Chapter 18, lecture 1

The lecture covers the functional comparison of endocrine vs. nervous systems, endocrine organ organization, hormone-receptor dynamics, receptor regulation, hormone circulation, and chemical hormone classes.

These two coordinating systems often work together. The nervous system uses neurotransmitters released locally at synapses, targeting muscle cells and neurons with instantaneous onset (\approx 10^{-3} s) and brief action.
The endocrine system uses hormones released into the interstitial fluid and blood, acting potentially distantly on virtually all body cells that express specific receptors. Its onset ranges from seconds to days, with duration also from seconds to days, depending on the hormone.

Five “pure” endocrine glands (e.g., Pituitary, Thyroid) secrete only hormones. Thirteen additional organs (e.g., Hypothalamus, Pancreas, Kidneys) have endocrine cell clusters with dual functions.
All are highly vascularized to facilitate hormone entry into the bloodstream. Only small quantities of hormones are needed for full effect.

Hormones (H⃝) circulate widely but act only on target cells expressing matching receptors (R⃝).
Each target cell typically has \approx 2000 \text{–} 100\,000 receptors, which, along with hormones, undergo continuous synthesis and degradation.

Up-regulation: Occurs when hormone levels are low, leading cells to increase R⃝ and heighten sensitivity.
Down-regulation: Occurs when hormone levels are high, causing cells to decrease R⃝ and reduce sensitivity.
The cellular response magnitude is proportional to the number of occupied receptors. This mechanism underlies drug tolerance, where chronic agonist exposure leads to down-regulation and requires higher doses.

Circulating hormones enter the bloodstream to act on distant targets, lasting minutes to hours before inactivation by the liver and excretion by the kidneys.
Local hormones remain in the interstitial fluid, acting at the site of release.
- Paracrine hormones: Act on neighboring cells.
- Autocrine hormones: Act on the same cell that secreted them.
Example: Interleukin-2 (IL-2) from helper T cells acts both paracrine (on B cells, cytotoxic T cells) and autocrine (on the secreting T cell itself).

Lipid-Soluble Hormones (Membrane-permeable):
- Poorly soluble in blood, usually travel bound to carrier proteins.
- Examples: Steroid hormones (from cholesterol: estrogen, cortisol), Thyroid hormones (T_3 & T_4 from tyrosine + iodine), Nitric Oxide (NO), Eicosanoids (from arachidonic acid).
Water-Soluble Hormones (Cannot cross plasma membrane, freely dissolve in blood):
- Examples: Amine hormones (single amino acid modifications: epinephrine, norepinephrine), Peptide / Protein hormones.
- Peptide: 3 \le n \le 49 amino acids (e.g., antidiuretic hormone, oxytocin).
- Protein: 50 \le n \le 200 amino acids (e.g., GH, insulin).