SECRETION OF LUTEINIZING-HORMONE CAUSED BY CONTINUOUS INFUSIONS OF LUTEINIZING-HORMONE RELEASING HORMONE IN LONG-TERM OVARIECTOMIZED RAT - EFFECT OF ESTROGEN PRETREATMENT

Continuous infusions of luteinizing hormone releasing hormone (LH-RH) into phenobarbitone-treated long-term ovariectomized rats, resulted in patterns of LH [luteinizing hormone] secretion which were determined by the blood LH-RH concentration. Infusions of 52 ng LH-RH/h caused steadily increasing plasma LH levels, which stabilized after about 2 h of infusion and were maintained for the rest of the experiment (9 h). A similar course of plasma LH concentration was obseved as a result of infusions of 104 ng LH-RH/h, though in this case LH concentrations reached higher levels than those induced by infusion of 52 ng LH-RH/h. Higher rates of LH-RH infusion (208 and 416 ng/h) induced clear-cut LH peaks, which reached their maximal plasma values after 2-3 h of infusion and then declined again until the end of the experiment when they were only slightly higher than the LH levels induced by infusions of 52 ng LH-RH/h. A similar series of LH-RH infusions given to ovariectomized rats pretreated with estradiol benzoate during 3 days (the rats were injected daily with 7 .mu.g steroid), produced a highly augmented response of the pituitary gland, but all LH-RH concentrations infused induced rather sharp LH peaks, reaching their maximum after 2-3 h of infusion. After 5 h of infusion the descending parts of all these peaks appeared to converge. In both control and estradiol benzoate-pretreated rats there appeared to be a linear relationship between the logarithm of the blood LH-RH concentration and the maximal plasma LH values on one hand, and the amount of LH secreted during the first 5 h of infusion on the other . It appeared that the longer the period of estrogen action, the more the response of the pituitary gland to a certain dose of LH-RH was enhanced.