GPR55 agonist lysophosphatidylinositol and lysophosphatidylcholine inhibit endothelial cell hyperpolarization via GPR-independent suppression of Na-Ca exchanger and endoplasmic reticulum Ca 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 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-dependent K channels of intermediate and small conductance. The hyperpolarizing responses to consecutive histamine applications are strongly reduced by NCX inhibition. In a Ca-re-addition protocol, bepridil, a NCX inhibitor, and KB-R7943, a blocker of reversed NCX, inhibit the hyperpolarizing responses to Ca-re-addition following Ca 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 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.