The Pattern of Gonadotropin-Induced Change in Plasma Testosterone, Testicular Esterified Cholesterol and Leydig Cell Lipid Droplets in Immature Mice1

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Abstract
The response of plasma testosterone to a series of twice-daily injections of human chorionic gonadotropin (hCG) over a 4 day period was measure 2 h after the last hCG injection in 4-week-old male C57Bl/6 mice. The largest elevations of plasma testosterone (approximately 10 ng/ml average) occurred after the first injection. After the second and third injections (14 and 26 h of hCG treatment), plasma testosterone remained near or below control levels (1–2 ng/ml average). Significant elevation of plasma testosterone reappeared at 50 h of treatment (approximately 5 ng/ml average) and remained at this level when measured at 74 h. By 98 h, the response (approximately 8 ng/ml average) approached that seen after the first injection. Cytologic observations revealed drastic depletion of Leydig cell lipid droplets during the first 14 h of hCG treatment. Lipid droplets remained scarce at longer treatment intervals. The concentration of esterified cholesterol in the testes of these mice declined by 50% during the first 14 h of hCG treatment and remained at the reduced level through longer treatment intervals. The simultaneous depletion of Leydig cell lipid droplets and testicular esterified cholesterol coincided with the onset of the refractory phase during which elevation of plasma testosterone in response to injection of hCG failed to occur. Subsequent recovery of the ability to elevate plasma testosterone occurred without restoration of either lipid droplets or esterified cholesterol reserves and was accompanied by apparent hypertrophy of the smooth endoplasmic reticulum.