Influence of serum and its lipoprotein fractions on progesterone synthesis and secretion by bovine luteal tissue in vitro

To study the ability of serum to affect progesterone synthesis and its subsequent secretion into the incubation medium, slices of bovine luteal tissue were incubated in the presence or absence of luteinizing hormone (LH) with various types and components of sera. When serum-free incubation medium was used, -30% of the progesterone produced was found in the incubation medium. Addition of LH to this serum-free medium greatly increased progesterone production, but did not increase the fraction of progesterone secreted into the incubation medium. Addition of 10% calf serum, heat-inactivated calf serum, dialyzed calf serum, fetal calf serum, or charcoal-treated serum to the medium increased the fraction of progesterone secreted into the incubation medium to -50%. Addition of LH to the serum-containing medium resulted in a highly significant increase in progesterone production, but the portion of the progesterone found in the incubation medium was no different from that found with serum alone. Removal of the lipoproteins (low and high density) from the serum did not alter the ability of the serum to increase the secretion. The increased secretion caused by the serum could be partially duplicated by bovine serum albumin (BSA). Addition of BSA to the incubation medium at levels equivalent to that found in 10% bovine serum increased the progesterone secreted from 30% to 42%, but could not duplicate the 50% secretion level achieved by the addition of the 10% serum preparations. These findings indicate that the ability of bovine tissue to secrete progesterone is increased in the presence of small amounts of serum, that this increased secretion is independent of progesterone synthesis, and that the increased secretion is not attributable to the lipoproteins in the serum. The data support the hypothesis that serum steroid-binding proteins facilitate the transport of steroid hormones from their site of synthesis into the blood stream.