L-Arginine

 Arginine (or L-Arginine) is an amino acid involved in numerous areas of human biochemistry, including ammonia detoxification, hormone secretion, and the immune system.
Arginine is also well known as a precursor to nitric oxide, a key component of endothelial-derived relaxing factor. The endothelium is the lining inside blood vessels and arginine supplements help make more nitric oxide. Therefore, arginine, by making more nitric oxide helps to relax and dilate blood vessels.
Nitric oxide is a messenger molecule involved in a variety of endothelium-dependent effects in the cardiovascular system. Because of arginine's nitric oxide-stimulating effects, it may potentially be useful in:

• Angina pectoris
• Congestive heart failure
• Hypertension
• Coronary heart disease
• Preeclampsia
• Intermittent claudication
• Erectile dysfunction

In addition, arginine has been studied for its role in:

• HIV/AIDS (improving the immune function)
• Athletic performance (increasing muscle mass and reducing adipose tissue body fat)
• Burns and trauma (Reducing the healing time of injuries, particularly bone, and quickening the repair time of damaged tissue)  
• Diabetes and syndrome X (Helps improve insulin sensitivity)
• Male and female infertility (Improves sperm production and motility, and increases circulation throughout the body, including the sex organs) 
• Interstitial cystitis
• Stimulating the release of the growth hormone
• Reducing the risk of heart disease and helps decrease blood pressure

L-arginine shows promise in the treatment and prevention of cardiovascular disease (including atherosclerosis, hypertension, hyperlipidemia and angina pectoris), in the treatment of some forms of male infertility and some kidney disorders and it is helpful in accelerating wound healing in some circumstances. It has demonstrated some positive immune-modulating and anticancer effects. There is evidence Arginine could be helpful in some men with erectile dysfunction and in some others with migraine, liver disease and primary ciliary dyskinesia. There is conflicting but mostly negative evidence related to claims that it can improve exercise performance and promote lean muscle mass. Numerous in vitro experiments have shown that L-arginine has effects on endothelial cells that could be expected to inhibit cardiovascular disease. Inferences have been drawn from these studies suggesting that L-arginine, through its nitric oxide activity, especially in the endothelial cells of the blood vessels, inhibits vasoconstriction, thrombolytic activity, cell proliferation, inflammation and other activities that promote cardiovascular disease.
Some of the promise of these in vitro studies has been realized in animal and clinical studies. In hypercholesterolemic animal models, L-arginine helps normalize lipids and vasodilatory response, inhibits platelet aggregation and formation of intimal lesions. Further, it has been seen in some of these animal studies to cause pre-existing lesions to regress.
Similarly, L-arginine has had significant positive effects in hypercholesterolemic and hypertensive humans. It has also been helpful in those with angina pectoris. In a recent long-term study, supplemental L-arginine, given for six months, resulted in significant improvement in coronary small-vessel endothelial function associated with a decrease in plasma endothelin concentrations.