The bio/immuno ratio of plasma luteinizing hormone does not change during the endogenous secretion pulse: reanalysis...

The purpose of this study was to find out whether the ratio of bioactive (B)/immunoreactive (I) LH varies during pulsatile LH secretion in healthy men when I-LH is determined using novel immunometric methods with improved sensitivity and specificity. Blood samples were collected for LH measurements from eight healthy men (aged 20-26 yr) at 20-min intervals over a period of 10 h. LH was measured in all samples using two novel sensitive and specific immunometric methods [Delfia, Wallac (sensitivity, 0.04 IU/L); and Amerlite, Amersham (sensitivity, 0.1 IU/L)] and for comparison with a conventional RIA. In addition, the plasma samples representing the LH secretion peaks and the preceding interpulse nadirs (two to five pairs of samples per subject; n = 23) were measured for B-LH using the mouse interstitial cell in vitro bioassay. The three immunoassay methods demonstrated the well known pulsatile mode of I-LH secretion in all subjects. A good correlation of B- and immunometric I-LH levels was observed (bioassay vs. Delfia, r = 0.89), but independent of the immunometric method used, no difference could be demonstrated between the B/I ratios of the mean basal and peak levels of LH (1.72 +/- 0.19 and 1.63 +/- 0.19 with Delfia, and 2.18 +/- 0.32 and 2.10 +/- 0.26 with Amerlite, respectively). Only when I-LH was measured by RIA was there a significant (P < 0.001) 2-fold increase in the B/I ratio in the secretion pulses. In conclusion, the previously documented increase in the B/I ratio of LH during the endogenous secretion pulses could not be documented when I-LH was measured, instead of RIA, using the novel immunometric methods. This finding further emphasizes the need for reassessment of the B/I ratio changes in LH in various physiological and clinical conditions.