Hormones can either stimulate or inhibit the release of certain hormones, affecting their target organs.

GnRH

• GnRH is secreted in a pulsatille pattern and will either favor the release of LH and or FSH.
• Furthermore puls frequency and amplitude will control the GnRH, and pulse frequency will change based on the menstural cycle.
• 1-GnRH will stimulates of synthesis and secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH), half life is 7 minutes and is destroyed by peptidase.

Control of GnRH

• GnRH is controlled by steroid feedback neurotransmitters: such as stimulatory Ne and Dopamine but also inhibitory Opioids and GABA.
• In addition GnIH will act directly on the ptuitary and inhibit gonadotrpins while inhibiting reproductive behaviors.

This lesson explores hormone release patterns, storage, solubility, and hormone types such as steroid, neurocrine, and local hormones. It covers hormones derived from tryptophan, tyrosine, and cholesterol.