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Many hormones released by endocrine organs travel down in the blood and bind to specific receptors on the target cells. What then breaks that binding of the molecule with the receptor ? ( thus inactivating further stimulation of the target cell )

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The binding is reversible typically; part of the potency of a drug is ow well and for how long it binds to its target. There's a natural equilibrium of binding and dissociation. Many drugs, once bound to their cognate receptor, cause a down regulation of their cognate receptor on the target cell. The bound/activated downstream signalling pathways may be inhibited by ubiquitination of the downstream signals themselves or upregulation of antagonists etc. The hormone itself has a half life, which is very important, thus levels naturally decrease and for some hormones this is incredibly rapid. Levels may decrease due to breakdown or excretion. Increase of binding hormones may decrease free hormone thus it's effect also.

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Nice answer ! What is down regulation ? –  biogirl Jul 28 '13 at 14:38
    
The number of receptors on cells are regulated by sometimes thousands of things. Often the hormone that activates a receptor in addition to its normal effects, also causes the numbers of its own receptors to go down. This is down regulation which prevents against too much of a hormone over a long period of time. –  AndroidPenguin Jul 28 '13 at 15:56
    
Another point: even if a receptor is not downregulated or inactivated, certain signaling and gene regulatory circuits can prevent prolonged response. –  WYSIWYG Jul 29 '13 at 3:31

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