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Prostaglandin E2 has a broad range of biological effects in diverse tissues that are mediated through its binding to specific receptors on plasma membranes. Four subtypes of receptors (EP1, EP2, EP3, and EP4) for PGE2 have been recognized on the basis of their respective responses to agonists and antagonists (Coleman et al. 1989). It has been proposed that the EP3 receptor mediates various very different effects of PGE2, such as inhibition of neurotransmitter release (Ohia and Jumblatt 1990), contraction of the uterus (Krall et al. 1984), inhibition of the reabsorption of sodium ions and water in kidney tubules (Nakao et al. 1989), and stimulation of catecholamine release from adrenal chromaffin cells (Tanaka et al. 1990). The EP3 receptor is a protein with seven transmembrane domains. The bovine EP3 receptor exists as