The effect of catecholamines on thyrotropin releasing hormone (TRH) induced prolactin and growth hormone (GH) release from sheep pituitary fragments was investigated in vitro. The addition of 8 ng/ml of TRH into the incubation medium caused a significant increase in prolactin and GH release. Forty ng/ml or larger doses of TRH induced greater release of prolactin, but not of GH. Addition of 100 ng/ml of norepinephrine HCl or dopamine HCl into the incubation medium significantly suppressed the basal prolactin release, but not GH release. TRH failed to stimulate either prolactin or GH release in the presence of 100 ng/ml of norepinephrine or dopamine, but the levels of prolactin and GH in the medium, after the combined addition of TRH and one of the catecholamines, appeared to be slightly higher than those observed for norepinephrine or dopamine alone. However, 1 μg/ml of dopamine completely prevented the increase in TRH induced prolactin and GH release, the levels of these two hormones failing to rise above the dopamine control. These results indicate that TRH stimulates the release of prolactin and GH by a direct action on the sheep pituitaries, and that catecholamines abolish this response.
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