Characterization of Na+-K+-2Cl- Cotransporter Activity in Rabbit Lacrimal Gland Duct Cells

We recently reported that isolated duct segments from rabbit lacrimal gland (LG) were able to secrete fluid in response to secretagogues, which were blocked completely by bumetanide. This suggests the functional involvement of Na+-K+-2Cl- cotransporter (NKCC1) in ductal fluid secretion. Therefore, the aim of this study was to investigate the activity profile of NKCC1 in isolated rabbit LG duct segments. Interlobular ducts were isolated from fresh rabbit LG tissue. Microfluorometry with the ammonium (NH4+)-pulse technique was used to elicit pH changes in duct cells, and the rate of bumetanide-sensitive cytosolic acidification after addition of NH4+ was used to quantify the activity of NKCC1. While basal activity of NKCC1 was undetectable, low cytosolic chloride (Cl-) level and hyperosmotic challenge (390 mOsm) were able to increase the activity of NKCC1. Carbachol (100 μM) had no significant effect on NKCC1 activity. Elevation of cytosolic calcium (Ca2+) level with Ca2+-ionophore (A 23187, 1 μM) did not cause any alteration in the activity of the cotransporter while direct activation of protein kinase C (phorbol myristate acetate, 100 nM) increased its activity slightly but in a significant manner. Addition of either forskolin (10 μM), cell-permeable cAMP analogue (8-bromo cAMP, 100 μM) or vasoactive intestinal peptide (200 nM) resulted in a significant increase in the activity of NKCC1. These results highlight the functional involvement of NKCC1 in LG duct secretion. These findings may facilitate our understanding of LG function and may contribute to the development of targeted pharmacologic interventions in case of dry eye disease.