Inhibitory effect of plasma obtained from hypophysectomized and control women on the assay of bioactive luteinizing hormone

The purpose of this study was to determine the effect of components of fema le plasma on the value of bioactive luteinizing hormone (LH), especially in the presence of low immunological LH value. Using both an immunoradiometri c assay (IRMA) and rat Leydig cell bioassay, immunoreactive (I) and bioacti ve (B) LH were assessed in plasma collected from women during a gonadotroph in releasing hormone (GnRH) test performed on day 7 of a spontaneous cycle. Two modes of response to an acute administration of GnRH were defined: nor mal production of gonadotrophins (group I) and excessive secretion (group I I) associated with a significant difference in the B/I-LH ratio between the two groups. The B/I-LH ratio did not vary with sampling time during the te st in either group. The addition of LH-free plasma collected from hypophyse ctomized women caused a 30% decrease in testosterone production compared to control values (in the presence or absence of hLH standard). A partial res toration of testosterone production was observed if plasma was first treate d with PEG 12%. The inhibitory factor(s) was also present in plasma from ov ulatory women, even after treatment by an antibody against the entire LH mo lecule. The effect of normal (A) or low I-LH plasma (B) on testosterone pro duction varied strongly according to the plasma volume added to the bioassa y, as well as to plasma treatments. Diethylether treatment caused a 50% dec rease in testosterone secretion for plasma B (but not for A) whereas a dimi nution of the steroidogenesis is observed after a PEG treatment of plasma A (but not for B), suggesting that different inhibitory factors are present in plasmas A and B. Therefore the LH bioactivity measured in the rat Leydig cell assay, in terms of testosterone output, seems to represent a balance between the LH molecule and the presence of inhibitory factors in the plasm a.