Hormones

Types

Amines

Derived from tyrosine

Peptides

Biggest group of endocrine hormones

Steroids

All come from cholesterol

Vitamin D also has a steroid structure

Control of Hormone Release and Mechanism of Action

Control of Hormone release

Humoral stimuli

Change of blood levels of ions and nutrients directly stimulates secretion of hormones

Neural stimuli

Sympathetic nervous system fibers stimulate adrenal medulla to secrete catecholamines

Hormonal stimuli

Hormones stimulate other endocrine organs to release their hormones

Hypothalamic hormones stimulate the release of most anterior pituitary hormones

Anterior pituitary hormones stimulate targets to secrete still more hormones

Hypothalamic-pituitary-target endocrine feedback loop:
hormones from the final target organs inhibit the release of the anterior pituitary hormones

Mechanism of Action

Target cells must have specific receptors, to which the hormone binds

ACTH receptors are only found on certain cells of the adrenal cortex

Thyroxin receptors are found on nearly all cells of the body

Target cell activation depends on three factors

Blood level of the hormone

Relative number of receptors on or in the target cell

Affinity of binding between receptor and hormone

Hormones influence the number of their receptors

Up-regulation

Target cells form more receptors in response to the hormone

Down-regulation

Target cells lose receptors in response to the hormone

Agonist

Drug that binds to a receptor and MIMICS actions of hormone or neurotransmitter

Antagonist

Drug that binds to a receptor and BLOCKS the actions of a hormone or neurotransmitter

Receptors

Ion Channel

GPCR & Enzyme-linked

Nuclear

Ligand-gated

Trigger enzyme reaction

Intracellular

Steroids and thyroid hormones bind to this receptor

Made of 5 separate polypeptides

Neurotransmitter

  • Binds to receptor
  • Opens (or closes) ion channel
  • Increases (decreases) intracellular ion level
  • Changes cell function

GPCR

Hormone

  • Binds receptor
  • Causes conformational change of receptor
  • Activates G protein

Activated G protein

  • Activates or inactivates an enzyme or an ion channel
  • Changes level of second messenger

Second messenger (first messenger is hormone or drug)

Affects cell function(s)

Cyclic AMP signaling mechanism

  • Activated kinases phosphorylate various proteins, activating some and inactivating others
  • cAMP is rapidly degraded by the enzyme phosphodiesterase
  • Intracellular enzymatic cascades have a huge amplification effect

Enzyme-Linked

Receptor Tyrosine Kinase (RTK)

Pair of two peptides linked together on surface of cell

Hormone

  • Binds to receptor
  • Stabilizes the receptor homodimer configuration
  • Allows trans-phosphorylation of tyrosine moeties

Hormone-receptor complex

  • Recruits signaling molecules
  • Affects cell functions

Hormone

  • Diffuses into cytoplasm
  • Binds to receptor
  • Forms hormone-receptor complex

Hormone-receptor complex

  • Enters nucleus
  • Binds to hormone response element
  • Increases or decreases synthesis of specific proteins

Pituitary Gland and Hypothalamus

Pituitary Gland

Posterior (neurohypophysis)

Anterior (adenohypophysis)

Nerve fibers and pituicytes (glial cells)

Hormone secreting epithelial cells

  • Downgrowth of hypothalamic neural tissue
  • Neural connection to the hypothalamus (hypothalamic-hypophyseal tract)
  • Nuclei of the hypothalamus synthesize the neurohormones oxytocin and antidiuretic hormone (ADH)
  • Neurohormones are transported to the posterior pituitary
  • Originates as an out-pocketing of the oral mucosa
  • Hypophyseal portal system

Hypophyseal portal system

  • Primary capillary plexus
  • Hypophyseal portal veins
  • Secondary capillary plexus
  • Carries releasing and inhibiting hormones to the anterior pituitary to regulate hormone secretion