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© 2016 Elsevier Inc. Ovariectomized pony mares were treated with vehicle (controls), estradiol-17β (estradiol), or dihydrotestosterone (DHT) daily for 21 days in summer, and the pituitary content of prolactin and growth hormone (GH) and the numbers of cells staining immunocytochemically for prolactin (lactotropes), GH (somatotropes), or both hormones (mammosomatotropes) were assessed. Treatments did not alter (P > .1) plasma prolactin concentrations over the 21 days. Estradiol treatment increased (P < .05) the pituitary content of prolactin several-fold relative to controls and DHT-treated ponies. The number of lactotropes was also increased (P < .05), as was the number of mammosomatotropes (P < .05). Treatment with DHT did not alter (P > .1) any prolactin characteristic. The number of somatotropes was not altered by either treatment (P > .1) nor was the pituitary content of GH (P > .1). Given that the mass of prolactin accumulation in the pituitary in response to estradiol was several-fold greater than the approximate doubling of the number of lactotropes and mammosomatotropes, it appears that estradiol administration to ovariectomized pony mares stimulates not only the number of prolactin-producing cells but also the amount of prolactin production per cell.

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Journal of Equine Veterinary Science

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