Dual internalization pathway for lutropin and choriogonadotropin in porcine Leydig cells in primary culture

Abstract The role of the high affinity receptor in the internalization of porcine lutropin (pLH) and human choriogonadotropin (hCG) by porcine Leydig cells in primary culture during short-term stimulation by the two hormones was investigated. The fate of the hormones was followed either by electron microscopy (with colloidal gold-labeled hormones) or by measurement of the cellular distribution of [ 125 I]pLH and [ 125 I]hCG. With both techniques, the internalization of pLH was found to be one order of magnitude greater than hCG, though the recycling rate of the high affinity receptors was the same with both hormones. However, when the cell surface was progressively depleted of its high affinity receptors by preincubation with increasing doses of hCG or pLH, the internalization of [ 125 I]pLH remained high and largely independent of the number of high affinity receptors still available on the cell surface, while that of [ 125 I]hCG was found to be proportional to this number. The endocytosis of [ 125 I]pLH could only be inhibited by the simultaneous presence of micromolar concentrations of unlabeled pLH, hCG or α or β subunits of ovine LH (oLH). The intact α-hCG subunit and the deglycosylated α-oLH subunit were less potent, while β-hCG and deglycosylated β-oLH had no significant effect. These results could be explained by the existence of a “carrier” or “scavenger” receptor for LH, but with a low affinity ( ≌3·10 6 M −1 ) and present in excess on the cell surface as compared to the high affinity receptor. The possible physiological significance of this receptor is discussed.