Abstract
Abstract
Mammary tissue cytosols (105,000g supernatants) were prepared from lactating or from nonlactating, nonpregnant cows, then incubated with [3H]dexamethasone alone (3 × 10-8 M) or in combination with various unlabeled hormones (1.9 × 10-7 or 1.9 × 10-6 M). Cortisol, corticosterone, and progesterone inhibited binding of [3H]dexamethasone, whereas testosterone, cortisone, and estradiol-17 β did not. Binding affinities of mammary cytosols from lactating and nonlactating cows for [3H]dexamethasone (0.5 to 100 nM) were virtually identical averaging 1.4 (± 0.3) and 2.0 (± 1.2) × 10-9 M, respectively. However, binding capacities of mammary tissue slices (fmole dexamethasone/μg DNA) were greater in lactating (2.3) than in nonlactating (1.5) cows. Specific cytosolic binding sites sedimented at 8 and 4 S on sucrose gradients. Whole mammary slices were incubated with 3 × 10-9 M [3H]dexamethasone alone or in combination with 10-13 to 10-5 × 3.8 M progesterone or 3 × 10-7 M dexamethasone, and cytoplasmic and nuclear fractions (1000g supernatants and pellets, respectively) were subsequently isolated. Specific cytoplasmic binding of [3H]dexamethasone in these slices was competitively reduced at concentrations in excess of 10-8 M progesterone in nonlactating tissue and at concentrations in excess of 10-7 M in lactating tissue. In contrast, when isolated cytosols were incubated directly, concentrations of progesterone needed to inhibit glucocorticoid binding were identical regardless of the physiological state of the mammary tissue. These data support the hypothesis that progesterone may be sequestered in milk fat and unavailable for competitive binding at the glucocorticoid binding site in lactating tissue but not in nonlactating tissue or in cytosol fractions which are relatively free of milk fat.
Get full access to this article
View all access options for this article.