Preferential release of bioactive luteinizing hormone in response to endogenous and low dose exogenous gonadotropin-releasing hormone pulses in man.

We used the rat interstitial cell testosterone (RICT) bioassay to assess biological LH activity secreted in response to endogenous and low dose exogenous GnRH pulses in normal men. The absence of nonspecific plasma effects in the LH bioassay was demonstrated by the finding of undetectable levels of LH bioactivity despite low but measurable immunoactivity in 10 hypogonadotropic men. Moreover, bolus injections of human LH in 6 hypogonadotropic men defined a curvilinear relationship between plasma bioactive and immunoactive LH concentrations, in which the extrapolated concentration of plasma bioactive LH at a zero dose of immunoactive LH was indistinguishable from zero. Zero bioactive LH intercepts were also found when physiological bio- and immunoactive LH concentrations derived from 7 intensively sampled normal men were subjected to linear regression using 2-dimensional error fitting. In these men, exogenous low dose (10 micrograms) iv GnRH administration resulted in preferential release of bioactive LH, with a consequent significant increase in the median plasma bio- to immunoactive (bioimmuno) LH ratio. This pattern mimicked that of endogenous LH pulsatility, in which median intrapulse bio:immuno LH ratios were significantly higher than median interpulse ratios in the same individuals (P = 0.006). Increases in spontaneous plasma bio:immuno LH ratios were not attributable to spurious rises in bioactive LH concentrations associated with decreases in serum immunoactive LH levels. Rather, sample cross-correlation analyses demonstrated positive correlations between bio- and immunoactive LH at lags of 0-40 min, indicating that both hormones increased or decreased concomitantly. These results demonstrate that LH is secreted physiologically in pulses of increased biological activity, presumably reflecting the release of a functionally compartmentalized LH pool relatively enriched in biologically active hormone. Accordingly, evaluation of the plasma bio:immuno LH ratio can provide a useful and sensitive index of qualitative changes in the LH molecule in response to endogenous (spontaneous) and exogenous GnRH stimulation.