RESEARCH PAPER: GPR55dependent and independent ion signalling in response to lysophosphatidylinositol in endothelial cells

Background and purpose: The glycerol-based lysophospholipid lysophosphatidylinositol (LPI) is an endogenous agonist of the G-protein-coupled receptor 55 (GPR55) exhibiting cannabinoid receptor-like properties in endothelial cells. To estimate the contribution of GPR55 to the physiological effects of LPI, the GPR55-dependent and -independent electrical responses in this cell type were investigated. Experimental approach: Applying small interference RNA-mediated knock-down and transient overexpression, GPR55dependent and -independent effects of LPI on cytosolic free Ca2+ concentration, membrane potential and transmembrane ion currents were studied in EA.hy296 cells. Key results: In a GPR55-dependent, GDPbS and U73122-sensitive manner, LPI induced rapid and transient intracellular Ca2+ release that was associated with activation of charybdotoxin–sensitive, large conductance, Ca2+-activated, K+ channels (BKCa) and temporary membrane hyperpolarization. Following these initial electrical reactions, LPI elicited GPR55-independent long-lasting Na+ loading and a non-selective inward current causing sustained membrane depolarization that depended on extracellular Ca2+ and Na+ and was partially inhibited by Ni2+ and La3+. This inward current was due to the activation of a voltage-independent non-selective cation current. The Ni2+ and La3+-insensitive depolarization with LPI was prevented by inhibition of the Na/K-ATPase by ouabain. Conclusions and implications: LPI elicited a biphasic response in endothelial cells of which the immediate Ca2+ signalling depends on GPR55 while the subsequent depolarization is due to Na+ loading via non-selective cation channels and an inhibition of the Na/K-ATPase. Thus, LPI is a potent signalling molecule that affects endothelial functions by modulating several cellular electrical responses that are only partially linked to GPR55. British Journal of Pharmacology (2010) 161, 308–320; doi:10.1111/j.1476-5381.2010.00744.x