Differential Ability of Thyrotropin-Releasing Hormone andN3im-Methyl-Thyrotropin-Releasing Hormone to Affect Prolactin and Thyrotropin Production in Primary Rat Pituitary Cell Cultures*

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Abstract
The effects of TRH and a methylated analog [pyroglutamyl-N3im-methyl-histidyl-proline-amide (MeTRH)] on the release of PRL and TSH were studied in dispersed primary cell cultures of rat pituitary glands. Both short term effects (occurring within 2–6 h) and long term effects (occurring within 3 days) were examined. Dose-response curves for TRH and MeTRH were indistinguishable for the short term release of PRL, with half-maximal values ranging from 2–5 nm for both compounds in different experiments. However, MeTRH was more active than TRH in releasing TSH; MeTRH released TSH with a half-maximal concentration of < 0.5 nm, whereas with TRH, half-maximal stimulation occurred at 3 nm. In long term studies, TRH increased both TSH and PRL accumulation in the medium, with half-maximal activity at 5 nm. Under these conditions, MeTRH was more active with both hormones; a halfmaximal TSH increase occurred at 0.4 nm, and a half-maximal PRL increase occurred at about 0.8 nm. Incubation with TRH for 3 days increased [3H]leucine incorporation into PRL to 140% of control values, while incubation with TRH for 2 h had effect. Incorporation of [3H]leucine into total protein was not affected at 2 h or 3 days. This indicated the short term effect was on the release of PRL alone, while the long term effect included increased PRL synthesis. Alteration of the structure TRH by methylation at the third nitrogen of the histidine increased activity of the peptide with respect to short term effects on TSH but not PRL; long term activity was increased by methylation for both PRL and TSH. Therefore, the TRHreceptor interaction required for the stimulation of PRL release is not the same as that required for stimulation of TSH release or PRL synthesis. (Endocrinology 106: 107, 1980)

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