The endothelium of resistance arteries: physiology and role in hypertension.

The endothelium plays a very important role in the regulation of vascular function by way of its barrier role, by interaction with circulating cells such as platelets, which may release vasoactive or growth regulating agents, and through production of substances which modulate vascular tone and smooth muscle cell growth, and which may also exert antithrombotic effects. The endothelium of resistance arteries, vessels critically involved in generating resistance to flow and which play an important role in hypertension, has been studied mainly from the point of view of generation of agents which regulate vascular tone and growth. Endothelium-derived relaxing factors such as nitric oxide, prostacyclin, hyperpolarizing factors (EDHF) and possibly C-type natriuretic peptide (CNP), are counteracted by endothelium-derived contracting factors, which include endothelins and contracting factors (EDCF) which are less well characterized and appear to be cyclooxygenase products. In experimental hypertension in animals, and in human essential hypertension, these mechanisms may be altered. There may be a reduced generation of endothelium-derived nitric oxide and enhanced production of EDCF. Some of the mechanisms involved in the role these agents play in the physiology of resistance arteries and pathologically in hypertension will be reviewed.