Endothelial control of vascular tone and growth.

Endothelial cells play an important regulatory role in the circulation as a physical barrier and as a source of a variety of regulatory substances. Endothelium-derived nitric oxide and prostacyclin are released in response to physical stimuli, hormones and platelet-derived substances and induce vascular relaxation and inhibition of platelet function. Certain substances can evoke a hyperpolarization of smooth muscle cells. In addition, endothelial cells can release several contracting factors (i.e. endothelin, thromboxane A2, angiotensin II, superoxide and unidentified endothelium-derived contracting factors), at least under certain conditions. Endothelial cells are also a source of growth inhibitors and promoters, such as heparin and heparin sulphates, platelet-derived growth factor and thrombospondin. Several vasoactive substances produced by the endothelium, such as nitric oxide, endothelin and angiotensin II may also play a role in the regulation of vascular growth. Thus, the endothelial layer can regulate vascular tone and growth. A dysfunction of these endothelium-dependent regulatory systems may play a role in cardiovascular diseases, such as hypertension and atherosclerosis.