Evidence for two populations of masked gonadotropin-binding sites in the corpus luteum of the rhesus monkey (Macaca mulatta).

To evaluate the possible existence of masked gonadotropin receptors in the corpus luteum, we characterized the effects of alcohols and neuraminidase on [125I]iodohuman LH binding to in vitro preparations of luteal tissue from the rhesus monkey and pseudopregnant rat. The presence of 1-8% (vol/vol) ethanol enhanced specific LH binding to macaque luteal particulates under steady state conditions (25 C, 20-h incubation), with a maximal effect at 8% ethanol (166% of control uptake; P less than 0.05). However, 1-8% ethanol had no effect on LH binding to rat luteal tissue. Higher concentrations of ethanol (20%) decreased LH binding relative to control values in both species. Ethanol modulation of LH binding to macaque luteal particulates and dispersed cells was a time- and temperature-dependent process. At 4 and 25 C, ethanol increased LH uptake at all times during a 32-h incubation. However, at 37 C, ethanol increased LH uptake at 30 min; binding peaked at 2 h and then returned to control levels within 20 h. The optimal concentration of ethanol for enhancing LH uptake was inversely related to the incubation temperature. The increase in LH binding to macaque luteal particulates in the presence of ethanol was reversible; binding returned to control levels if ethanol was removed before the addition of labeled LH. Longer straightchain alcohols (butanol, pentanol, and octanol) were progressively more potent than ethanol in enhancing LH binding to macaque luteal particulates and dispersed luteal cells. Pretreatment of luteal particulates from either the rat or monkey with neuraminidase increased LH uptake, with a maximal effect (160% of control) at 1 mg/ml enzyme. Scatchard analyses revealed that both ethanol and neuraminidase increased (P less than 0.05) the number of LH-binding sites without altering the affinity for gonadotropin. Moreover, the effects of ethanol and neuraminidase were additive, i.e. increased LH binding during combination of the two treatments approximated the sum of the individual effects. The data suggest that two distinct populations of LH-binding sites are masked within the membranes of the monkey corpus luteum. The ability of two markedly different agents, alcohol and neuraminidase, to increase LH binding indicates that diverse mechanisms may modulate the masking/unmasking of gonadotropin receptors in target cell membranes. Finally, the inability of ethanol to enhance LH binding in the rat suggests species differences in the receptor population or milieu of luteal membranes.