Differential ligand binding and agonist-induced regulation characteristics of the two rainbow trout GH receptors, Ghr1 and Ghr2, in transfected cells

Previously, we isolated and characterized two distinct GH receptor (GHR)-encoding mRNAs, ghr1 and ghr2, from rainbow trout. In this study, Chinese hamster ovary-K1 cells were individually transfected with plasmids that contained cDNAs encoding rainbow trout ghr1 or ghr2. High affinity binding of ¹²⁵I-salmonid GH (sGH) by the expressed receptors was saturable, displaceable, and ligand selective. Whole-cell binding analysis revealed a single class of binding site; for Ghr1 K_d=8 nM, for Ghr2 K _d=17 nM. While salmonid prolactin (sPrl) displaced ¹²⁵I-sGH from both Ghr1 and Ghr2, the affinity of either receptor subtype for sPrl was substantially less than for sGH; salmonid somatolactin, another member of the GH-PRL family, did not displace labeled sGH except at pharmacological concentrations. ¹²⁵I-sGH was internalized by Ghr1- and Ghr2-expressing cells in a time-dependent manner; the maximum internalization reached was 71% for Ghr1 and 55% for Ghr2. Long-term exposure (24 h) of transfected cells to sGH up-regulated surface expression of both Ghr1 and Ghr2; however, sGH induced surface expression of Ghr1 to a greater extent than that of Ghr2. These results indicate that rainbow trout ghrs display both overlapping and distinct characteristics that may be important for ligand selection and differential action in target organs.