Dynamics of Luteinizing Hormone Release after Intravenous Administration of Crude Stalk-Median Eminence Extract or Synthetic Gonadotropin-Releasing Hormone 1

Summary

The comparative ability of crude acidic extracts of stalk-median eminence (SME) and of synthetic Gn-RH to induce the release of LH in ewes was examined. Intravenous injections of 4 SME equivalents resulted in a rapid increase in serum levels of LH ranging from 27 to 54 ng/ml and with maximal levels occurring from 3 to 6 min postinjection. Injection (iv) of 10 μg synthetic Gn-RH resulted in a prolonged release of LH into the circulation with serum levels of LH ranging from 21 to 49 ng/ml and with maximum levels occurring from 36 to 84 min postinjection. Administration of 280 ng synthetic Gn-RH (the quantity of Gn-RH contained in 4 SME equivalents) resulted in only a slight increase in serum LH (6 ± 3 ng/ml) with the maximum level at 9.7 ± 3.6 min post-injection. Injection of crude SME extracts after partial purification by ammonium sulfate precipitation or by gel-filtration resulted in only slight increases in serum levels of LH. Radioimmunoassay of the crude SME extracts indicated that over 90% of the increase in serum levels of LH observed after injection of crude SME could be accounted for by contamination of LH in the original extract.

Levels of Gn-RH in serum increased 10 to 60 pg/ml within 3 min after iv injection of crude SME extract or 280 ng synthetic Gn-RH and returned to baseline within 15 min. After iv administration of 10 μg synthetic Gn-RH serum levels of Gn-RH increased to 964 ± 216 pg/ml and returned to baseline within 60 min. Since a substantial release of endogenous LH occurred only after administration of 10 μg synthetic Gn-RH it is suggested that levels of Gn-RH in the hy-pophyseal portal circulation must approach 900 pg/ml to induce an increase in serum levels of LH comparable to the preovulatory surge.