Multiple functions of Na/K-ATPase in dopamine-induced salivation of the Blacklegged tick, Ixodes scapularis

Tick salivation during blood-feeding is crucial for successful tick feeding, which includes the roles in the osmoregulation and in secretion of bioactive components for compromising the hosts’ immune responses.  Control of salivary secretion involves the control by autocrine dopamine activating two dopamine receptors: D1 and Invertebrate-specific D1-like (InvD1L) dopamine receptors.  The D1 receptor mediates epithelial fluid transport for excretion, while InvD1L activates myoepithelial cells for emptying acini lumen.  This study investigated Na/K-ATPase as a downstream component of D1-mediated epithelial physiology.  The gene structure of Na/K-ATPase and its expression patterns were characterized for increased expression in the post blood-feeding.  Immunoreactivity for Na/K-ATPase were found in all three types of acini: basal infolding of lamellate cells in type I, abluminal interstitial (epithelial) cells in type II, and labyrinth-like infolding structures in type III acini.  A high dose of ouabain, a Na/K-ATPase specific blocker, abolished dopamine-induced salivary secretion via suppressing the influx of fluid in type III acini.  A mild dose of ouabain resulted in a slightly reduced volume of highly hyperosmolar secretion, suggesting an additional ouabain action is for inhibition of ion resorptive function of Na/K-ATPase in type I acini.  Dopamine/ouabain were not involved in activation of protein secretion, while basal constitutive level of protein was found in the dopamine-induced saliva.  We propose that the dopamine-dependent primary saliva formation, mediated by Na/K-ATPase in type III and type II acini, is followed by a dopamine-independent resorptive function of Na/K-ATPase in type I acini located in the proximal end of the salivary duct.