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 Dermacentor variabilis salivary ducts dilated when dopamine-stimulated and that dilation was nitric oxide-mediated. Stimulation with dopamine resulted in an increased diameter (19.7%) compared to unstimulated ducts (P<0.005). Pretreatment with L-NAME, an inhibitor of nitric oxide synthase, or cytochalasin D abolished the dilation. Addition of L-arginine to L-NAME-treated ducts partially restored the ability to dilate. A cuticular coil composed of a series 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 µm in unfed ducts to 1.0 µm from partially fed ducts (P<0.05). When the ducts from partially fed females were stimulated with dopamine, the periodicity increased further to 1.75 µm (P<0.05), suggesting the coils moved further apart in response to stimulation. Prominent folds lining the lumen of unstimulated ducts were less pronounced in stimulated preparations, suggesting that the cuticle stretches, thereby increasing lumen size. Actin was localized in epithelial cells as a honeycomb pattern that we suggest links the epithelial cells to the rings. Together, these data support the following hypothesis: stimulated ducts dilated during fluid production; dilation involved an actin-based system, and was mediated by nitric oxide. Dilation of the duct may enhance its role as a reservoir for saliva produced by the acini during the period between imbibition and salivation.