The endothelial cells can release both relaxing and contracting substances. The former include prostacyclin and endothelium-derived relaxing factor (EDRF, which most likely is nitric oxide, or a nitrosoderivative releasing nitric oxide, derived from L-arginine). Candidates as endothelium-derived contracting factors (EDCF) include superoxide anions thromboxane A2 and the peptide endothelin. Endothelium-derived relaxing factor causes relaxation of vascular smooth muscle by activation of the soluble form of guanylate cyclase which leads to an accumulation of cyclic GMP; it also reduces platelet adhesion and aggregation. The latter effect is synergistic with the inhibition evoked by prostacyclin. The release of EDRF and prostacyclin plays a key role in the protective role of the endothelium against vasospasm and the unwanted coagulation of blood. Indeed, thrombin and aggregating platelets are potent stimuli for the release of EDRF. The platelet-products responsible are the adenine nucleotides, ADP and ATP, which activate P2y-purinergic receptors on the endothelial cells and 5-hydroxytryptamine (serotonin) that stimulates 5-HT1-like serotonergic receptors. The response to serotonin, but not that to the adenine nucleotides, is mediated by a pertussis toxin-sensitive mechanism. When endothelial cells regenerate, or are cultured, they selectively lose the pertussis toxin-sensitive mechanism of release, which results in a marked decrease in sensitivity to exogenous and platelet-released serotonin. As a consequence, the endothelial cells exhibit a considerably reduced response to aggregating platelets. This phenomenon, which can be exacerbated by hypercholesterolemia, favors ongoing platelet aggregation and vasospasm, and constitutes a first step toward atherosclerosis.