In vivo mechanisms of acetylcholine-induced vasodilation in rat sciatic nerve

Abstract

We examined the importance of nitric oxide (NO), endothelium-derived hyperpolarizing factor (EDHF), and neurogenic activity in agonist-induced vasodilation and baseline blood flow [i.e., nerve microvascular conductance (NMVC)] in rat sciatic nerve using laser Doppler flowmetry. Agonists were acetylcholine (ACh) and 3-morpholinosydnonimine (SIN-1). Vasodilation occurring despite NO synthase (NOS) and cyclooxygenase inhibition and showing dependence on K+ channel activity was taken as being mediated by EDHF. NOS and cyclooxygenase inhibition with N ω-nitro-l-arginine (l-NNA) + indomethacin (Indo) revealed two phases of ACh-induced vasodilation: an initial, transient l-NNA + Indo-resistant vasodilation, peaking at 23 ± 6 s and lasting 145 ± 69 s, followed by sustainedl-NNA + Indo-sensitive vasodilation. l-NNA alone did not affect sustained ACh-induced vasodilation but decreased baseline NMVC by 55%. In the presence of l-NNA + Indo, the K+ channel blocker tetraethylammonium (TEA) inhibited transient ACh-induced vasodilation by 58% and reduced baseline NMVC by 25%. SIN-1-induced vasodilation increased fourfold in the presence of l-NNA, whereas the specific guanylyl cyclase inhibitor 1 H-(1, 2,4)oxadiazolo(4,3-α)quinoxalin-1-one abolished it. However, in homogenates of rat sciatic nerve, SIN-1-stimulated soluble guanylyl cyclase (sGC) activity was unaffected by l-NNA. TTX affected neither SIN-1- nor ACh-induced vasodilation. In conclusion, ACh-induced vasodilation consisted of two components, the first partially mediated by EDHF and the second by a vasodilatory prostanoid + NO. Baseline NMVC was dependent on NO and EDHF. Althoughl-NNA enhanced SIN-1-induced vasodilation, it had no effect on sGC-activity.