Endothelial nitric oxide modulates perivascular sensory neurotransmission in the rat isolated mesenteric arterial bed

1 A possible role of nitric oxide (NO) as a modulator of capsaicin-sensitive sensory neurotransmission in blood vessels was investigated in the rat isolated mesenteric arterial bed. 2 Electrical field stimulation (EFS) of methoxamine-preconstricted mesenteric beds elicited frequency-dependent vasorelaxation mediated by capsaicin-sensitive sensory nerves. N-G-nitro-(L)-arginine methyl ester ( (L)-NAME, 10 and 300 mum) and 7-nitroindazole (7-NI, 100 muM), inhibitors of nitric oxide synthase (NOS), augmented sensory neurogenic vasorelaxation. (D)-NAME (300 mum), 6-aminoindazole (100 mum) and N-psi-propyl-(L)-arginine (50 nM), a selective inhibitor of neuronal NOS, were without effect. The effect of 10 muM (L)-NAME was reversed by (L)-arginine (I mm), the substrate for NOS. 3 (L)-NAME (300 pm) and 7-NI (100 muM) had no significant effect on vasorelaxations to calcitonin gene-related peptide (CGRP), the principal motor neurotransmitter of capsaicin-sensitive sensory nerves in rat mesenteric arteries, or to capsaicin, indicating a prejunctional action. The inhibitors of NOS had no effect on vasorelaxation to forskolin, but augmented vasorelaxation to sodium nitroprusside (SNP). 4 Removal of the endothelium augmented sensory neurogenic vasorelaxation, but did not affect vasorelaxation to CGRP, indicating a prejunctional action of endothelial NO. 5 In the absence of endothelium, (L)-NAME (300 muM) inhibited, and 7-NI (100 muM) caused no further augmentation of sensory neurotransmission. 6 SNP (100 muM), a nitric oxide donor, attenuated sensory neurogenic relaxations to EFS. 7 In rat isolated thoracic aortic rings, (L)-NAME (100 muM) and 7-NI (100 muM) attenuated concentration-dependent relaxations to acetylcholine. 8 These data show that NO modulates sensory neuro transmission evoked by EFS of the rat isolated mesenteric arterial bed, and that when NO synthesis is blocked sensory neurogenic relaxation is augmented. The source of NO is the vascular endothelium.