PLA2 and TRPV4 channels regulate endothelial calcium in cerebral arteries

We previously demonstrated that endothelium-derived hyperpolarizing factor (EDHF)-mediated dilations in cerebral arteries are significantly reduced by inhibitors of PLA(2). In this study we examined possible mechanisms by which PLA(2) regulates endothelium-dependent dilation, specifically whether PLA(2) is involved in endothelial Ca2+ regulation through stimulation of TRPV4 channels. Studies were carried out with middle cerebral arteries (MCA) or freshly isolated MCA endothelial cells (EC) of male Long-Evans rats. Nitro-L-arginine methyl ester (L-NAME) and indomethacin were present throughout. In pressurized MCA, luminally delivered UTP produced increased EC intracellular Ca2+ concentration ([Ca2+](i)) and MCA dilation. Incubation with PACOCF(3), a PLA(2) inhibitor, significantly reduced both EC [Ca2+](i) and dilation responses to UTP. EC [Ca2+](i) was also partially reduced by a transient receptor potential vanilloid (TRPV) channel blocker, ruthenium red. Manganese quenching experiments demonstrated Ca2+ influx across the luminal and abluminal face of the endothelium in response to UTP. Interestingly, PLA(2)-sensitive Ca2+ influx occurred primarily across the abluminal face. Luminal application of arachidonic acid, the primary product of PLA(2) and a demonstrated activator of certain TRPV channels, increased both EC [Ca2+](i) and MCA diameter. TRPV4 mRNA and protein was demonstrated in the endothelium by RT-PCR and immunofluorescence, respectively. Finally, application of 4 alpha-phorbol 12,13-didecanoate (4 alpha PDD), a TRPV4 channel activator, produced an increase in EC [Ca2+](i) that was significantly reduced in the presence of ruthenium red. We conclude that PLA(2) is involved in EC Ca2+ regulation through its regulation of TRPV4 channels. Furthermore, the PLA(2)-sensitive component of Ca2+ influx may be polarized to the abluminal face of the endothelium.