Prostanoid IP receptors coupled to Gs are thought to be the primary target for prostacyclin (PGI(2)) analogues. However, these agents also activate prostanoid EP1-4 receptor subtypes to varying degrees, which are positively (EP2/4) or negatively (EP3) coupled to adenylate cyclase through Gs or Gi, respectively. We investigated the role of these receptors in modulating relaxation to PGI(2) analogues cicaprost, iloprost and treprostinil in pre-contracted segments of rat tail artery. Prostanoid IP (RO1138452), EP4 (GW627368X), EP3 (L-798106), EP1-3 (AH6809), and EP1 (SC-51322) receptor antagonists were used to determine each receptor contribution. The role of Go was investigated using pertussis toxin (PTX), while dependence on cAMP was determined using adenylate cyclase (2'5'dideoxyadenosine, DDA) and protein kinase A (2'-O-monobutyryladenosine- 3',5'-cyclic monophosphorothioate, Rp- isomer, Rp-2'-O-MB-cAMPS) inhibitors, and by measurement of tissue cAMP. All analogues caused relaxation which was significantly (P