Role of the Phospholipase C Pathway and Calcium Mobilization in Oxytocin-Induced Contraction of Lacrimal Gland Myoepithelial Cells

PURPOSE. We reported that oxytocin (OXT), added to freshly prepared lacrimal gland lobules, induced myoepithelial cell (MEC) contraction. In other systems, OXT activates phospholipase C (PLC) generating Inositol 1,4,5-trisphosphate (IP3) which increases intracellular calcium concentration ([Ca2+](i)) causing contraction. The aim of the current study was to investigate the role of this pathway in OXT-induced contraction of MEC. METHODS. Tear volume was measured using the cotton thread method. Lacrimal gland MEC were isolated and propagated from alpha-smooth muscle actin (SMA)-green fluorescent protein (GFP) mice, in which MEC express GFP making them easily identifiable. RNA and protein samples were prepared for RT-PCR and Western blotting for G protein expression. Changes in [Ca2+](i) were measured in Fura-2 loaded MEC using a ratio imaging system. MEC contraction was monitored in real time and changes in cell size were quantified using ImageJ software. RESULTS. OXT applied either topically to surgically exposed lacrimal glands or delivered subcutaneously resulted in increased tear volume. OXT stimulated lacrimal gland MEC contraction in a dose-dependent manner, with a maximum response at 10(-7) M. MEC express the PLC coupling G proteins, G alpha q and G alpha 11, and their activation by OXT resulted in a concentration-dependent increase in [Ca2+](i) with a maximum response at 10(-6) M. Furthermore, the activation of the IP3 receptor to increase [Ca2+](i) is crucial for OXT-induced MEC contraction since blocking the IP3 receptor with 2-APB completely abrogated this response. CONCLUSIONS. We conclude that OXT uses the PLC/Ca2+ pathway to stimulate MEC contraction and increase lacrimal gland secretion.