GPR81, also known as hydroxycarboxylic acid receptor 1 (HCAR1), is a recently de-orphanized receptor that is activated by lactic acid (EC50 ~ 1.5 mM) but not by other hydroxycarboxylic acids. 3-chloro-5-hydroxy Benzoic Acid (3-chloro-5-hydroxy BA) is an agonist of GPR81 (EC50 = 16 μM) that is inactive against the related GPR109a (HCAR2) receptor. It is similarly effective at GPR81 receptors from a variety of mammalian species and is bioavailable, stimulating lipolysis (increased serum free fatty acids) in mice fed high fat chow for 10 weeks. 3-chloro-5-hydroxy BA, like lactate, blocks basal ghrelin secretion by primary gastric mucosal cells.
