Pure natural porcine LHRH and synthetic LH-RH were administered iv to immature male rats by a quick injection or by infusion for a 4-hr period. Blood was collected 15 min or 60 min after quick injection, or immediately after the termination of the prolonged infusion. Serum was separated and assayed for LH and FSH by radioimmunoassay. A quick iv injection of 0.2 μg or 1 μg of synthetic LH-RH increased serum LH levels significantly at 15 min, but not at 60 min after injection. The magnitude of response was dose-related. Serum FSH was also increased, but the increase was very slight, and no dose-response relationship was observed. On the other hand, a 4-hr infusion of either 0.2 μg or 1 μg of synthetic LH-RH induced a considerable rise of both serum LH and FSH levels and the magnitude of rise for both serum LH and FSH was dose-related. A four-hr infusion of either 1 μg or 3 μg of natural pure or of synthetic LH-RH stimulated LH and FSH release near maximally, as indicated by the marginally significant or insignificant dose-response relationships at these dose levels. These results indicate that LH-RH decapeptide can stimulate a considerable release of FSH as well as of LH in vivo if given by prolonged infusion. This supports our concept that LH-RH decapeptide is also the only or the principal FSH-RH.
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