Progesterone-binding globulin and testosterone-binding activity in guinea pig serum during pregnancy: relationship to progesterone and oestrogens.

Progesterone levels in serum have been determined throughout pregnancy in guinea-pigs by a competitive protein-binding technique, using pregnant guinea pig plasma protein as binding agent. The concentrations of this protein (progesterone-binding globulin, PBG) as well as the testosterone-binding activity (TBA) have been quantitated by means of equilibrium dialysis. In order to correlate these parameters to the endogenous oestrogen production, the urinary oestrogen excretion was recorded. The concentration of progesterone, PBG and TBA showed a sharp rise on days 14-18 of gestation, reaching a maximal level on days 30-44. The progesterone level thereafter declined significantly towards parturition, followed by a sharp drop post partum. PBG and TBA followed a similar course, the decline towards parturition, however, being non significant. Neither progesterone, PBG nor TBA were significantly correlated to the urinary oestrogen excretion. The concentration of PGB was remarkably high, amounting to 1-2 x 10(-5) M during most of the guinea pig pregnancy. This binding capacity generally exceeded the endogenous progesterone concentration by a factor of 20, as calculated on a molar basis. A mean of 64% of the binding sites was available for testosterone binding, corresponding to a free binding capacity of 400 mug testosterone per 100 ml.