Evidence that dilation of isolated salivary ducts from the tick Dermacentor variabilis (Say) is mediated by nitric oxide

We used pharmacological methods to test the hypothesis that female Dermacen tor variabilis salivary ducts dilated when dopamine-stimulated and that dil ation was nitric oxide-mediated. Stimulation with dopamine resulted in an i ncreased diameter (19.7%) compared to unstimulated ducts (P<0.005). Pretrea tment with L-NAME, an inhibitor of nitric oxide synthase, or cytochalasin D abolished the dilation. Addition of L-arginine to L-NAME-treated ducts par tially restored the ability to dilate. A cuticular coil composed of a serie s of concentric rings ran the length of the duct adjacent to the epithelial cell layer. In stimulated ducts, the center-to-center periodicity of these rings increased from 0.59 mu m in unfed ducts to 1.0 mu m from partially f ed ducts (P<0.05). When the ducts from partially fed females were stimulate d with dopamine, the periodicity increased further to 1.75 mu m (P<0.05), s uggesting the coils moved further apart in response to stimulation. Promine nt folds lining the lumen of unstimulated ducts were less pronounced in sti mulated preparations, suggesting that the cuticle stretches, thereby increa sing lumen size. Actin was localized in epithelial cells as a honeycomb pat tern that we suggest links the epithelial cells to the rings. Together, the se data support the following hypothesis: stimulated ducts dilated during f luid production; dilation involved an actin-based system, and was mediated by nitric oxide. Dilation of the duct may enhance its role as a reservoir f or saliva produced by the acini during the period between imbibition and sa livation. (C) 2000 Elsevier Science Ltd. All rights reserved.