Prostanoid IP receptors coupled to Gs are thought to be the primary target for 
prostacyclin (PGI(2)) analogues. However, these agents also activate prostanoid 
EP(1-4) receptor subtypes to varying degrees, which are positively (EP(2/4)) or 
negatively (EP(3)) 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), EP(4) (GW627368X), EP(3) (L-798106), 
EP(1-3) (AH6809), and EP(1) (SC-51322) receptor antagonists were used to 
determine each receptor contribution. The role of G(i/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 
<0.1μM. In conclusion, prostanoid EP(3) receptors via G(i/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.