Short-loop feedback of luteinizing hormone: dose-response relationships and specificity.

The short-loop feedback control of rabbit luteinizing hormone (rLH) was studied by using a highly specific radioimmunoassay (RIA) system for rLH which does not react with human LH. Permanent intravenous catheters were placed in adult female New Zealand White rabbits at the time of castration. Highly purified human luteinizing hormone (hLH) was injected intravenously at doses of 0.1, 0.3, 0.5, 1, 2, 3, 4, 10, 50, and 100 IU into unanesthetized animals 1 to 16 days following castration. Blood samples were obtained at -20, 0, 5, 10, 20, 30, 60, 120, and 180 minutes via catheter, and rLH and rabbit follicle-stimulating hormone (rFSH) levels were determined by RIA. Doses between 1 and 100 IU of hLH produced a prompt decrease in rLH (within 5 minutes); the maximal response occurred within 20 to 30 minutes. Calculated as integrated area of response between 0 and 180 minutes, a dose-response relationship existed between 0.5 and 4.0 IU of hLH. Human LH, even at high doses of 50 IU, produced no changes in endogeneous rFSH. This is the first description of an entirely specific control system for LH, separate from FSH. The short-loop feedback control system for LH is sensitive to levels of LH estimated to be present in eugonadal animals.