Modulation of the vasodilator action of SIN-1 by the endothelium

We studied the influence of endothelium-derived relaxing factor (EDRF) on sydnonimine (SIN-1)-induced vasodilatation and the accumulation of cyclic GMP in the rabbit femoral artery. The potency of SIN-1 to elicit vasodilatation in norepinephrine-contracted femoral arteries was significantly enhanced in the absence of the endothelium or following impairment of the synthesis of EDRF with gossypol or NG-nitro-L-arginine, whether the application of SIN-1 was intra- or extraluminal. The increase in cyclic GMP in the femoral segments by a combination of SIN-1 and endothelium-derived relaxant factor (released by the endothelium of either the rabbit thoracic aorta or the femoral artery) was significantly less than the sum of the increases in cyclic GMP induced by each agent alone. In contrast, stimulation of purified soluble guanylate cyclase by submaximal concentrations of SIN-1 was additive with the effect of EDRF, released from acetylcholine-stimulated rabbit aortas. This indicates the absence of a direct interaction between the factor and SIN-1 at the level of soluble guanylate cyclase. The interaction seems to be specific for cyclic GMP-mediated responses, since cyclic AMP-induced dilatations elicited by isoproterenol were not affected by the presence of the endothelium. The results indicate that the endothelium can modulate the vascular reactivity to SIN-1. This modulation may be mediated either by EDRF or by another endothelial substance that alters the metabolism or the action of cyclic GMP in vascular smooth muscle.