Suppression of plasma luteinizing hormone-releasing hormone by administration of human chorionic gonadotropin in castrated women

Suppression of plasma luteinizing hormone-releasing hormone by administration of human chorionic gonadotropin in castrated women

FERTILITY AND STERILITY Copyright c 1982 The American Fertility Society Vol. 38, No.2, August 1982 Printed in U.sA. Suppression of plasma luteinizin...

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FERTILITY AND STERILITY Copyright c 1982 The American Fertility Society

Vol. 38, No.2, August 1982 Printed in U.sA.

Suppression of plasma luteinizing hormone-releasing hormone by administration of human chorionic gonadotropin in castrated women

Akira Miyake, M.D.* Toshihiro Aono, M.D. Yasuhiro Kawamura, M.D. Keiichi Kurachi, M.D. Department of Obstetrics and Gynecology, Osaka University Medical School, Osaka, Japan

A gonadotropin short-feedback mechanism has been found in rats,! rabbits,2 and humans,3 but it is unknown whether the site of gonadotropin action is the hypothalamus or the pituitary. In this investigation the site of gonadotropin shortfeedback action was examined by observation of change in plasma luteinizing hormone-releasing hormone (LH-RH) following human chorionic gonadotropin (hCG) injection in castrated women. MATERIALS AND METHODS

Sixteen women 36 to 48 years of age who had undergone total hysterectomy and bilateral oophorectomy more than 2 weeks previously volunteered for this study. Consent was obtained from all of the volunteers. Six of 16 women were given 10,000 IU of hCG (Mochida Pharmaceutical Co., Tokyo, Japan) as a single intramuscular (1M) injection at 8:00 A.M., 5 women received 40,000 IU ofhCG in the same way, and 5 others received 1M injections of saline as a control. Heparinized blood samples were collected before and 30 minutes, 1 hour, 2 hours, and 4 hours after injections. Plasma LH-RH and hCG concentrations were deter-. mined by double-antibody radioimmunoassay (RIA) methods as reported previously.4 For LHRH assay, 1 ml of plasma was extracted with 3 ml Received February 2, 1982; revised and accepted May 3, 1982. *Reprint requests: Akira Miyake, M.D., Department of Obstetrics and Gynecology, Osaka University Medical School, Osaka 553, Japan. VOl. 38, No.

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August 1982

of methanol immediately after blood samples had been drawn, the supernatant was evaporated to dryness, and the residue was dissolved in 0.6 ml of 0.01 M phosphate-buffered saline containing 1% bovine serum albumin immediately before assay. By this RIA method, the minimum detectable dose of LH-RH was 0.6 pg/tube, the intraassay and interassay coefficients of variation were 13.8% and 15.3%, and the mean recovery was 85.6 ± 3.2% in the range of 5 to 100 pg/ml. Statistical analysis of data was performed with the use of the Student's t-test. RESULTS

The mean plasma hCG levels after injection of 10,000 IU and 40,000 IU ofhCG rapidly increased and reached 166.7 mlU/ml and 650 mIU/ml, respectively, 4 hours after the injection. The plasma immunoreactive hCG level after the injection of saline was less than 0.5 mlU/ml. There were significant differences in these hCG levels in the three groups 4 hours after the injections. The mean plasma levels of immunoreactive LH-RH before the injections were 10.2 pg/ml, 11.8 pg/ml, and 10.9 pg/ml, respectively; and these values were not significantly different. The changes of plasma levels of immunoreactive LH-RH after hCG injection are shown in Figure 1. The plasma LH-RH decreased significantly (P < 0.05) after the injection of 40,000 IU hCG, compared with the levels before and those of the control group; 68.7% after 2 hours, and 50.6% after 4 hours. The plasma LH-RH concentration in the group given . Miyake et. aL

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Time (h) Figure 1 Mean (± SE) changes in plasma levels of immunoreactive LH-RH expressed as percentages of preinjection levels after the injections of 10,000 IU or 40,000 IU of hCG or saline in castrated women. Statistical analysis was performed with the use of Student's t-test between groups injected with 40,000 IU of hCG and saline. *p < 0.05.

injections of 10,000 IV hCG was not significantly different from the concentration before the injection or the plasma LH-RH level in the control group that had received saline. DISCUSSION

The present data on human beings clearly demonstrate that exogenous hCG results in significant suppression of plasma immunoreactive LH-RH. These data confirm the results in rats by Nallar and Biscardi 1 ; that is, plasma LH-RH, determined by bioassay, was significantly decreased in hypophysectomized rats injected with LH. A significant decrease in the hypothalamic content of bioassay able follicle-stimulating hormone-releasing hormone (FSH-RH) was observed in rats treated with FSH by Frashini et a1. 5 and rats

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given implants of FSH in the median eminence by Corbin and Story.6 These data suggest that gonadotropins may exert effects, in part, on the hypothalamus. The present data also suggest that the effect of hCG on the concentration of plasma immunoreactive LH-RH may depend on the dose ofhCG. We 7 have previously shown that the suppression of plasma LH was dependent on the dose of exogenous hCG in human beings. Patritti-Laborde and Ode1l2 reported a dose-response relationship between exogenous human LH and the plasma LH concentration in rats. Thus, the decrease of plasma LH after exogenous gonadotropins may be caused by depletion of hypothalamic LH-RH. REFERENCES 1. Nallar R, Biscardi AM: Hypophysial feed-back in the hy-

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pothalamic regulation of luteinizing hormone secretion. Experientia 29:878, 1973 Patritti-Laborde N, Odell WD: Short-loop feedback of luteinizing hormone: dose-response relationships and specificity. Fertil Steril 30:456, 1978 Miyake A, Tanizawa 0, Aono T, Yasuda M, Kurachi K: Suppression of luteinizing hormone in castrated women by the administration of human chorionic gonadotropin. J Clin Endocrinol Metab 43:928, 1976 Miyake A, Kawamura Y, Aono T, Kurachi K: Changes in plasma LRH during the normal menstrual cycle in women. Acta Endocrinol (Copenh) 93:257, 1980 Frashini F, Motta M, Martini L: A "short" feedbacl-mechanism controlling FSH secretion. Experientia 24:270, 1968 Corbin A, Story JC: "Internal" feedback mechanism: response of pituitary FSH and of stalk-median eminence follicle stimulating hormone-releasing factor to median eminence implants of FSH. Endocrinology 80:1006, 1967 Miyake A, Aono T, Kinugasa T, Tanizawa 0, Kurachi K: The time course change after castration in short-loop negative feedback control of LH by hCG in women. Acta Endocrinol (Copenh) 88:1, 1978

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