Role of prostanoid IP and EP receptors in mediating vasorelaxant responses to PGI2 analogues in rat tail artery: Evidence for Gi/o modulation via EP3 receptors

Prostanoid IP receptors coupled to Gs are thought to be the primary target for prostacyclin (PGI2) 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 PGI2 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 Gi/o 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 < 0.01) inhibited by RO1138452; with maximum response to cicaprost, iloprost and treprostinil reduced by 51%, 66% and 37%, respectively. GW627368X had no effect when used alone, but in combination with RO1138452, caused a rightward shift of the curves for cicaprost and iloprost but not treprostinil. PTX treatment potentiated relaxation to all 3 analogues (P < 0.01), as did L798106 and AH6809 but not SC-51322. Basal cAMP levels were higher in PTX-treated tissues and DDA- and Rp-2'-O-MB-cAMPs--sensitive responses increased to analogue concentrations < 0.1 μM. In conclusion, prostanoid EP3 receptors via Gi/o negatively modulate prostanoid IP receptor-mediated relaxation to cicaprost, iloprost and treprostinil. However, other pathways contribute to analogue-induced vasorelaxation, the nature of which remains unclear for treprostinil.