1. Biphasic effect of gonadotropin-releasing hormone and its agonist analog (HOE766) on in vitro testosterone production by purified rat Leydig cells
J Y Browning, R D'Agata, A Steinberger, H E Grotjan Jr, E Steinberger Endocrinology. 1983 Sep;113(3):985-91. doi: 10.1210/endo-113-3-985.
GnRH and GnRH agonists have stimulatory and inhibitory effects on testicular testosterone secretion both in vivo and in vitro. To determine whether they are exerted directly on the Leydig cells and to explore the temporal relationships, we examined the effects of acute (3 h) and chronic (24-72 h) exposure of purified (greater than or equal to 80%) rat Leydig cells to GnRH and its agonist analog HOE766 (D-Ser-t-BU6,des-Gly-NH2 10LHRH ethylamide; Hoechst, Frankfurt, Germany) on testosterone production. GnRH and HOE766 enhanced basal testosterone secretion by freshly isolated or cultured Leydig cells. HOE766 was at least 100 times more potent than GnRH. However, exposure of Leydig cells to HOE766 for 24 h or longer lead to a significant reduction in hCG responsiveness without altering basal testosterone secretion. Both the stimulatory and inhibitory effects were dose related, with a maximal response elicited by 10(-9) M HOE766. HOE766 reduced Leydig cell sensitivity to hCG (ED50) stimulation, but did not alter the slope of the dose-response curves. Thus, GnRH and its agonist appear to have a dual and biphasic effect on the Leydig cells. Acute exposure stimulates basal testosterone secretion (and occasionally the hCG response), whereas chronic exposure decreases the response to hCG stimulation. These data provide additional evidence that GnRH has a direct effect on Leydig cell steroidogenesis.
2. Effects of gonadotropin-releasing hormone receptor blockade on rat testicular gonadotropin and lactogen receptors, steroidogenesis, and responses to human chorionic gonadotropin stimulation
I T Huhtaniemi, K J Catt Endocrinology. 1985 Jan;116(1):281-7. doi: 10.1210/endo-116-1-281.
Testicular endocrine regulation was studied in adult male rats after treatment with a potent GnRH antagonist analog [N-Ac-D-p-Cl-Phe1,2, D-Trp3, D-Lys6, D-Ala10-GnRH (Ant.)] given in doses of 1 mg/kg at 0, 12, and 24 h. One group of animals also received an injection of human CG (hCG) (600 IU/kg) at 12 h, and all animals were killed at 36 h for hormone and receptor (R) measurements. Treatment with Ant. blocked greater than 95% of the pituitary and testicular GnRH-R, and decreased serum LH concentration by greater than 90%. Testicular lactogen-R content was decreased by 60% (P less than 0.01), but there was no change in LH-R and FSH-R concentrations. Ant. decreased serum and testicular testosterone levels by 90%, and testicular capacity to produce testosterone in vitro by 50% (P less than 0.01). No decrease was observed in production rates of cAMP and progesterone. hCG alone abolished testicular LH-R, decreased lactogen-R by 55% (P less than 0.01), and GnRH-R by 65% (P less than 0.01). Desensitization of cAMP and testosterone production, and an increase in progesterone-testosterone ratio, were seen after hCG. hCG + Ant. treatment resulted in R, cAMP, and steroid responses that were indistinguishable from those seen after hCG alone. These findings indicate that: 1) Ant.-induced hypogonadotropism decreases testicular lactogen-R concentration and testosterone production; 2) testicular GnRH-R and lactogen-R are subject to heterologous down-regulation by hCG; and 3) inhibition of the putative GnRH-mediated regulation of testis by Ant. blockade of the GnRH-R does not change testicular response to hCG-treatment in vivo. Hence, the present observations still leave the physiological role of testicular GnRH-R open.
3. Action of inhibitory LH-RH analogs in rat pituitary and luteal cell cultures
J Spona, R Müntz, K Nicolics, J Seprödi, I Teplan Endocrinol Exp. 1984 Jun;18(2):101-7.
The effect of LH-RH antagonists on LH-RH stimulated LH release was studied in primary rat pituitary cell culture and the inhibition by LH-RH antagonists of HCG provoked progesterone production was investigated in primary rat luteal cell culture. Antagonists used in this study were Ac-D-Trp1, D-Phe(Cl)2, D-Lys6, D-Ala10-LH-RH(I1) and the respective D-Lys6 isophthaloyl dimer (I2). LH-RH activity was noted to be reduced to 1/12 by I1 and to 1/23 by I2 in the pituitary cell culture system. The LH-RH inhibitors did not possess intrinsic LH releasing activity up to 10(-6) mol 1(-1). Incubation of rat luteal cells with HCG in the presence of LH-RH, I1 or I2 resulted in a smaller progesterone release than that observed in the absence of the peptides. ED50 of HCG was noted to be 5.7 X 10(-13) mol 1(-1) and 10(-8) mol 1(-1) LH-RH, I1 or I2 caused a shift of ED50 to 6.3 X 10(-12) mol 1(-1), 1.2 X 10(-12) mol 1(-1) and 7.9 X 10(-13) mol 1(-1), respectively. The present investigation is the first demonstration of reduction by LH-RH antagonists of gonadal steroid production. The present results suggest the use of such inhibitory LH-RH analogs in the treatment of hormone dependent tumors such as prostatic carcinoma and would not cause a transient rise of gonadal steroids as seen by the use of LH-RH agonists.
