Osmoregulatory functions of V-ATPase and Na/K-ATPase in the salivary gland of the blacklegged tick, Ixodes scapularis Say

Tick salivary secretion during blood-feeding is crucial for successful tick feeding, which includes excretory water, ions, and bioactive components compromising the hosts’ immune responses.  Control of salivary secretion involves a command signal, dopamine, which is the most potent inducer of tick salivation.  We have recently found that dopamine activates salivary secretion by orchestrating two different physiological responses through two distinct dopamine receptors.  In addition, the study demonstrated that two different types of cells in the salivary gland acini are responsible for each of the diverging physiological pathways: epithelial cells for inward fluid transport and myoepithelial cells for expelling fluid out through the acinar ducts.  We were further interested in the downstream physiology of the dopamine receptors.  In this study, we investigated two candidate genes, encoding Na/K-ATPase and the B subunit of V-ATPase.  Both genes were highly expressed in the salivary glands.  Immunoreactivities revealed that the B subunit of V-ATPase is mainly expressed in myoepithelial cells and in the basal neck region of acini, and Na/K-ATPase is expressed in epithelial cells of acini.  Ouabain, a Na/K-ATPase blocker, significantly suppressed dopamine induced inward fluid transport in a dose-dependent manner (10 nM – 100 µM), whereas bafilomycin, V-ATPase blocker, did not suppress the inward fluid transport.  The most plausible interpretation for these observations is that Na/K-ATPase is the major machinery driving inward fluid transport, whereas V-ATPase is potentially involved in fluid expulsion, via contraction of the myoepithelial cell and opening of the acinar valve.