Oxytocin receptors expressed and coupled to Ca2+ signalling in a human vascular smooth muscle cell line

1 In a human vascular smooth muscle cell line (HVSMC), binding experiments with [H-3]-arginine(8)-vasopressin (AVP) have shown the existence of a homogeneous population of binding sites with affinity (K-d value) of 0.65 nM and a maximum number of binding sites (B-max) of 122 fmol mg(-1) protein. 2 Nonlabelled compounds compete for [H-3]-AVP binding in the HVSMC membrane with an order of potency of oxytocin > lyspressin greater than or equal to AVP > Thr(4), Gly(7)-oxytocin > (beta-mercapto-beta-beta-cyclopentamethylenepropionyl-O-Me Tyr(2), Arg(8)) vasopressin > desmopressin > OPC21268 > OPC31260. This order was markedly different from that observed in rat vascular smooth muscle cells (A10), a well-established V-1A receptor system. 3 In HVSMC both oxytocin and AVP increased inositol 1,4,5-trisphosphate (IP3) production and [Ca2+](i) response, but the efficacy of the responses was greater for oxytocin than AVP. 4 Reverse transcription-polymerase chain reaction (RT-PCR) assay detected only oxytocin receptor but not V-1A Or V-2 receptors in HVSMC, whereas only V-1A receptors were found in A10 cells. 5 In conclusion, in HVSMC only oxytocin receptors are expressed among the vasopressin receptor family, and they coupled to phosphatidyl inositol (PI) turnover/Ca2+ signalling. This unexpected observation should provide new insight into the functional role of the oxytocin receptor in a human vascular smooth muscle cell line.