Naloxone increases bioactive LH in man: evidence for selective release of early LH pool.

Opioid peptides inhibit LH secretion and the opiate antagonist naloxone provokes increases in plasma LH levels by release of endogenous GnRH from the hypothalamus. To explore the effect of endogenously released GnRH on the mobilization of bioactive LH pools, the bioactive LH response to a single iv bolus dose of 20 mg naloxone has been evaluated and compared to the immunoactive pattern of the hormone in eight young normal male volunteers. Blood samples were withdrawn at 15, 30, 45, 60, 90, 120 min after naloxone injection and LH levels were measured by RIA and rat interstitial cell testosterone (RICT) bioassay. A significant increase in both bio and immuno active LH was observed in all subjects after 15-30 min (p less than 0.05 to p less than 0.001), reaching maximal levels at 30-60 min for both forms of the hormone. The time course of the bioactive LH response magnified the immunoactive LH pattern, and the maximum fold increases were 1.4 and 1.3 fold (62.4 +/- 5.5 SE and 25.0 +/- 3.7 SE mIU/ml) from basal bio and immuno LH levels of 25.9 +/- 4.3 SE and 11.1 +/- 2.0 SE mIU/ml respectively. An early single peak response of bio and immunoactive LH was observed in six subjects while a biphasic pattern was observed in two subjects with a clearly defined and prominent early pool followed by a second pool of higher magnitude. Both bio and immunoactive LH levels began to decline at 45-60 min, but in most subjects remained significantly elevated by about 30% above the basal values at 120 min