Luteinizing Hormone-Receptor Interaction in the Testis: Progressive Decrease in Reversibility of the Hormone-Receptor Complex

Abstract

The interaction of LH and other hormones with specific receptor sites is accompanied by a progressive change in the degree of reversibility of the hormone-receptor complex. This process was investigated by analysis of in vitro binding of [¹²⁵]hCG to 15,000 × g fractions of rat testis homogenate. During the first hour of incubation at 37 C, dilution and washing of the particles gave about 20% less specific binding than when particles were incubated with excess unlabeled hormone for a further 1 h before dilution and washing. This effect was no longer demonstrable after 4 h, suggesting that the hormone-receptor complex became less reversible to dilution with time. A decrease in reversibility was also noted when hormone-receptor complexes formed by preincubation with [¹²⁵I]hCG at 37 C were dissociated with MgCl₂ at successive intervals after binding. Thus, 90% of the complexes formed in the first 10 min could be dissociated by 1 M MgCl₂, whereas about 50% of the complexes was dissociable at 10–p20 min, and only 20% was dissociable at 120 min. In testis fractions equilibrated with 1 nM unlabeled hCG at 24 C, available LH receptors decreased to 30% over 16 h, and the proportion of occupied sites that could be dissociated with 2 M MgCl₂ also showed a serial decrease. The apparent loss of LH sites during prolonged incubation was attributable to occupancy by progressively less dissociable hormone. The absence of true LH receptor loss during exposure to gonadotropin in vitro at 24 C suggests that receptor regulation is a function of the intact, metabolically active Leydig cell. The progressive formation of LH-receptor complexes with increased resistance to dissociation was temperature dependent, being more rapid at 37 C, and was coincident with the early biological actions of the gonadotropins. The progression from loose to tight binding of gonadotropins in the testis may represent an initial step in the processing and degradation of the hormone-receptor complex.