GPR55-dependent and -independent ion signalling in response to lysophosphatidylinositol in endothelial cells.
OBJECTIVE
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.


METHODS
Applying small interference RNA-mediated knock-down and transient overexpression, GPR55-dependent and -independent effects of LPI on cytosolic free Ca(2+) concentration, membrane potential and transmembrane ion currents were studied in EA.hy296 cells.


RESULTS
In a GPR55-dependent, GDPbetaS and U73122-sensitive manner, LPI induced rapid and transient intracellular Ca(2+) release that was associated with activation of charybdotoxin-sensitive, large conductance, Ca(2+)-activated, K(+) channels (BK(Ca)) 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 Ca(2+) and Na(+) and was partially inhibited by Ni(2+) and La(3+). This inward current was due to the activation of a voltage-independent non-selective cation current. The Ni(2+) and La(3+)-insensitive depolarization with LPI was prevented by inhibition of the Na/K-ATPase by ouabain.


CONCLUSIONS
LPI elicited a biphasic response in endothelial cells of which the immediate Ca(2+) 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.
Bondarenko
Alexander
A
Institute of Molecular Biology and Biochemistry, Center of Molecular Medicine, Medical University of Graz, Graz, Austria.
Waldeck-Weiermair
Markus
M
Naghdi
Shamim
S
Poteser
Michael
M
Malli
Roland
R
Graier
Wolfgang F
WF
D002118:Calcium / Q000378:metabolism; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D002460:Cell Line; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D042783:Endothelial Cells / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D006801:Humans; D017136:Ion Transport / Q000187:drug effects / Q000502:physiology; D008246:Lysophospholipids / Q000494:pharmacology / Q000502:physiology*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D024681:Potassium Channels, Calcium-Activated / Q000378:metabolism; D034741:RNA, Small Interfering; D043882:Receptors, Cannabinoid; D043562:Receptors, G-Protein-Coupled / Q000819:agonists / Q000037:antagonists & inhibitors / Q000235:genetics / Q000502:physiology*; D012964:Sodium / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors; D014162:Transfection
C524885:GPR55 protein, human; D008246:Lysophospholipids; D024681:Potassium Channels, Calcium-Activated; D034741:RNA, Small Interfering; D043882:Receptors, Cannabinoid; D043562:Receptors, G-Protein-Coupled; C025449:lysophosphatidylinositol; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D002118:Calcium