CHARACTERIZATION OF GROWTH HORMONE-RELEASING HORMONE (GHRH) BINDING TO CLONED PORCINE GHRH RECEPTOR

To study structure-activity relationships of growth hormone-releasing hormone (GHRH), a competitive binding assay was developed using cloned porcine adenopituitary GHRH receptors expressed in human kidney 293 cells. Specific binding of [His(1),I-125- Tyr(10),Nle(27)]hGHRH(1-32)-NH2 increased linearly with protein concentration (10-45 mu g protein/tube). Binding reached equilibrium after 90 min at 30 degrees C and remained constant for at least 240 min. Binding was reversible to one class of high-affinity sites (K-d = 1.04 +/- 0.19 nM, B-max = 3.9 +/- 0.53 pmol/mg protein). Binding was selective with a rank order of affinity (IC50) for porcine GHRH (2.8 +/- 0.51 nM), rat GHRH (3.1 +/- 0.69 nM), [N-Ac-Tyr(1),D-Arg(2)]hGHRH(3-29)-NH2 (3.9 +/- 0.58 nM), and [D-Thr(7)]GHRH(1-29)-NH? (189.7 +/- 14.3 nM), consistent with their binding to a GHRH receptor. Nonhydrolyzable guanine nucleotides inhibited binding. These data describe a selective and reliable method for a competitive GHRH binding assay that for the first time utilizes rapid filtration to terminate the binding assay.