A word is needed about how signal transduction pathways are shut off. As expected, the key event is usually the cessation of receptor activation. Because organic second messengers are rapidly inactivated (for example, cAMP by phosphodiesterase) or broken down intracellularly, and because calcium is continuously being pumped out of the cell or back into the en-doplasmic reticulum, increases in the cytosolic concentrations of all these components are transient events and continue only as long as the receptor is being activated by a first messenger.
A major way that receptor activation ceases is by a decrease in the concentration of first messenger molecules in the region of the receptor. This occurs as the first messenger is metabolized by enzymes in the vicinity, taken up by adjacent cells, or simply diffuses away.
In addition, receptors can be inactivated in two other ways: (1) The receptor becomes chemically altered (usually by phosphorylation), which lowers its affinity for a first messenger, and so the messenger is released; and (2) removal of plasma-membrane receptors occurs when the combination of first messenger and receptor is taken into the cell by endocytosis. The processes described here are physiologically controlled. For example, in many cases the inhibitory phosphorylation of a receptor is mediated by a protein
Synthesis of different transcription factors v mRNA
Synthesis of proteins that mediate the cell's response to the first messenger (for example, proliferation and differentiation)
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.