Hormonal Regulation of Type I Insulin-Like Growth Factor Receptors of Leydig Cells in Hypophysectomized Rats*

The effects of hCG and various pituitary hormones on type I insulin-like growth factor (IGF) receptors of purified Leydig cells of hypophysectomized rats were studied. The number of type I IGF receptors of Leydig cells obtained from hypophysectomized rats (18.0 +/- 1.5 fmol/10(6) cells) was lower than that in normal rats (54.6 +/- 5.3 fmol/10(6) cells; P less than 0.05). After a single administration of hCG (10 U, ip), specific binding of [125I]IGF-I to purified Leydig cells increased 3-fold. Scatchard analyses of the binding data suggested that increased binding was the result of an increase in receptor number, whereas binding affinity remained unaltered. Type I IGF receptor increased within 12 h and remained persistently elevated 96 h after hCG treatment. Administration of hCG (10 U, ip) daily for 5 days increased type I IGF receptor levels to 73.2 +/- 8 fmol/10(6) cells (P less than 0.001). FSH caused a small but significant increase in type I IGF receptors. Concomitant administration of FSH and hCG further enhanced IGF-I-binding capacity. IGF-I-binding affinity of Leydig cells treated with FSH or FSH plus LH was not significantly different from that in the control hypophysectomized rats. Daily administration of GH for 5 days also upregulated type I IGF receptors, whereas PRL had no effect. FSH, GH, and PRL administration had no effect on serum testosterone levels. Serum testosterone levels increased to 3.99 +/- 0.35 ng/ml after 5 days of treatment with hCG. Concomitant administration of FSH and hCG caused a further increased in serum testosterone levels (6.13 +/- 0.46 ng/ml; P less than 0.01). The present study suggests that type I IGF receptors of Leydig cells can be up-regulated by LH, FSH, and GH. However, hCG/LH seems to be the most important factor in maintaining and regulating type I IGF receptors of Leydig cells. Steroidogenic and growth-promoting effects of hCG and pituitary hormones on Leydig cells may be mediated by increased type I IGF receptors.