Oxytocin receptors expressed and coupled to Ca 2+ signalling in a human vascular smooth muscle cell line

1 In a human vascular smooth muscle cell line (HVSMC), binding experiments with [ 3 H]‐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 [ 3 H]‐AVP binding in the HVSMC membrane with an order of potency of oxytocin > lyspressin ≥ AVP > Thr 4 , Gly 7 ‐oxytocin > (β‐mercapto‐β‐β‐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 (IP 3 ) production and [Ca 2+ ] 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/Ca 2+ signalling. This unexpected observation should provide new insight into the functional role of the oxytocin receptor in a human vascular smooth muscle cell line.