Endocrine System 3

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Mechanism of Hormone Action

Water Soluble Hormones

Water soluble hormones cannot cross the plasma membrane and therefore have to use second messengers, e.g. cyclic AMP.
  1. Water soluble hormone binds to membrane receptor, e.g. adenylate cyclase via G protein
  2. AC catalyses the synthesis of cAMP
  3. cAMP changes metabolism by activating enzymes and phosphorylating membrane proteins (opening channels).
  4. cAMP is deactivated by PDE (phosphodiesterase)

Lipid Soluble Hormones

Lipid soluble hormones (steroid hormones) can diffuse across the plasma membrane and bind to and activate receptors in the cytoplasm or nucleus. The activated receptor-hormone complex alters gene expression and directs the synthesis of new proteins. This alters cellular metabolism.

Thyroid Hormones

Thyroid hormones are transported across the cell membrane and bind to receptors at the mitochondria and nucleus. They form the hormone-receptor complex, which binds to DNA, much like steroid hormones. Genes are activated, new proteins are synthesised and the target cell response is activated.

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