INITIAL CHARACTERIZATION OF MASKED GONADOTROPIN RECEPTORS IN THE CORPUS LUTEUM OF THE RHESUS MONKEY (MACACA MULATTA) (MEMBRANE FLUIDITY, FLUORESCENCE POLARIZATION).
AuthorDANFORTH, DOUGLAS ROBERT.
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PublisherThe University of Arizona.
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AbstractThis study was designed to evaluate the possible existence of masked gonadotropin binding sites in the corpus luteum of the rhesus monkey. Pretreatment of macaque luteal particulates and cells with neuraminidase increased LH binding. In vitro exposure to alcohols also enhanced LH binding to these preparations. Ethanol modulation of LH binding was a time- and temperature-dependent process. The optimal concentration of ethanol for enhancing LH uptake was inversely proportional to the incubation temperature. Longer straight-chain alcohols were more potent than ethanol in increasing LH binding. Ethanol and neuraminidase increased the number of binding sites with no affect on affinity. Moreover, the effects of ethanol and NA were additive. Since alcohols and temperature are modulators of membrane fluidity, we examined the hypothesis that the unmasking of gonadotropin binding sites may be related to changes in the fluid state of the lipid bilayer of the luteal membrane. First, membrane fluidity was estimated from the fluorescence polarization of the membrane probe diphenylhexatriene. Conditions which resulted in enhanced gonadotropin binding (1-8% ethanol, increased temperature), increased the fluidity of luteal membranes. Moreover, changes in gonadotropin binding were highly correlated (r = -0.97) with changes in membrane fluidity under these conditions. Pretreatment of luteal particulates with neuraminidase had no apparent effect on membrane fluidity. Second, gonadotropin receptors were removed from the luteal membrane by detergent solubilization, and the effects of ethanol on soluble receptors were compared to those on receptors associated with the lipid bilayer. Solubilization resulted in the recovery of 50% more gonadotropin binding sites than are available in particulate preparations of the corpus luteum; these sites displayed lower affinity for gonadotropin. Moreover, conditions which increase LH binding to luteal particulates (1-8% ethanol at 25C) decreased LH uptake by soluble receptors. The data suggest that two 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 the diverse mechanisms may modulate the masking/unmasking of gonadotropin receptors in target cell membranes. As such, changes in membrane fluidity may play an important role in this response.