Identification and characterization of two receptors for SchistoFLRFamide on locust oviduct

An in vitro binding assay, using [125I][Y1]SchistoFLRFamide (YDVDHVFLRFamide), an iodinated analogue of SchistoFLRFamide, was developed to demonstrate and characterize putative receptors for SchistoFLRFamide associated with the locust oviduct. Two receptors were revealed by the binding assay. The high-affinity receptor had a Kd of 9.52 +/- 1.15 x 10(-10) M and a Bmax of 14.5 +/- 1.2 fmol/mg membrane protein; the low-affinity receptor had a Kd of 1.86 +/- 0.16 x 10(-7) M and a Bmax of 540 +/- 43 fmol/mg membrane protein. Binding to both receptors was saturable, specific, and reversible, and competitively inhibited by [Y1]SchistoFLRFamide, SchistoFLRFamide, and ADVGHVFLRFamide. Binding was Ca2+ or Mg2+ dependent and was completely inhibited by 2 mM EDTA. The receptors showed a regional distribution, with the majority of the high-affinity receptors associated with the upper lateral oviducts, which receive little or no innervation, and most of the low-affinity receptors associated with the lower lateral and common oviducts, which receive extensive innervation.