Bovine Prolactin, Growth Hormone, Thyroxine and Corticoid Response to Thyrotropin-Releasing Hormone12

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Abstract
Preliminary experiments utilizing lactating cows and dairy bulls demonstrated release of prolactin by a single iv injection of 100 Mg synthetic thyrotropin—releasing hormone (TRH). Thereafter, a second experiment described more completely the endocrine changes occurring in the bovine after TRH. Twenty—five, 50 or 100 μg synthetic TRH was administered via jugular cannulae to each of four lactating cows. Serum prolactin immediately before TRH averaged 3.4 ng/ml. Prolactin averaged 21.1, 9.2 and 23.9 ng/ml at 2 min after 25, 50 or 100 μg TRH, respectively. Comparable values at the prolactin peak were 68.0, 23.4 and 60.6 and intervals to the prolactin peak were 10, 8 and 8 min, respectively. Serum prolactin increased in each cow after each dose of TRH, and returned to baseline within 2 hr. Serum thyroxine (μg/100 ml) increased linearly (p < 0.01) for 5 hr, but the increase was not linearly related to dose of TRH. In contrast, growth hormone increased proportionally to the dose of TRH. Total serum glucocorticoids increased after TRH in some cows, but a consistent pattern was not established. Prolactin release from steer pituitary explants cultured in vitro was unchanged with 0, 10 or 100 ng TRH/ml medium 199. Thus, while TRH increases bovine serum prolactin, growth hormone and thyroxine, failure of TRH to release prolactin in vitro does not support the view that TRH is the prolactinreleasing factor. (Endocrinology92: 471, 1973)

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