The mode of action of LHRH agonists on the rat Leydig cell

The relationship between LHRH agonist-receptor interaction and subsequent stimulation of steroidogenesis has been investigated using dispersed adult rat Leydig cells. Binding of 125I-labelled LHRH agonist to these cells was rapid and was readily and completely reversible, which contrasted with the binding of 125I-labelled hCG which was never completely reversible and which became progressively less reversible with increase in time. Following 125I-LHRH agonist-receptor interaction for 5 min at 21 degrees C, over 80% of the bound hormone dissociated at a fast rate (t 1/2 1.5 min) and the remainder at a slower rate (t 1/2 20 min). Following LHRH agonist-receptor interaction for a longer period (45 min) at 21 degrees C, only the slower of these 2 components of dissociation was evident, although binding was still completely reversible. Evidence was also obtained that following preincubation of Leydig cells for 2 hr at 34 degrees C with a near-saturating concentration (2000 pg/ml) of unlabelled LHRH agonist, all or most of the bound hormone dissociated during subsequent incubation at 21 degrees C. Incubation of Leydig cells at 34 degrees C in the continuous presence of either a low dose (1 pM) of hCG or a high dose (2000 pg/ml) of LHRH agonist resulted in similar percentage stimulation of testosterone secretion with a similar time lag (2 h) before this increase was evident. Preincubation with this dose of hCG for as little as 15 or 30 min still led to increased testosterone secretion following removal of the free hormone. In contrast, preincubation with LHRH agonist for up to 2 h was without subsequent stimulatory effect, presumably due to dissociation of the bound hormone from its receptors. Therefore, for LHRH agonist to stimulate Leydig cell steroidogenesis in vitro, it must be present continuously in the incubation medium, and the implications of this with respect to the secretion and mode of action of 'testicular LHRH' in vivo are discussed.