Influence of cyclic nucleotides on receptor binding, immunological activity, and microheterogeneity of human choriogonadotropin synthesized in placental tissue culture.

The regulation of the biosynthesis of choriogonadotropin (hCG) in tissue culture by human first trimester placenta in the presence of the following cyclic nucleotides and 3-isobutyl-1-methylxanthine (IBMX) was studied (concentrations in parentheses): IBMX (0.1 mM), cAMP (1 mM) + IBMX (0.1 mM), cGMP (0.1 mM) + IBMX (0.1 mM), 8-bromo-cAMP (0.5 mM) and 8-bromo-cGMP (0.5 mM). The medium concentration of hCG follows an optimum curve at all conditions, showing highest values at day 3 of the culture. The efficacy of the substances to cause an increase in the hCG medium concentration was in the following order: IBMX less than control less than 8-bromo-cGMP = 8-bromo-cAMP less than cGMP + IBMX less than cAMP + IBMX. The synthesized hCG was examined with respect to its receptor binding activity (LH/hCG receptor of rat testes), the activities to stimulate adenylate cyclase as well as testosterone biosynthesis in purified mouse Leydig cells, the immunological activity, and the microheterogeneity in isoelectric focusing. Only in the presence of 8-bromo-cAMP, 8-bromo-cGMP, and cGMP + IBMX was hCG synthesized, which differs significantly in the investigated properties from hCG of the control cultures. Only in the presence of 8-bromo-cGMP is the ratio of receptor binding activity/immunological activity optimal (near 1). In the presence of both 8-bromo-cAMP and 8-bromo-cGMP, microheterogeneity of hCG in isoelectric focusing was diminished and the synthesis of more acidic hCG subpopulations was favoured.