Since recombinant hormones are considered as safer and more reliable in their bioactivity than extractive hormones, the recently available human recombinant luteinizing hormone (r-hLH), will probably replace hCG in the near future, for clinical purposes. This prompted us to investigate whether or not, and by which mechanisms, r-hLH can induce a desensitization of signal transduction and/or an up-regulation of steroidogenic capacity in Leydig cells. The effects of a 30 min to 24 h exposure to r-hLH (10(-9) M) on the differentiated functions of cultured immature porcine Leydig cells were studied by measuring the following parameters: LH/hCG receptor number and mRNA, hCG-, cholera toxin- and forskolin-induced cAMP production, G protein alphas subunit content of the membrane, hCG-, cholera toxin-, forskolin-, 8Br-cAMP-, 22R-OH-cholesterol-, progesterone-, 170H-progesterone-, DHEA-, delta4-androstenedione-induced testosterone secretion and StAR, 3beta-HSD, cytochrome P-450scc and P-450c17 mRNAs. hCG binding sites and LH/hCG receptor mRNA were slowly down regulated by r-hLH, reaching 47+/-1 and 18+/-7% of control at 24 h, respectively. Down-regulation of both hCG- and cholera toxin-induced cAMP production occurred earlier and was more marked, and at 24 h represented only 2.7+/-0.5 and 12.5+/-3.6% of control. Due to the synergistic effect of r-hLH and forskolin on cAMP production, the forskolin-induced cAMP was higher in r-hLH treated than in control cells, but this response also declines with time and was, at 24 h, only 32% of that observed at 30 min. This decreased cAMP production was associated with a less marked decline in the amount of membrane content of Galphas protein. The testosterone production in response to hCG, cholera toxin, forskolin and 8Br-cAMP declined to reach a nadir at 6 h but increased thereafter and at 24 h was significantly higher than in control cells. In contrast, the conversion of several precursors into testosterone remained stable or increased slightly during the first hours of r-hLH treatment and significantly increased at 24 h and this was associated with an increase of StAR, 3beta-HSD, P-450scc and P-450c17 mRNAs. Taken together, the present results indicate that, despite the marked down-regulation of transmembrane signaling, r-hLH increased the steroidogenic capacity of Leydig cells by increasing the expression of several genes encoding the proteins involved in testosterone synthesis.