Biology · Ch 20 — Chemical Coordination and Integration
Mechanism of Hormone Action
Mechanism of Hormone Action
Having surveyed the individual endocrine glands and their hormones, it is important to understand how a hormone, once it has been carried by the blood to a target cell, actually produces its physiological effect there. Because hormones differ sharply in their basic chemical nature, they also differ sharply in how they cross (or fail to cross) the plasma membrane of a target cell, and this single chemical difference determines which of two broadly distinct mechanisms of action a given hormone will use.
Protein and peptide hormones — which include, among the hormones covered in this chapter, growth hormone, TSH, ACTH, FSH, LH, prolactin, oxytocin, vasopressin, insulin, glucagon and parathyroid hormone, as well as the catecholamines adrenaline and noradrenaline, which though not strictly proteins behave in essentially the same way — are water-soluble and are too large, or too polar, to cross the lipid-rich plasma membrane of a target cell directly. These hormones therefore act entirely at the cell surface. A protein/peptide hormone binds to a specific receptor protein embedded in the plasma membrane, without itself ever entering the cell. This binding event on the outside of the cell is then relayed to the inside of the cell by a signal transduction pathway, commonly involving an associated membrane-bound G-protein and a membrane enzyme such as adenylate cyclase, which together generate a small, diffusible intracellular signalling molecule known as a second messenger — cyclic AMP (cAMP) is the best-known example, though calcium ions and inositol trisphosphate (IP3) serve a similar role for certain other hormones. This second messenger then activates a cascade of enzymes already present within the cell, rapidly altering the cell's ongoing metabolic activity — for example, activating or deactivating particular enzymes already present, or opening ion channels — without requiring any new protein to be synthesised. Because this mechanism works entirely through modifying existing cellular machinery, the resulting response typically begins within seconds to minutes of hormone binding, but is also relatively short-lived once the hormone is no longer present, since the second-messenger signal is rapidly broken down once its source is removed. …
What this figure shows. A side-by-side diagram contrasting protein-hormone action (left) and steroid-hormone action (right) on a target cell. On the left, a protein hormone is shown binding a specific receptor embedded in the plasma membrane on the cell's outer surface, without itself entering the cell; this binding activates a G-protein and an associated membrane enzyme (e.g. adenylate cyclase), generating a diffusible second messenger (e.g. cyclic AMP) inside the cytoplasm, which in turn activates a cascade of enzymes altering the cell's ongoing metabolic activity rapidly but only transiently. On the right, a small, lipid-soluble steroid hormone is shown diffusing directly across the plasma membrane and binding a receptor protein located inside the cytoplasm or nucleus; the resulting hormone-receptor complex is shown entering (or already positioned within) the nucleus and binding a specific hormone-response element on the DNA, switching the transcription of a particular gene on or off and s …