Effect of thyrotropin releasing hormone and some of its histidine analogs on the cardiovascular system and prolactin release in the conscious rat.

The cardiovascular and endocrine activity of three analogs of thyrotropin releasing hormone (TRH), 4-nitro-imidazole TRH (4-nitro-TRH), 2-trifluoro-methyl-imidazole TRH (2-TFM-TRH) and 4-trifluoro-methyl-imidazole TRH (4-TFM-TRH), was compared to TRH in conscious rats. Injection of TRH or the three analogs (1 mg/kg or 5 mg/kg) into the arterial line induced increases in mean arterial pressure, pulse pressure and heart rate and raised plasma prolactin (PRL). None of the analogs were more potent than TRH in inducing cardiovascular changes. The 4-TFM-TRH was significantly less potent than the 2-TFM-TRH in increasing blood pressure, while the nitro-TRH was more potent than the 2-TFM-TRH in producing tachycardia. TRH induced a two-fold increase in PRL at the 5 mg/kg dose, while both the fluorinated analogs elicited a 4 to 5 fold increase in PRL at the higher dose. The present results suggest that the receptors for TRH-elicited PRL release differ from TRH-receptors involved in its cardiovascular actions.