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PRL and GH biosynthesis was monitored in sheep in response to steroid treatment. Ovariectomized ewes were treated for 10 days with one of the following: oil vehicle, progesterone, estradiol cyclopropionate, or progesterone and estradiol in combination. The incorporation of radioactive leucine into PRL and GH was measured by electrophoresis on sodium dodecyl sulfate-polyacrylamide gels and immunoprecipitation. PRL production increased from 26% of total protein synthesis to 39% in estrogen-treated animals (an increase to 150% of control values), while GH synthesis was maintained at 5% of the total protein. Progesterone alone had no significant effect on either peptide and did not interfere with the estrogen-induced increase in PRL synthesis. To examine effects at the messenger RNA (mRNA) level, RNA was isolated from pituitaries and translated in the wheat germ system with radioactive leucine. Both immunoprecipitated PRL and GH cell-free products were larger than their respective standards on sodium dodecyl sulfate-polyacrylamide gels. Competition experiments with purified hormone standards and double label tryptic digests show that pre-PRL and pre-GH contain the amino acid sequences of the hormone standards. With steroid treatment, mRNA levels for pre-GH and pre- PRL paralleled protein biosynthesis with respect to both direction and magnitude of the response, resulting in increases in translatable pre-PRL mRNA to 170% of control levels with estrogen administration. The magnitude of the PRL response to estrogen in this species (70%) is much lower than that reported in the rat (300%), suggesting less stringent estrogen control for PRL production in the sheep
Shupnik et al. (Thu,) studied this question.