How nitric oxide helps regulate biological function

By Jacob Adams Ph.D. • Noxsan

ILLUSTRATION 1

What do wound healing, drugs for ED, biofilm quorum sensing, fruit ripening, and the 1998 Nobel Prize in Physiology and Medicine have in common? nitric oxide. This remarkable diatomic gas plays an outsized role in regulating biological processes everywhere, from bacteria to animals to humans. Nitric Oxide is involved in everything from memory and learning to skin tone and most importantly wound healing.

Accelerating or resuming wound healing after injury, surgery or illness and restoring the animal to full health as quickly as possible is a primary goal in veterinary care. Fast healing means less stress for the animal, less risk of infection and less stress and expense for the pet owner. Nitric oxide is known to be a critical part of the normal wound healing cascade – a discovery that led to the Nobel Prize mentioned above. The therapeutic value of nitric oxide in wound healing derives from the regulation of the inflammatory response, cell proliferation, collagen formation, antimicrobial activity and angiogenesis (Figure 1).

The enzyme nitric oxide synthase converts L-arginine to L-citrulline, releasing nitric oxide in vivo. Three isoforms of the enzyme have been characterized. Wound healing is mainly controlled by endothelial nitric oxide synthase, which is mainly expressed in the skin and blood vessels. Endothelial nitric oxide synthase is activated by thrombin when a wound occurs and then orchestrates the cascade of processes required for wound closure.

Nitric oxide is used by the body in two ways after clotting to kill or remove pathogens in the wound bed to prevent infection. At low levels, nitric oxide acts as a signaling molecule that promotes the growth and activity of immune cells. At high concentrations, nitric oxide induces broad spectrum damage to pathogens caused by nitrosative and oxidative reactions. The damage suffered by pathogens exposed to these chemical processes is extensive. Few bacteria can escape the antimicrobial effects of nitric oxide. Nitric oxide also controls immune cell signaling and the biochemical reactions used to defend against bacteria, fungi, viruses and parasites.

Nitric oxide is a biological signal that controls the spread of biofilms to the more vulnerable form of planktonic bacteria. Biofilms are notoriously tenacious and generally very resistant to antimicrobials and antibiotics. Plankton bacteria are much more sensitive to antimicrobials and antibiotics. Nitric Oxide upregulates the expression of endogenous collagenase, which autolytically debrides the wound to further promote the healing process.

Nitric oxide coordinates proliferation, differentiation and apoptosis in a range of cell types involved in wound healing. When tested, nitric oxide donors significantly increase fetal bovine serum-induced thymidine incorporation into human dermal fibroblast DNA and enhance fibroblast growth factor- or platelet growth factor-induced DNA synthesis. Nitric oxide has been shown to stimulate endothelial cell proliferation, protect endothelial cells from apoptosis, and mediate the production of vascular endothelial growth factor (VEGF). These effects of nitric oxide on endothelial cells control angiogenesis, the formation of new capillaries. The resulting increased blood flow promotes the transport of proteins into the wound bed and thus facilitates wound healing. Low nitric oxide levels increase keratinocyte proliferation. Nitric Oxide coordinates increased collagen synthesis and deposition in the final stages of wound healing. Treatment with nitric oxide donors has been shown to increase collagen formation from fibroblasts and conversely decrease collagen formation following inhibition of nitric oxide synthase.

The role of nitric oxide in wound healing is multifaceted, its presence and amount supplied are critical at every stage of wound healing. The challenge in medicine has been to safely, efficiently and effectively deliver the right amount of nitric oxide to the point of need, the wound bed, to control and promote the healing process.

Several nitric oxide delivery systems for wound care have been developed. The success of nitric oxide delivery systems in treating laboratory models of wounds and infections spurs further development of this promising technology.

Gas streams containing nitric oxide have been shown to be effective in long-term clinical practice for the therapy of wounds and inflammatory diseases. Placement in the wound bed may require patient immobilization during treatment. Nitrous oxide in this form is supplied in cylinders and requires proper storage. Intermittent exposure may be practiced to avoid long periods of immobilization, with administration occurring over multiple visits.

  • Generation of nitric oxide by acidified nitrite

Nitric Oxide can be produced when combined with a weak acid like citric acid, with wound healing studies showing promising results. The length of exposure and the sensitivity of the skin to low pH must be taken into account. Consistent, biologically appropriate dosing requires care as the reaction produces an immediate increase in nitric oxide concentration. Advances in this approach have been made by immobilizing the acid to carefully control the mixing of nitrite salts. Long-term controlled delivery at biologically appropriate concentrations is the current focus to improve this delivery method.

  • Low molecular and macromolecular nitric oxide dispenser

Nitric Oxide donors in many forms including organic nitrates, nitrites, S-nitrosothiols, nitrosamines, N-diazeniumdiolates and metal nitric oxide complexes have been developed to release nitric oxide on demand. The donors are often encapsulated into or conjugated to a variety of biomaterial vectors to design delivery systems. Nitric oxide release depends on the environmental conditions in the wound to activate the delivery system, although a combination with a release agent can be used to improve control. Nitric oxide synthase mimics have also been developed to catalyze the production and release of nitric oxide from L-arginine. Each approach has its own activation and delivery method, and most have shown promising results for improved wound healing, at least in the laboratory.

  • Nitric Oxide by Electrochemical Activation

Nitric oxide can be generated from sodium nitrite by electrochemical reduction. This system is capable of delivering nitric oxide at a constant, biologically relevant concentration for long periods of time. To date, the use of powered systems or expensive electrodes has limited the usefulness of the system. A new electrochemical system was introduced that is activated by the addition of water. Once activated, the electrochemical system produces a biologically relevant level of nitric oxide for several days. The first practical application of this approach has just been published by Noxsano in wound dressings optimized to deliver nitric oxide at levels that promote blood flow, angiogenesis, proliferation and epithelialization, resulting in enhanced wound healing (Figure 2).

This educational center article was written by Noxsano.