Serum Prolactin and Growth Hormone Responses to Thyrotropin Releasing Hormone in Postpubertal Cattle

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Abstract
Serum prolactin increased 9-20 and growth hormone [GH] 2.5-4 times following administration of thyrotropin releasing hormone [TRH] in 6 heifers during an estrous cycle, in 26 heifers during pregnancy, and in 16 cows during lactation. Stage of the estrous cycle or pregnancy did not affect quantity of prolactin or GH released after administration of TRH. There was a tendency for the quantity of prolactin released by TRH to be greater at 2 mo. than at 4, 6, 8, or 10 mo. of lactation. During lactation, GH release after TRH was greatest at 2 mo. Prolactin release after TRH was 6-16 times greater in summer than in spring or fall in pregnant heifers and approximately 3 times greater in spring, summer and fall than during the winter in lactating cows. Season did not affect release of GH after TRH. Season of year more importantly affects capacity of the anterior pituitary to release prolactin than stages of the estrous cycle, pregnancy, or lactation. On the other hand, season does not affect induced release of GH by TRH. There is an increased capacity of the anterior pituitary to release GH during early lactation, coincident with maximal milk yield.