In vivo acute testicular testosterone response to injection of luteinizing hormone in the rat fetus.

The acute in vivo testosterone response to LH stimulation and its change during late fetal life were determined in the rat. In 18.5-day-old fetuses, testicular testosterone content was increased in a dose- and time-dependent manner after fetal subcutaneous LH injection. The maximum response was small: the testicular content and plasma concentration were increased by 200% and 250% over basal values respectively, while they were increased 1100% and 1200% in adult rats. Similarly, comparable low responses were obtained after subcutaneously injecting the fetuses with human chorionic gonadotropin (hCG) and after injecting LH into the vitelline vein. Between days 18.5 and 21.5 of fetal life, the testosterone levels in the testis and plasma of uninjected or PBS-injected fetuses decreased and were comparable in both groups. In maximally LH-stimulated fetuses, the testicular content did not change with age, and plasma concentration was lower on day 21.5 than on day 18.5. Since the number of Leydig cells increases 1.5 to 2-fold between days 18.5 and 21.5, these results show an age-related decrease in basal and maximally LH-stimulated in vivo testosterone secretions per Leydig cell during late fetal life.