VASOCONSTRICTION AND INCREASED FLOW - 2 PRINCIPAL MECHANISMS OF SHEAR STRESS-DEPENDENT ENDOTHELIAL AUTACOID RELEASE

The mechanisms by which nitric oxide (NO) and prostacyclin (PGI2) are released from endothelium-intact rabbit femoral arteries under resting conditions and after stimulation by either shear stress or acetylcholine (ACh) were investigated. The concentration of NO in the effluate was determined by monitoring the NO-mediated stimulation of purified soluble guanylyl cyclase, and that of PGI2 was done using a specific radioimmunoassay for its stable hydrolysis product, 6-ketoprostaglandin F1alpha. NO release under static (no-flow) conditions and in the absence of a stimulus accounted for 10-15% of the maximum release of NO from luminally perfused segments stimulated with ACh and was attenuated by removal of extracellular Ca2+. A six- to sevenfold increase in shear stress (from 0.15 to 1 dyn/cm2), generated either by vasoconstriction at constant flow or by an increase in flow at constant diameter, elicited a five- to sevenfold increase in NO release, which was correlated with increasing shear stress. The same increase in shear stress also enhanced the release of PGI2 from the femoral artery segments by 11- to 12-fold. Removal of extracellular Ca2+ abolished the shear stress-dependent PGI2 release but did not affect that of NO. In contrast, the ACh-stimulated NO release was strongly inhibited in the absence of extracellular Ca2+ (78% inhibition). Charybdotoxin, an inhibitor of Ca2+-activated K+ channels, and glibenclamide, an inhibitor of the ATP-sensitive K+ channel, had no effect on the shear stress-dependent release of NO. However, removal of sialic acids from the endothelial glycocalyx by neuraminidase selectively inhibited the shear stress-dependent NO release (64% inhibition) but had no effect on the release of NO induced by ACh or shear stress-dependent PGI2 release. Thus distinct signal transduction pathways may exist in native endothelial cells, by which physical and receptor-dependent stimuli are transformed into vasoactive signals. Moreover, these findings support the concept that the continuous release of NO and PGI2 from the endothelium, which is enhanced by an increase in shear stress, acts as an attenuating principle to counteract neurogenic and myogenic vasoconstriction in vivo.