Hydroxycarboxylic acid receptor 1 (HCA1), formerly known as G protein-coupled receptor 81 (GPR81), is a protein that in humans is encoded by the HCAR1 gene.[5][6] HCA1, like the other hydroxycarboxylic acid receptors HCA2 and HCA3, is a Gi/o-coupled G protein-coupled receptor (GPCR).[7][8] The primary endogenous agonist of HCA1 is lactic acid (and its conjugate base, lactate).[7][8]