No direct correlation between binding of sex hormones and calcium pump activity by rat liver microsomal preparations

In the previous paper (J. steroid Biochem. 16 (1982) 437-446. [5]), we demonstrated that in vitro liver microsomal preparations of adult male rats possessed binding sites specific for progesterone (Prog) of high affinity (KD approximately 25.2 nM) and high capacity (*Nmax approximately 6.43pmol/mg of microsomal protein), using 1.30 mM NaCl-based incubation buffer. To explore the biological roles of liver microsomal Prog binding, we investigated the effects of such binding on liver microsomal Ca2+ pump activity. Firstly, we obtained results similar to those previously obtained concerning the characteristics of microsomal Prog binding using 100 mM KCl-based incubation buffer, usually used for experiments on microsomal Ca2+ pump activity. For microsomal 45Ca2+ uptake we also obtained results similar to those already demonstrated be several investigators. That is to say liver microsomal 45Ca2+ uptake was markedly increased by the addition of 30 mM oxalate and 5 mM ATP, and was not inhibited by the addition of 5 mM NaN3 into the incubation buffer. However, the addition of 1.0 microM Prog, as well as 17 beta-hydroxy-5 alpha-androstan-3-one (5 alpha-DHT) and estradiol-17 beta (E2 --17 beta), which should be a sufficiently saturable concentration for liver microsomal binding capacity for Prog was 5-10 times higher than that for 5 alpha-DHT and E2-17 beta. In addition, Prog (1.0 microM) had little effect on 45Ca2+ release from prelabeled microsomes. In conclusion, we suggest, therefore, that there is no direct correlation between binding of sex hormones and Ca2+ pump activity by rat liver microsomal preparations.