Modulation of human myometrial PGE2 receptor by GTP characterization of receptor subtype

We studied PGE2 specific binding sites in human myometrial microsomes prepared from uterine specimens obtained by hysterectomy (women between 38 and 55 years of age). Competition experiments showed that the potency order for various prostaglandins (PGs) was: PGE2 > or = PGE1 >> PGF2 alpha > Iloprost > or = Carbacyclin >> ZK 110841 (PDG2 analogue). These relative affinities indicated that the receptor was of the EP type. In kinetic experiments GTP, GppNHp and GTP gamma S increased the rate of PGE2 binding (steady state was reached more rapidly in the presence of nucleotides) but maximal specific binding was not significantly different. Complete dissociation could not be obtained, even in the presence of GTP. Only 50% of maximal binding was readily dissociable. The dissociation rate was 4.56.10(-4) sec-1 (half time of about 660 sec) and in the presence of GTP analogues it was slightly increased (k-1 = 7.16 10(-4) sec-1, half time 420 sec.). Scatchard analysis of saturation curves showed an increase in ligand receptor affinity in the presence of GTP or nucleotide analogues: the Kd shifted from 9.66 +/- 2.8.10(-9) M to 4.96 +/- 1.25.10(-9) M, but the number of binding sites did not change significantly (310 +/- 37 to 350 +/- 17 fmol/mgP). The effect of GTP was observed at a concentration of 5.10(-4)M. GppNHp and GTP gamma S were effective at 1.10(-5) M. Pretreatment of myometrial membranes with pertussis or cholera toxins had no effect on PGE2 binding to membrane sites. Our conclusion is that GTP induced conversion of a population of low affinity sites into a population of higher affinity sites. This effect of guanine nucleotides was described in adipocytes and kidney medulla. Competition studies with PGE2 analogues (sulprostone, 17-phenyl-omega-trinor PGE2, M&B 28,767, misoprostol, butaprost) showed that this receptor mediates a contractile response and is probably an EP3 subtype.