Specificity and Mechanism of Biphasic Action of Glucocorticoids onα-Lactalbumin Production by Rat Mammary Gland Explants*

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Abstract
RU26988 [11.beta.,17.beta.-dihydroxy-17.alpha.-(1-propynyl)androsta-1,4,6-trien-3-one], a specific type II (glucocorticoid) ligand with negligible affinity for type I (mineralocorticoid) receptors, increased .alpha.-lactalbumin production in mammary gland explants from midpregnant rats when cultured in vitro in the presence of insulin (5 .mu.g/ml) and rat PRL (1 .mu.g/ml). The dose-response curve was biphasic, over a 3-300 nM dose range, with maximum at 3-10 nM, followed by a progressive decline toward or to control levels at 300 nM. A similar dose-response curve was obtained with another type II-specific ligand, RU28362 [11.beta.,17.beta.-dihydroxy-6-methyl-17.alpha.-(1-propynyl)androsta-1,4,-6-trien-3-one], and with dexamethasone. This biphasic response had been previously reported for mouse mammary gland explants using cortisol, which binds to both type I and type II receptors. Our experiments, therefore, show that occupancy of type II receptors alone can be responsible for the stimulation at low steroid concentrations, followed by the decrease from peak levels observed at higher steroid concentrations. On the basis of these data, we propose a model to account for these findings, based on ligand binding to a single class of receptors and the putative existence of both turn-on and turn-off glucocorticoid regulatory elements in the nucleus.

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