Short-term kinetics of LRH-induced LH-release in the long-term ovariectomized rat

In a first series of experiments plasma concentrations of luteinizing hormone (LH) were measured at 10 min intervals during 2 h of constant rate infusion of luteinizing hormone-releasing hormone (LRH; 104 ng/ h) in phenobarbitone-anaesthetized long-term ovariectomized (OVX) rats, treated with oil or oestradiol-benzoate (OeB). From these data the mean LH secretion rates during the sampling intervals were calculated using a one-compartment model for the elimination of LH from the plasma. It was found in the OeB-primed OVX rats that during the initial 30–40 min of infusion the LH release is high but constant. Thereafter it shows a further increase. In the oil-treated OVX rats a similar biphasic LH release pattern was found, but in these animals it was preceded by an initial phase of very high LH release, lasting a few minutes.In another series of experiments a second LRH infusion (again 104 ng/h) was given to OeB-primed OVX rats, starting 1.5 h after the discontinuation of a first LRH infusion lasting either 1, 3.5 or 20 h. The resulting secondary LH responses were smaller the longer the first infusion had lasted, but the LH secretion pattern was similar with all three time schedules and resembled the triphasic pattern observed during the first experiment in the oil-treated OVX rats, rather than the biphasic pattern of the OeB-primed OVX rats.These results indicate that the LH response to LRH of OVX rats (either treated with OeB or oil), like that of the cyclic rats, exhibits a phase of constant LH release. It is generally assumed that during this period conditions, necessary for the subsequent further increase of the LH secretion, are generated. It is concluded that these conditions largely disappear during a 1.5 h non-stimulus period. It is also concluded that the short initial phase of very high LH secretion is due to recent exposure of the LH-secretory system to stimulatory amounts of LRH