Abstract Body: Vascular tone is impacted by the endothelium's ability to detect both mechanical and chemical stimulation. The Leucine-Rich Repeat-Containing protein 8a, (LRRC8A), was previously identified as a required component of the mechanoresponsive endothelial LRRC8 complex regulating AKT-endothelial nitric oxide synthase (eNOS) signaling and vascular function. While LRRC8A is broadly expressed, LRRC8B, C, D and E have tissue-restricted expression. Here, we identified 2 single nucleotide polymorphisms (SNPs) in Lrrc8c that are highly associated with diastolic and systolic blood pressure in human Genome-Wide Association Studies (GWAS) and Phenome Wide Association Studies (PheWAS), implicating LRRC8C as a required LRRC8 heteromer that regulates endothelial function. While LRRC8A/B/C/D are expressed in endothelium, co-immunoprecipitation experiments from lung endothelium using LRRC8A-3xFlag knock-in mice and endothelium-specific LRRC8A-3xFlag overexpression mice revealed the endothelial LRRC8 complex to be composed largely of LRRC8A/B/C heteromers. LRRC8A/B/C knock-out studies in mice and knock-down studies in human umbilical vein endothelial cells (HUVECs) show co-dependent expression of LRRC8A/B/C proteins, but not LRRC8D. Functionally, LRRC8A and LRRC8C depletion reduces endothelial LRRC8 currents, reduces AKT-eNOS signaling, increases myogenic tone, impairs eNOS dependent vasodilation, and exacerbates angiotensin-induced hypertension. These data identify LRRC8A, LRRC8B and LRRC8C as components of the endothelial LRRC8 complex and reveal LRRC8C as having a non-redundant role in regulating endothelial AKT-eNOS, vascular relaxation and susceptibility to hypertension.