Osmoregulatory functions of Na/K-ATPase in the salivary glands 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.  A candidate gene, encoding Na/K-ATPase, which is highly expressed in the salivary glands, was investigated.  Immunoreactivity revealed that Na/K-ATPase is expressed in epithelial cells of acini.  Ouabain, a Na/K-ATPase blocker, significantly suppressed both dopamine induced inward fluid transport in a dose-dependent manner (10 nM – 100 µM) and dopamine induced salivary secretion at high dose (10µM).  We measured the salivary contents to determine Na, K, and Cl ion, and protein concentrations.  Treatment of ouabain at the low dose, which still allows dopamine-mediated salivary secretion, produced the saliva with hyperosmolar, but with same amount of protein as the control saliva.  The results suggest that ouabain-sensitive Na/K-ATPase is the main downstream pathway for dopamine response in the epithelial cells of salivary gland for water transport, but not for protein secretion.