Problems Associated with I-125 Oxytocin Binding to Membrane Receptors

Radioiodinated oxytocin prepared by the lactoperoxidase method exhibited a substantial biologic activity in uterotonic assay of the rat uterus. 125I-oxytocin was bound to the uterine membrane particulate fraction, but the unlabelled oxytocin did not inhibit the binding of 125I oxytocin to the membrane fraction of rat uterus. Cold iodinated oxytocin, however, inhibited the 125I-oxytocin binding to the membrane fraction of rat uterus in proportion to its concentration. These results suggest that 125I-oxytocin is not a suitable radioligand for oxytocin receptor binding study.