Genetic ablation of calcium-independent phospholipase A2β causes hypercontractility and markedly attenuates endothelium-dependent relaxation to acetylcholine

Abstract

Activation of phospholipases leads to the release of arachidonic acid and lysophospholipids that play prominent roles in regulating vasomotor tone. To identify the role of calcium-independent phospholipase A2β (iPLA2β) in vasomotor function, we measured vascular responses to phenylephrine (PE) and ACh in mesenteric arterioles from wild-type (WT; iPLA2β+/+) mice and those lacking the β-isoform (iPLA2β−/−) both ex vivo and in vivo. Vessels isolated from iPLA2β−/−mice demonstrated increased constriction to PE, despite lower basal smooth muscle calcium levels, and decreased vasodilation to ACh compared with iPLA2β+/+mice. PE constriction resulted in initial intracellular calcium release with subsequent steady-state constriction that depended on extracellular calcium influx. Endothelial denudation had no effect on vessel tone or PE-induced constriction although the dilation to ACh was significantly reduced in iPLA2β+/+vessels. In contrast, vessels from iPLA2β−/−constricted by 54% after denudation, indicating smooth muscle hypercontractility. In vivo, blood pressure, resting vessel diameter, and constriction of mesenteric vessels to PE were not different in iPLA2β−/−vessels compared with WT mouse vessels. However, relaxation after ACh administration in situ was attenuated, indicating an endothelial inability to induce dilation in response to ACh. In cultured endothelial cells, inhibition of iPLA2β with ( S)-(E)-6-(bromomethylene)tetrahydro-3-(1-naphthalenyl)-2 H-pyran-2-one (BEL) decreased endothelial nitric oxide synthase phosphorylation and reduced endothelial agonist-induced intracellular calcium release as well as extracellular calcium influx. We conclude that iPLA2β is an important mediator of vascular relaxation and intracellular calcium homeostasis in both smooth muscle and endothelial cells and that ablation of iPLA2β causes agonist-induced smooth muscle hypercontractility and reduced agonist-induced endothelial dilation.