Acute responses of Leydig cells to hCG: evidence for early hypertrophy of Leydig cells.

The cross-sectional area of Leydig cells has been studied in rats following a single injection of 100 IU hCG. Leydig cell size was not changed within 2 h of injection (89.73 +/- 3.02 micron 2) but had increased significantly by 12 h (114.76 +/- 3.57 micron 2). Leydig cell size continued to increase until 24 h when a maximum cross-sectional area of 186.30 +/- 4.72 micron 2 was measured. This area was significantly greater than that recorded after 1 week of daily hCG treatment (143.18 +/- 6.25 micron 2, P less than 0.05). At 48 and 72 h Leydig cell size decreased slightly and was not significantly different from that seen after chronic hCG treatment. A single injection of 100 IU hCG induced a biphasic serum testosterone response with peaks in serum testosterone at 2 h and 72 h, the intervening nadir suggesting a period of in vivo refractoriness. A corresponding period of Leydig cell refractoriness in vitro was also demonstrated 12 h following a single injection of 100 IU hCG and persisted for 48 h. Further injections of hCG on day 2 or 3 did not induce a second biphasic serum testosterone pattern or the accompanying period of refractoriness, indicating that Leydig cell refractoriness is a temporary phenomenon which cannot be maintained during persistent stimulation with hCG. The absence of further refractory periods to hCG during recurrent stimulation may be related to the trophic action of the hormone on Leydig cells.