Progesterone receptor of adult rabbit lung

Properties of progestin binding sites in adult male rabbit lung cytosol were analyzed using [3H] promegestone ([3H]R5020). At concentrations of 0.05-10 nM, [3H]R5020 bound to two saturable sites with differing affinities: a high affinity site (Kd = 0.34 nM, Bmax = 57 fmol/mg protein) and a moderate affinity site (Kd = 60 nM, Bmax = 540 fmol/mg protein). [3H]R5020 binding, under conditions selected to favor binding to the high affinity site, was reversible, protease-sensitive and inhibited in a concentration-dependent manner by steroids with the following order of potency: R5020 greater than norgestrel greater than norethindrone greater than progesterone greater than norethindrone acetate greater than norethynodrel greater than deoxycorticosterone greater than lynestrenol greater than quingestanol greater than testosterone greater than 17 beta-estradiol greater than cortisol. Upon sucrose density gradient ultracentrifugation analysis, a peak of [3H]R5020 binding activity was observed in the 6-7S region in the absence of KCl and in the 3-4S region in the presence of KCl. The progestin-binding activity of adult male rabbit lung cytosol thus possesses those characteristics conventionally accorded to a hormone receptor and suggests that lung tissue may be progesterone-responsive.