Functional difference between two isoforms of rat kidney prostaglandin receptor EP₃ subtype

We have cloned two isoforms of rat kidney prostaglandin E₂ receptor EP₃ subtype (rEP_3A and rEP_3B), which differ only in their cytosolic carboxyl-terminal tails (30 and 29 amino acids, respectively). The aim is to clarify the functional difference between two rEP₃ receptor isoforms by examining formation of adenosine 3',5'-monphosphate (cAMP) and change in cytosolic free calcium ([Ca²⁺]i) in cultured cells transiently transfected with cloned rEP_3A or rEP_3B receptor cDNA. In immortalized renal distal tubule cells (TKC2), vasopressin(VP) stimulated cAMP formation, and the cAMP formation was significantly attenuated by a non-peptide VP receptor antagonist, OPC-31260. The VP-induced increase in cAMP formation was also attenuated by overexpression of rEP_3A receptor but not that of rEP_3B receptor. On the other hand, in COS-7 cells transfected with rEP_3B receptor cDNA, PGE₂ induced an increase in [Ca²⁺]i, but no increase in [Ca²⁺]i was observed in the cells transfected with rEP_3A cDNA. In conclusion, rEP_3A receptor is suggested to antagonize VP (V₂) receptor by inhibiting cAMP formation, whereas rEP_3B receptor is linked with Ca²⁺ messenger system.