second messenger, molecule inside cells that acts to transmit signals from a receptor to a target. The term second messenger was coined upon the discovery of these substances in order to distinguish them from hormones and other molecules that function outside the cell as “first messengers” in the transmission of biological information. Many second messenger molecules are small and therefore diffuse rapidly through the cytoplasm, enabling information to move quickly throughout the cell. As elements of signaling pathways, second messengers can serve to integrate information when multiple independent upstream inputs influence the rates of synthesis and degradation of the second messenger. In addition, second messengers can have multiple downstream targets, thereby expanding the scope of signal transmission.
A large number of second messenger molecules have been characterized, including cyclic nucleotides (e.g., cyclic adenosine monophosphate, or cAMP, and cyclic guanosine monophosphate, or cGMP), ions (e.g., Ca2+), phospholipid-derived molecules (e.g., inositol triphosphate), and even a gas, nitric oxide (NO). The calcium ion Ca2+ has a critical role in the rapid responses of neurons and muscle cells. At rest, cells maintain a low concentration of Ca2+ in the cytoplasm, expending energy to pump these ions out of the cell. When activated, neurons and muscle cells rapidly increase their cytoplasmic Ca2+ concentration by opening channels in the cell membrane, which allow Ca2+ ions outside the cell to enter rapidly.
The cyclic nucleotide cAMP is synthesized by adenylyl cyclase enzymes, which are downstream of heterotrimeric G-proteins (guanine nucleotide binding proteins) and receptors. For example, when epinephrine binds to beta-adrenergic receptors in cell membranes, G-protein activation stimulates cAMP synthesis by adenylyl cyclase. The newly synthesized cAMP is then able to act as a second messenger, rapidly propagating the epinephrine signal to the appropriate molecules in the cell. This stimulatory signaling pathway leads to the production of effects such as increasing rate and force of contraction of the heart that are characteristic of epinephrine. Caffeine also enhances the action of cAMP by inhibiting the enzyme phosphodiesterase, which degrades cAMP; the enhancement of cAMP activity contributes to the general stimulatory action of caffeine. As a gas, nitric oxide (NO) is distinct among second messengers in being able to diffuse across cell membranes, which allows signal information to cross into neighbouring cells.