DISTINCT EFFICACIES FOR 2 ENDOGENOUS LIGANDS ON A SINGLE COGNATE GONADOLIBERIN RECEPTOR

A cDNA encoding a putative gonadoliberin receptor was cloned from the pituitary of the African catfish, Conceptual translation predicts a pr otein of 379 amino acids which shows typical characteristics of GTP-bi nding-protein-coupled receptors. The isolated cDNA was stable expresse d in human embryonic kidney (HEK) 293 cells which were used for studie s on gonadoliberin-activated second messenger systems (inositol phosph ate production; increase in cAMP and/or intracellular Ca2+). The isola ted cDNA encoded a functional receptor, designated catfish gonadoliber in receptor (cfGnRH-R), which had an amino acid sequence similarity of 38% with mammalian gonadoliberin receptors. In contrast to its mammal ian counterparts which lack an intracellular carboxy-terminal domain, the cfGnRH-R contains an additional 49 amino acid residues, From the t wo endogenous gonadoliberins in African catfish, chicken gonadoliberin -II had a several hundredfold higher potency than catfish gonadoliberi n to activate cfGnRH-R-associated second messenger systems in transfec ted HEK 293 cells. This is in line with the previously deter mined hig her gonadotropin-release capacity of chicken gonadoliberin-II in catfi sh. Stimulation of second messenger systems with chicken gonadoliberin -II, but not with catfish gonadoliberin, resulted in a biphasic effect and chicken gonadoliberin-II led to a higher maximum stimulation than catfish gonadoliberin. Challenging cfGnRH-R simultaneously with chick en gonadoliberin-II and catfish gonadoliberin did not lead to additive effects. in contrast, two types of mutual inhibitory effects were rec orded, These data indicate that a single cognate cfGnRH-R couples with distinct efficacies to signal transduction systems upon stimulation b y the two endogenous gonadoliberins which, in addition, may interact n egatively.