CYCLO(HIS-PRO) A METABOLITE OF THYROTROPIN-RELEASING-HORMONE - SPECIFIC BINDING TO RAT-LIVER MEMBRANES

[3H]cyclo(His-Pro) bound with high affinity (59 nM) to a single class of sites in rat liver plasma membranes, without significant tracer degradation during equilibration for 60 min at 0.degree. C. Binding was specific and saturable (3.9 pmol/mg protein), and were increased by the addition of K+, Mg++ and Na+ at optimal concentrations, but not of Ca++ at all concentrations tested. In vivo administration of cyclo(His-Pro), but not thyrotropin-releasing hormone, to rats caused the downregulation of cyclo(His-Pro)-binding sites with decreases in specific binding numbers but did not change binding activity.