Regulation of Cortisol Uptake in Mammary Tissue of Cows

Mammary tissue explants from four nonlactating, nonpregnant cows were placed into culture with media containing various combinations of insulin, prolactin, growth hormone, 17β-estradiol, dexamethasone, and progesterone. Combinations of insulin, prolactin, growth hormone, or 17β-estradiol had no effect on cytoplasmic or nuclear uptake of tritiated cortisol compared with values at zero time. Combinations containing dexamethasone or progesterone reduced cytoplasmic and nuclear uptake of tritiated cortisol. To examine inhibition by progesterone of binding of tritiated cortisol, mammary tissue from each of four lactating, nonpregnant and four nonlactating, nonpregnant cows were placed in flasks containing tissue culture medium 199, tritiated cortisol (2 ng/ml), and progesterone at concentrations of 0, 10⁻¹², 10⁻¹¹, 10⁻¹⁰, 10⁻⁹, 10⁻⁸, 10⁻⁷, 10⁻⁶, or 10⁻⁵ × 6.4 M. Cytoplasmic uptake of tritiated cortisol into nonlactating tissue decreased linearly as progesterone increased, whereas tritiated cortisol uptake in lactating tissue did not decrease until progesterone exceeded 10⁻⁷ M. We postulated progesterone is sequestered in milk fat of cytoplasm of lactating tissue whereas in nonlactating tissue progesterone is available to compete with cortisol at sites of cortisol binding.