Characterization of calcium signals provoked by lysophosphatidylinositol in human microvascular endothelial cells

The lipid molecule, lysophosphatidylinositol (LPI), is hypothesised to form part of a novel lipid signalling system that involves the G protein-coupled receptor GPR55 and distinct intracellular signalling cascades in endothelial cells. This work aimed to study the possible mechanisms involved in LPI-evoked cytosolic Ca2+ mobilization in human brain microvascular endothelial cells. Changes in intracellular Ca²⁺ concentrations were measured using cell population Ca²⁺ assay. LPI evoked biphasic elevation of intracellular calcium concentration, a rapid phase and a sustained phase. The rapid phase was attenuated by the inhibitor of PLC (U 73122), inhibitor of IP₃ receptors, 2-APB and the depletor of endoplasmic reticulum Ca²⁺ store, thapsigargin. The sustained phase, on the other hand, was enhanced by U 73122 and abolished by the RhoA kinase inhibitor, Y-27632. In conclusion, the Ca²⁺ signal evoked by LPI is characterised by a rapid phase of Ca²⁺ release from the endoplasmic reticulum, and requires activation of the PLC-IP₃ signalling pathway. The sustained phase mainly depends on RhoA kinase activation. LPI acts as novel lipid signalling molecule in endothelial cells, and elevation of cytosolic Ca²⁺ triggered by it may present an important intracellular message required in gene expression and controlling of vascular tone.