2.1 OXIDATIVE STRESS
The term oxidative stress is often used to imply a condition in which cells are exposed to excessive levels of either molecular oxygen or chemical derivatives of oxygen called reactive oxygen species; (Kojda et al., 1999). This occurs when molecules of oxygen are reduced to various subunits such as water, the production of superoxide anion radical, hydrogen peroxide and hydrogen radicals (Geller, D.A., et al. 1993). On the other hand ROS has different effects on individual blood vessels and also play a very important role on the physiological and pathological aspects of the vessels. The main ROS in blood vessels are superoxides. Super oxides are formed from the remains of reduced oxygen which is catalysed by two enzymes which are NADPH oxidase and xanthine oxidase (Taniyama et al., 2003). Superoxides are able to act on different cells but it can produce ROS by reacting with other substances, for e.g. superoxides can react with Nitric oxide to produce peroxynitrite which is potentially deleterious of ROS (Helmut, 1997). This happens by the use of superoxide dismutase enzyme, by the use of this enzyme a production of stable ROS is formed such as hydrogen peroxide (H2O2) which is then broken down into water (H2O) and oxygen (O2), this occurs in all types of vascular cells (Taniyama et al., 2003)
Figure 1: Formation of free radicals in biological systems (Vet med, 2014)
2.2 ROLE OF REACTIVE OXYGEN SPECIES
The major source of reactive oxygen species in both endothelial and vascular smooth muscle cells are membrane bound oxidases, which utilise NADH and NADPH as substrates (Kojda et al., 1999).
Impaired endothelium dependent vasorelaxation:
ROS plays a very important role in the endothelium dysfunction and an addition of lipid lowering drugs with antioxidants causes the reduction in the endothelium dysfunction, and therefore Glutathione Peroxidase (GPx) is important when removing ROS and this produces a weakened endothelium dependent vasorelaxation (Taniyama et al., 2003).
Activation of adhesion molecules:
In mammalian endothelium inert inflammatory surfaces are present but only on exposure to various pro-inflammatory stimuli, and this induces the activation of adhesion molecules and leads to the adhesion of monocytes on the endothelium cells and thus produces atherosclerotic formations. Some adhesion molecules are dependent on ROS and so NO acts as an antioxidant as NO has an antioxidant property which reduces expressions.
Angiogenesis is the physiological process by which new blood vessels form from pre-existing vessels this occurs in repair and in the growth of tumour cells such as cancer. ROS is involved in the proliferation and migration of endothelial cells and can intervene in lymphocyte activated tubulogenesis which is the formation of tubules in epithelial cells. ROS is also able to act as in-between the angiogenic growth factors.
Growth of vascular smooth muscle cells:
ROS is involved in the...