Lectin-stimulated sheep and cow lymphocytes were used to test whether inhibitors of classical steroid receptors block suppressive effects of progesterone and whether effects of progesterone vary with physiological status. Neither RU 38486 nor RU 43044 blocked the inhibitory effects of progesterone on lymphocyte proliferation. Rather, these antagonists were themselves inhibitory. Effects of progesterone and antagonists were additive: the percentage inhibition caused by progesterone was similar whether antagonists were present or not. The degree of lymphocyte proliferation and the inhibitory effects of progesterone were of the same magnitude for pregnant/lactating cows, pregnant/non-lactating cows, postpartum/lactating cows and cyclic/non-lactating cows. In conclusion, progesterone does not appear to inhibit lymphocyte proliferation through actions that involve classical steroid receptors. There was no evidence that lymphocyte proliferation in the cow is suppressed during pregnancy or that the inhibitory effects of progesterone increase during pregnancy.
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