Pharmacologic characterization of the oxytocin receptor in human uterine smooth muscle cells

[ 3 H]‐oxytocin was used to characterize the oxytocin receptor found in human uterine smooth muscle cells (USMC). Specific binding of [ 3 H]‐oxytocin to USMC plasma membranes was dependent upon time, temperature and membrane protein concentration. Scatchard plot analysis of equilibrium binding data revealed the existence of a single class of high‐affinity binding sites with an apparent equilibrium dissociation constant ( K d ) of 0.76 n M and a maximum receptor density ( B max ) of 153 fmol mg −1 protein. The Hill coefficient ( n H ) did not differ significantly from unity, suggesting binding to homogenous, non‐interacting receptor populations. Competitive inhibition of [ 3 H]‐oxytocin binding showed that oxytocin and vasopressin (AVP) receptor agonists and antagonists displaced [ 3 H]‐oxytocin in a concentration‐dependent manner. The order of potencies for peptide agonists and antagonists was: oxytocin>[Asu 1,6 ]‐oxytocin>AVP= atosiban>d(CH 2 ) 5 Tyr(Me)AVP>[Thr 4 ,Gly 7 ]‐oxytocin>dDAVP, and for nonpeptide antagonists was: L‐371257>YM087>SR 49059>OPC‐21268>SR 121463A>OPC‐31260. Oxytocin significantly induced concentration‐dependent increase in intracellular Ca 2+ concentration ([Ca 2+ ] i ) and hyperplasia in USMC. The oxytocin receptor antagonists, atosiban and L‐371257, potently and concentration‐dependently inhibited oxytocin‐induced [Ca 2+ ] i increase and hyperplasia. In contrast, the V 1A receptor selective antagonist, SR 49059, and the V 2 receptor selective antagonist, SR 121463A, did not potently inhibit oxytocin‐induced [Ca 2+ ] i increase and hyperplasia. The potency order of antagonists in inhibiting oxytocin‐induced [Ca 2+ ] i increase and hyperplasia was similar to that observed in radioligand binding assays. In conclusion, these data provide evidence that the high‐affinity [ 3 H]‐oxytocin binding site found in human USMC is a functional oxytocin receptor coupled to [Ca 2+ ] i increase and cell growth. Thus human USMC may prove to be a valuable tool in further investigation of the physiologic and pathophysiologic roles of oxytocin in the uterus.British Journal of Pharmacology (2000) 129 , 131–139; doi: 10.1038/sj.bjp.0702996