Shear stress, the endothelium and the balance between flow-induced contraction and dilation in animals and man.

Dilation is the most commonly observed diameter change in blood vessels when intraluminal flow increases. However, at very high and low levels of vascular tone the response is constriction. This complex response seems designed to ensure that time-averaged vascular tone levels are restricted to an intermediate range. Flow dilation is initiated predominantly at the surface of the endothelium, probably by conformational change in macromolecules of the extracellular matrix such as glycosaminoglycans. This is associated with changes in ion binding--flow is exquisitely sodium sensitive, and subsequent alteration in cellular function. In the rabbit basilar artery the inward rectifying potassium channel of the endothelium cell is opened by shear stress increase leading to dilation and the voltage-dependent calcium channel of the smooth muscle cells with constriction. In this blood vessel, at any rate, the final response to flow change seems to be predominantly the consequence of the interaction between these two processes.