Effect of Synthetic Luteinizing Hormone Releasing Hormone (LH-RH) on Plasma Testosterone Level

Hypothalamic luteinizing hormone releasing hormone (LH-RH) has been synthesized recently, by means of which the hypothalamic-pituitary-gonadal dynamics have been elucidated with much useful informations.<BR>We have studied the effect of LH-RH on plasma testosterone levels both in men and rats.<BR>Health adult men, aged 20-30, apparently being of normal gonadal function, were given 100μg and 400μg of LH-RH by a single intravenous injection.<BR>Plasma testosterone was measured serially before and after the injection by a specific radioimmunoassay using an antiserum produced against testosterone-3-bovine serum albumin. Cross-reactivity of this antiserum with other steroids is shown in TABLE I.<BR>The injection of 100μg of LH-RH was not effective to elevate plasma testosterone level in all the normal subjects, in spite of a significant increase in circulating LH concentrations. On the contrary, 400μg of LH-RH raised plasma testosterone significantly in all the normal subjects tested, although the amount of the increase in testosterone was not so large even when 400μg of LH-RH was administered (Fig. 1). It seems necessary, therefore, to use 400μg of LH-RH when plasma testosterone is to be measured as an index of the gonadal function.<BR>Further studies were carried out to clarify the the reason why plasma testoste- rone response to LH-RH is not so marked.<BR>For this purpose, male Wister rats, from 70 to 90 days of age, weighing between 150 and 200 gm, were used.<BR>I. Testes removed immediately from normal rats were decapsulated and transferred to incubation vials containing 4.0ml of Krebs-Ringer bicarbonate buffer with glucose (0.2%) and bovine serum albumin (0.5%). Incubation was performed at 37°in a metabolic shaker under 95% O₂-5% CO₂. After 60min, the media were replaced by fresh buffer added hormone samples and the final incubation was performed.<BR>Testosterone was determined by radioimmunoassay.<BR>The following results were obtained.<BR>(1) Ten pituitary halves removed from intact male rats were incubated by the method of Saffran and Schally, in 10ml of Krebs-Ringer bicarbonate buffer containing 10ng of LH-RH/ml. After 4 hours, the LH released into the incubation medium was measured by bioassay using rat testis in vitro steroidogenic system. As shown in Fig. 2, the incubation medium of pituitary halves possessed the ability of testosterone production. This suggests that the LH released from the pituitary by exgenous LH-RH has a steroidogenic potency in the testis.<BR> (2) Dibutiryl cyclic AMP caused an increase in testosterone production by rat testis in vitro (Fig. 3). This finding coincides with the previous studies which suggested that cyclic AMP mediates at least some parts of the action of LH.<BR> (3) The time course of testosterone production by rat testis in response to HCG was studied in vitro. No response in testosterone production was observed within 60min, but a marked response after 60min (Fig. 4). The reason for this delayed response is still obscure. It may be due to the experimental condition or to the age of the rats.<BR>II. Rats were anesthetized with pentobarbital and the jugular veins on both sides were exposed. LH-RH was given through a jugular vein and blood sample was drawn from the opposite one. Plasma testosterone began to increase 30 min after LH-RH administration, and a higher response was observed (Fig. 5)<BR>In summary, LH-RH, 400μg in dose, caused a rise in plasma testosterone in normal men, although the rise was slow and not so marked. Testosterone production of rat testis in vitro in response to HCG was also delayed. The reason for this delayed response is still obscure. Evidence was obtained which suggests a possible role of cyclic AMP in mediating LH action in testis.