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Downregulation is the process by which a cell decreases the quantity of a cellular component, such as RNA or protein, in response to an external variable. An increase of a cellular component is called upregulation. An example of downregulation is the cellular decrease in the number of receptors to a molecule, such as a hormone or neurotransmitter, which reduces the cell's sensitivity to the molecule. This phenomenon is an example of a locally acting negative feedback mechanism. An example of upregulation is the increased number of cytochrome P450 enzymes in liver cells when xenobiotic molecules such as dioxin are administered (resulting in greater degradation of these molecules). Some receptor agonists may cause downregulation of their respective receptor(s), while most receptor antagonists may temporally upregulate their respective receptor(s). The disequilibrium caused by these changes often causes withdrawal when the long-term use of a medication or drug is discontinued. However, the use of certain receptor antagonists may also damage receptors faster than they upregulate (internalization of receptors due to antagonism). Upregulation and downregulation can also happen as a response to toxins or hormones. An example of upregulation in pregnancy is hormones that cause cells in the uterus to become more sensitive to oxytocin. == Receptor downregulation == 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Downregulation and upregulation」の詳細全文を読む スポンサード リンク
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