Variation in Responsiveness to Synthetic LH-Releasing Factor (LRF) in Proestrous and Diestrous-3 Rats

Synthetic LRF, prepared by solid phase methodology and purified by ion exchange chromatography, was administered under pentobarbital anesthesia at 1500 hr to 4-day cyclingrats in proestrus and 5-day cycling rats on thethird day of diestrus (D-3) in single intravenousdoses from 100 to 1000 ng per animal. SerumLH at 15, 30, 90 min and 20 hr after LRF wasmeasured by radioimmunoassay and ovaries andampullae were removed at 20 hr and examinedmicroscopically for ovulation. Although a 5 ngdose of this LRF produced a 9-fold increase inLH in the ovariectomized rat pretreated withestrogen and progesterone, 100 ng LRF on D-3of the 5-day cycle failed to elevate serum LHabove the minimum detectable level of 60 ng/ml. An apparent small rise in LH after 250 ng LRFon D-3 was not statistically significant, but significantelevations were observed after 500, 750 and 1000 ng LRF. By contrast LH levelswere significantly increased during proestrus ofthe 4-day cycle after 100, 250, 500, 750 and 1000ng LRF, with post-LRF values ...