Where does a hormone's receptor sit?

• A hormone acts on a target tissue by binding to specific proteins, hormone receptors, which are present only in target tissues.
• Receptors on the cell membrane of target cells are membrane-bound receptors; those inside the cell are intracellular receptors, mostly nuclear receptors found in the nucleus.
• A hormone binding to its receptor forms a hormone-receptor complex. Each receptor is specific to one hormone only.
• Forming the complex sets off biochemical changes in the target tissue; this is how hormones regulate target tissue metabolism and hence physiological functions.
• By chemical nature hormones fall into four groups: (i) peptide, polypeptide and protein hormones, such as insulin, glucagon, pituitary and hypothalamic hormones; (ii) steroids, such as cortisol, testosterone, estradiol and progesterone; (iii) iodothyronines, the thyroid hormones; (iv) amino-acid derivatives, such as epinephrine.
• A hormone that binds a membrane-bound receptor normally stays outside the target cell. The binding produces second messengers (such as cyclic AMP, IP₃ and Ca⁺⁺) inside the cell, and these regulate cellular metabolism.
• Hormones that bind intracellular receptors, such as steroid hormones and iodothyronines, mostly regulate gene expression or chromosome function: the hormone-receptor complex interacts with the genome. The combined biochemical actions produce physiological and developmental effects.

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