Pituitary responsiveness to luteinizing hormone-releasing hormone (LH-RH) during pregnancy in the rat.

Effects of LH-RH on serum LH levels were studied during pregnancy in rats. On days 10, 15, 18 or 21 of pregnancy, a catheter was placed at the jugular vein under light ether anesthesia. Sixty minutes after the surgery, LH-RH (1 microgram/300 microliters/300 g body weight) was injected through the catheter without anesthesia. Blood samples were collected through the catheter before, and 15 and 30 minutes after LH-RH injection. Basal serum LH levels were lower on days 15 and 18 than on days 10 and 21 of pregnancy. LH-RH injections caused significant increases in serum LH levels on any days of pregnancy, showing the maximum levels at 15 minutes after injection. The LH response was highest on day 10 and gradually decreased towards day 21. LH responses were significantly lower on days 18 and 21 than on days 10 or 15. These results suggested that the lowering of serum LH levels from days 10 to 15 might be due to the decrease in the responsiveness of the pituitary to LH-RH.