Reevaluation of immunoreactive gonadotropin-releasing hormone (GnRH) levels in general circulation in women: changes in levels and episodic patterns before, during and after gonadotropin surges

In order to reevaluate the earlier varying data regarding circulatory gonadotropin-releasing hormone (GnRH), we assayed extracted GnRH from the plasma frequently collected at mid-cycle in 11 women. For the analysis of episodic GnRH patterns and basal levels, blood samples were obtained at 6 h intervals for 72 h and at 15 min intervals for 2 h every 12 h throughout the experimental period. All blood samples were assayed for GnRH and selected samples for LH, FSH, estradiol and progesterone. For GnRH assay, 5 or 6 ml of blood was mixed with 60 mg of ethylenediaminetetraacetic acid, disodium salt, and 3 mg of phenylmethylsulfonyl floride immediately after blood collection. These enzyme inhibitors prevented the destruction of GnRH in the blood at room temperature for at least 4 h. Plasma GnRH was extracted through several steps including florisil absorption, acidic extraction and washing with organic solvent. Nonspecific immunoreactivity in the plasma was markedly decreased through this extraction process. Our assay values (approximate range, 0.1-2.0 pg/ml) of plasma GnRH in normal women corresponded to the low range of those obtained by others who used the alcohol extraction method. The basal levels of GnRH did not change significantly throughout 3 different periods, i.e., before, during and after the LH surges, and fluctuated between a small range of 0.11 and 1.44 pg/ml. Although the peak levels of GnRH observed in its episodic patterns did not change between the periods before and during the LH surges, they decreased significantly after the LH surge compared with those seen during the LH surges (0.93 +/- 0.07 vs 1.17 +/- 0.09 pg/ml, p less than 0.05). The present data demonstrate that immunoreactive GnRH in the extracted peripheral plasma does not change significantly in its mean, basal and peak levels during the periovulatory period except for a minor but significant decrease in the peak levels shortly after an LH surge.