Guanine nucleotides modulate TRH-receptor binding in sheep anterior pituitary

Binding of thyrotropin-releasing hormone (TRH, thyroliberin) to receptors in chilled membrane resuspensions from sheep anterior pituitary glands is reduced in affinity almost 2-fold by micromolar concentrations of guanosine 5'-triphosphate (GTP) and the hydrolysis-resistant analog guanyl-5'-yl imidodiphosphate (GppNHp) added to tissue during a 10-min preincubation at 37 degree C. Guanosine 5'-diphosphate (GDP) produced a similar effect at 1 mM, while GMP, ATP, ADP and AMP appeared relatively inactive. The number of TRH-binding sites was not affected by the nucleotide treatments. These results are consistent with reports of guanine nucleotide effects on other receptor types and with evidence suggesting an adenylate cyclase mechanism for some of TRH's effects in the anterior pituitary.