In Vivo Studies on the Mode of Action of LH

The mode of action of LH on testosterone biosynthesis in male rats and mice has been studied in vivo. In rat's experiments, testicular vein blood was collected and testosterone in the plasma was measured by fluorometry of Ohsawa et al (1969). After intravenous administration of LH testosterone output started to increase within three minutes and reached the maximum level in ten minutes. Thus, the effect of LH on testosterone secretion from rat's testis has been revealed to be quite rapid, like the effect of ACTH on adrenal cortex. The similar results have been obtained in mice. After intratesticular injection of LH, the content of testosterone in mouse testis started to increase within three minutes and reached the plateau in ten minutes.Cyclic-3′, 5′ AMP, proposed mediator of action of LH in the Leidig cell, has been tested in the same assay system as described above, concerning the capacity of stimulation of testosterone synthesis in the male animals. Administration of cyclic AMP caused a prompt and significant increase in testosterone production in testes of both specieses. Cyclic AMP armarentiv mimics the action of LH.Inhibitors of protein and nucleic acid synthesis have also been tested in the same system. Cytosine-arabinoside, an inhibitor of DNA synthesis, and actinomycin D, an inhibitor of m-RNA synthesis, have been shown to have no influence on the action of LH on testosterone synthesis. On the contrary, cycloheximide, a known inhibitor of protein synthesis, completely blocks the steroidgenic activity of LH as well as cyclic AMP. From these results, it is postulated that LH exerts its action on the testosterone biosynthesis through the formation of cyclic AMP, followed by rapid synthesis of unknown regulator protein.