Role of Nitric Oxide in Biology
Since it was first discovered to play a role in the dilation of blood vessels many new roles for Nitric Oxide (NO) have been discovered. Nitric oxide has been found to be produced by virtually every cell type in the body and plays an important role in controlling the normal function of cells as well as in regulating larger scale processes such as the nervous and immune systems. Some of these biological roles for NO are described in more detail below.
The Immune System
Nitric oxide plays many important roles in the immune system. It is produced in high amounts from specialised cells of the immune system called macrophages. Following a bacterial infection, for example, the body produces chemicals known as cytokines which activate the cells of the immune system, including macrophaes, and help guide them to the site of infection. The high amounts of nitric oxide produced by the macrophages is actually toxic to the bacteria and plays an important role in their destruction (see image on the right). The production of nitric oxide in this way also help protect against other types of infection including viruses and parasites.
However, too much nitric oxide production has also been implicated in conditions where the immune system is too active - diseases like arthritis and the so-called autoimmune diseases.
The Nervous System
Nitric oxide has been shown to be involved in both the central and peripheral nervous system. Of the three types of enzyme that produce nitric oxide in humans, one type is found almost exclusively in the nervous system. It appears to play a role in promoting the transfer of nerve signals from one neuron to another. It does this be stimulating the release of molecules called neurotransmitters which are released from one nerve cell, diffuse across the gap between the cell and stimulate the neighbouring nerve cell to transmit the signal. Nitric oxide has been implicated in diseases of the nervous system like Parkinson's and Alzheimer's.
Nitric oxide is involved in many aspects of reproduction. It is thought to play a role in the implantation of the early embryo in the uterus and it functions to relax blood vessels and thereby helps to regulate maternal blood pressure. During pregnancy nitric oxide may also play a role in promoting the formation of new blood vessels, a process known as angiogenesis. It is also known to be an important survival factor for specialist cells called trophoblasts which form the placenta. There is also evidence that complications of pregnancy such as preeclampsia may be associated with reduced production of nitric oxide.
In addition drugs such as Viagra help overcome erectile dysfunction by affecting nitric oxide signalling.
A wide range of cellular activity can be regulated by nitric oxide including cell division, cell survival and cell movement.
Most cells have an in-built self-destruct system or cell suicide mechanism. This mechanism, usually called apoptosis or programmed cell death, exists to prevent damaged or infected cells from affecting the proper functioning of the rest of the tissue. Once triggered the apoptotic pathway leads to the breakdown of the structure of the cell in an organised manner, leading to a cell that is smaller and more neatly "packaged" ready for removal by cell of the immune system.
Nitric oxide has been shown to inhibit apoptosis and therefore is important in promoting cell survival. However, high doses of nitric oxide have been reported as being toxic to many cell types and in these circumstances may promote cell death instead.
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