Endocrine Glands and Hormones

Endocrine Glands and Hormones

• Endocrine Glands
  • Ductless glands that secrete hormones directly into the bloodstream.
  • Hormones travel via the bloodstream to target cells, which have specific receptor proteins for those hormones.
  • Various organs like the heart, liver, kidneys, and adipose tissue also secrete hormones.
  • Neurohormones produced by the hypothalamus help regulate body metabolism, growth, and reproduction.

Major Endocrine Glands

• Adipose Tissue: Secretes leptin, which suppresses appetite.
• Adrenal Cortex:
  • Produces glucocorticoids (primarily cortisol) affecting glucose metabolism.
  • Mineralocorticoids, mainly aldosterone, promote sodium retention and potassium excretion (target: kidneys).
• Adrenal Medulla: Secretes epinephrine, impacting heart and dilation of bronchioles.
• Heart: Produces atrial natriuretic hormone, promoting sodium and water excretion.
• Hypothalamus: Secretes releasing and inhibiting hormones regulating anterior pituitary functions.
• Kidneys: Produce erythropoietin, stimulating red blood cell production.
• Ovaries: Produce estradiol-17β and progesterone, affecting female reproductive systems.
• Thyroid Gland: Produces thyroxine (T4) and triiodothyronine (T3) for regulating metabolism and development.

Chemical Classification of Hormones

• Amines: Derived from tyrosine and tryptophan (e.g., adrenal hormones).
• Polypeptides and Proteins: Include hormones like insulin and oxytocin.
• Glycoproteins: Long polypeptides associated with carbohydrates (e.g., FSH, LH).
• Steroids: Lipids derived from cholesterol (e.g., testosterone, cortisol).

Hormone Actions and Mechanisms

• Polar Hormones: Water-soluble and cannot penetrate cell membranes (e.g., epinephrine, insulin).
• Nonpolar Hormones: Lipophilic and can cross cell membranes (e.g., thyroid hormones).
• Prohormones and Prehormones:
  • Prohormones: Inactive hormones that need modification (e.g., insulin).
  • Prehormones: Require modifications within target cells to become active (e.g., thyroxine).

Hormone Interactions

• Antagonistic Effects: Hormones act in opposite ways (e.g., insulin vs. glucagon).
• Synergistic Effects: Combined effects of multiple hormones amplify a response (e.g., milk production).
• Permissive Effects: Presence of one hormone enhances the action of another (e.g., estradiol's effect on progesterone).

Hormone Concentrations and Responses

• Half-life: Time for hormone concentration to reduce by half.
• Threshold Levels: Tissues respond only when hormone levels reach a physiological norm.
• Priming Effects: Increase in receptor number enhances response to hormones.
• Desensitization: Prolonged high concentrations may reduce receptor numbers leading to decreased sensitivity.

Mechanisms of Hormone Action

• Lipophilic Hormones: Bind to intracellular receptors leading to transcriptional regulation in target cells.
• Second Messenger Systems: Water-soluble hormones activate second messengers for effect (e.g., cAMP, IP3 pathway).
  • cAMP: Activated by adenylate cyclase, influencing protein kinases and cellular metabolism.
  • Phospholipase C: Produces DAG and IP3, leading to calcium release and activation of other proteins.
  • Tyrosine Kinase: Used by insulin and growth factors, catalyzes phosphorylation of target proteins.

Pituitary Gland Overview

• Composed of an anterior lobe (glandular) and posterior lobe (neural tissue).
• Anterior Pituitary Hormones:
  • Growth hormone (GH): Stimulates growth and metabolism.
  • Thyroid-stimulating hormone (TSH): Stimulates thyroid hormone secretion.
  • Adrenocorticotropic hormone (ACTH): Stimulates cortisol production in adrenal cortex.
• Posterior Pituitary Hormones: Secretes hormones made in the hypothalamus (ADH and oxytocin).

Adrenal Glands

• Located atop kidneys; consist of adrenal cortex (glandular) and medulla (neural).
  • Cortex: Produces corticosteroids affecting metabolism and immune response.
  • Medulla: Secretes epinephrine and norepinephrine as part of the fight-or-flight response.

Thyroid Gland

• Located below the larynx, produces thyroid hormones essential for metabolism and development.
• Calcitonin: Secreted by parafollicular cells, lowers blood calcium levels.

Pancreas and Blood Glucose Regulation

• Produces insulin (lowers glucose) and glucagon (raises glucose).
  • Insulin: Decreases blood sugar levels by enhancing glucose uptake.
  • Glucagon: Increases blood glucose levels by stimulating glycogen breakdown.

Pineal Gland

• Sits in the brain, secretes melatonin related to circadian rhythms and seasonal responses.

Paracrine and Autocrine Regulation

• Autocrine: Signal acts on the same cell type.
• Paracrine: Signal acts on adjacent different cell types.

Prostaglandins and Related Actions

• Eicosanoids with roles in inflammation, immune responses, and regulation of physiological functions.
• Inhibitors: NSAIDs like aspirin block synthesis, reducing inflammation and pain but potentially causing side effects.