Studies on cytosol progestin-binding components in the utero-placental unit of the pregnant hamster

Abstract

Two progestin-binding components in the cytosol fraction of hamster utero-placental unit at day 6 of pregnancy have been studied by dextran-chareoal absorption methods. Component A exhibits high affinity ( K A = 0.7 ± 0.2 × 10 9M −1 for progesterone and 0.5 ± 0.1 × 10 9M −1 for promegestone, a highly potent synthetic progestogen), is thermolabile and sensitive to sulphydryl-blocking reagent; its hormone specificity, as measured by relative binding affinity (RBA) is: promegestone and progesterone chlormadinone and norethisterone testosterone estradiol dexamethasone and cortisol. Injection of estradiol benzoate (2 mg/kg, s.c., 24 h before sacrifice) increases the amount of binding sites, whereas a reduction of the latter is observed following an injection of progesterone (4 mg/animal, s.c., 24 h before sacrifice). Component B has lower affinity ( K A 2.5 × 10 7 M −1 for progesterone and 4.0 × 10 7 M −1 for promegestone respectively), is heat-insensitive and susceptible to cortisol competition; “ in vivo” pretreatment with cortisol (10 mg/kg, s.c., 24 h before sacrifice) reduces the amount of binding sites whereas estradiol benzoate and progesterone are ineffective. It is concluded that hamster utero-placental unit contains a progestin-binding molecule, component A, with many of the attributes of a true receptor.