Suppression by LRH of the stimulatory effect of oestradiol on the secretion of LH by the rat pituitary gland.

Ovariectomized rats were infused with varying doses of luteinizing hormone-releasing-hormone (LRH). Some of the rats were also treated with oestradiol benzoate (EB). The effects of these pre-treatments on the in vitro release of luteinizing hormone (LH) were studied. The following parameters of in vitro LH release were measured: the autonomous secretion rate; the secretion rate following maximum stimulation with LRH, and the total quantity of LH released during the 6-hour experiment. The in vivo pre-treatments with LRH and EB dose-dependently decreased the pituitary LH content as well as all three of the above parameters of in vitro LH secretion. There was a linear relationship between the pituitary LH content and the three parameters of in vitro LH release. These parameters were therefore expressed as percentage of the pituitary LH content to give the relative LH secretion rates. The three parameters were thereby corrected for LRH/EB-induced changes in the pituitary LH content. In this way we obtained information on the effects of LRH and EB on the state of the LH release mechanisms of the gonadotropes. EB potentiated the LRH-induced depletion of the pituitary LH stores at all in vivo LRH infusion rates. The effect of EB on the quantity of LH released during perifusion in vitro, however, varied with the previous LRH infusion rates. After LRH infusion rates lower than about 120 ng/h (which establishes plasma concentrations of about 70 ng/l) EB enhanced the stimulated quantity of LH released. After higher rates of LRH infusion, EB lowered the amount of LH released.(ABSTRACT TRUNCATED AT 250 WORDS)