GPR55 agonist lysophosphatidylinositol and lysophosphatidylcholine inhibit endothelial cell hyperpolarization via GPR-independent suppression of Na+-Ca2 + exchanger and endoplasmic reticulum Ca2 + refilling

Lysophosphatidylinositol (LPI) and lysophosphatidylcholine (LPC) are lipid signaling molecules
that induce endothelium-dependent vasodilation. In addition, LPC suppresses acetylcholine (Ach)-
induced responses. We aimed to determine the influence of LPC and LPI on hyperpolarizing
responses in vitro and in situ endothelial cells (EC) and identify the underlying mechanisms.
Using patch-clamp method, we show that LPI and LPC inhibit EC hyperpolarization to histamine
and suppress Na
+
/Ca
2+
exchanged (NCX) currents in a concentration-dependent manner. The
inhibition is non-mode-specific and unaffected by intracellular GDPβS infusion and tempol, a
superoxide dismutase mimetic. In excised mouse aorta, LPI strongly inhibits the sustained and the
peak endothelial hyperpolarization induced by Ach, but not by SKA-31, an opener of Ca
2+
-
dependent K
+
channels of intermediate and small conductance. The hyperpolarizing responses to
consecutive histamine applications are strongly reduced by NCX inhibition. In a Ca
2+
-re-addition
protocol, bepridil, a NCX inhibitor, and KB-R7943, a blocker of reversed NCX, inhibit the
hyperpolarizing responses to Ca
2+
-re-addition following Ca
2+
stores depletion. These finding
indicate that LPC and LPI inhibit endothelial hyperpolarization to Ach and histamine
independently of G-protein coupled receptors and superoxide anions. Reversed NCX is critical for
ER Ca
2+
refilling in EC. The inhibition of NCX by LPI and LPC underlies diminished
endothelium-dependent responses and endothelial dysfunction accompanied by increased levels of
these lipids in the blood.