Flow-dependent regulation of coronary vasomotor tone.

Large coronary artery calibres are regulated by humoral factors (autacoids) released from the endothelial cell lining. Several hormones, transmitters and platelet-derived products like serotonin and ATP or acetylcholine, noradrenaline, and histamine, stimulate such an autacoid release and thus an endothelium-mediated dilation in addition to their direct constrictor effect on the vasculature. Another mechanical stimulus is the flow-induced shear stress acting as viscous drag upon the endothelial surface along with the pulsatile stretching of the endothelial lining, which causes, in a moment to moment fashion, a continuous adjustment of coronary calibres. With endothelial function impaired or absent as in atheromatosis or after balloon catheter denudation this adjustment is compromised. Thus unbalanced constrictor effects may become effective especially in the presence of a simultaneously reduced platelet antiaggregation.