Mechanoreception by the endothelium: mediators and mechanisms of pressure- and flow-induced vascular responses.

Mechanoreception, a widely distributed sensory modality, has been shown to be present in certain blood vessels. Changes in physical forces, like sudden increase of transmural pressure or flow velocity (shear stress), trigger changes in blood vessel diameter; the former reduces it while the latter increases vessel caliber. These changes in diameter, which are the result of contraction and relaxation of vascular smooth muscle in the blood vessel media, can serve the purpose of physiological regulation of blood flow (autoregulation) and protection of the intima against damages from high shear forces. The precise location of mechanosensor(s) and the mechanism of mechanoreception and signal transduction are poorly understood. Accumulating evidence suggests that the endothelium may be a site of mechanoreception and that changes in the synthesis/release of endothelium-derived relaxing (EDRF, EDHF, PGI2) and contracting factors (EDCF) result in altered vascular smooth muscle tone and vessel caliber. Increased shear stress stimulates the release of EDRF and PGI2 probably via activation of a K+ channel (inward rectifier) in endothelial cell membrane. Endothelium-dependent vascular contraction evoked by increased transmural pressure may be the result of (1) reduced release of EDRF (canine carotid artery) and (2) stimulation of the release of a still unidentified EDCF(s) (feline cerebral artery). Thus the endothelium can serve as pressure and flow sensor and is capable of transducing changes in mechanical forces into changes of vascular smooth muscle tone by modulating the release of endothelium-derived vasoactive factors. The physiological importance of the mechanoreception by endothelial cells in the intact circulation remains to be determined.