With advancing age, there is a reduction in the ability of arteries to dilate in humans and in rodents. This is related to a reduction in nitric oxide (NO) in the inner lining of arteries (the endothelium.) This is partly due to reduced production of an enzyme called NO synthase (NOS) and partly due to a reduced supply of NO secondary to NO degradation by antioxidants.
The reduced production of NO is most likely due to a reduced supply of an essential enzyme in NO synthesis, endothelial tetrahydrobiopterin (BH4.) BH4 may be decreased with aging due to BH4 oxidation. The definite cause is uncertain at present.
Low arginine may be the result of increased arginase activity. Low arginine levels have been suggested as a possible cause of blood vessel dilation problems. There is some evidence from research on rodents to support this idea. BH4 is 60% deficient in the small arteries of the leg muscles of aging rats. Using a BH4 precursor, sepiapterin, increased the ability of the small leg arteries to dilate and increased circulation by 52%.
Human studies showed that a single high oral dose of BH4 increased the circulation of an arm artery. These studies relate to the importance of BH4 in blood vessel dilation in aging. Further studies are needed.
CONCLUSION: BH4 deficiency, not arginine deficiency, seems to be related to endothelial dilation and circulation problems of aging. BH4 use partly restores the circulation problem. Clinical studies are needed to see if BH4 works for restoring the vascular endothelial dysfunction of aging humans.
NOTE: Arginine is important to the functioning of the immune system. It has a similar function in plants. Arginase is an enzyme which makes urea and ornithine from arginine. NO synthase is an enzyme which forms NO from arginine.
PMID: 18388135.
Summary #337.
