Impairment of gonadotropin binding occurs during membrane rigidification in plasma membrane samples prepared from regressed rat corpora lutea.
The effect upon human chorionic gonadotropin (hCG) binding of a 90-min incubation of plasma membranes prepared from the corpora lutea of control and prostaglandin F2 alpha injected rats was studied. After incubation for 90 min with 1 mM CaCl2 at 40 degrees C, single point hCG binding assays at room temperature revealed a significant decrease in the degree of binding of approximately 50% in membrane samples prepared from regressed corpora lutea. The binding decrease in regressed samples did not occur if the incubation temperature was reduced to 35 degrees C or if calcium ion was replaced with magnesium. Scatchard analyses indicated that the decrease in binding capacity was the result of a loss of gonadotropin receptors rather than an affinity shift. Specific activities of two membrane-bound enzymes (Na+-K+ ATPase, 5'-nucleotidase) did not change in a correlative fashion during the incubation. In previous studies the same in vitro conditions caused a substantial and significant decrease in membrane fluidity, as determined by fluorescence polarization. Thus it appears that the membrane rigidification is of a specific nature and interferes with gonadotropin binding during luteolysis.