Demonstration of specific binding of prolactin by porcine corpora lutea.

Subcellular fractions from porcine corpora lutea of the reproductive cycle and pregnancy were shown to have specific binding sites for ovine prolactin (oPRL). Aside from oPRL, only ovine and bovine growth hormone preparations competed with [125I]iodo-oPRL for its binding site. These cross reactions were at a level consistent with the prolactin contamination of these preparations. Rat growth hormone, FSH, LH, TSH, insulin, and ACTH exhibited negligible cross-reactivity. Both corpora hemorrhagica and albicantia had lower specific binding of [125I]iodoPRL than did active corpora lutea of the reproductive cycle, while corpora lutea of pregnancy demonstrated a nearly 5-fold increase in specific binding compared with that of the cycle. Corpora lutea from animals with larger fetuses (greater gestational age) bound the most prolactin. Analysis of data from cold competition studies employing weighted non-linear least-square fitting to a three-parameter model, showed high-affinity binding of oPRL with an association constant (Ka, 23 C) of 2.0 X 10(9)M-1 for the binding site of the corpus luteum of the cycle. The Ka shows no appreciable change with pregnancy. In contrast, the binding site concentration (N) increases markedly from less than 10 fmol/mg protein in corpora lutea from non-pregnant animals to approximately 40 fmol/mg protein for animals at a gestational stage of 40-46 days. The observed Ka's are similar to values obtained for the prolactin binding site in porcine granulosa cells harvested from unruptured follicles and to the prolactin-binding site in the mammary gland.