Characterization of Prostaglandin Receptors in the Bovine Corpus Luteum Cell Membranes

The presence of gonadotropin and prostaglandin E receptors in the cell membranes of bovine corpora lutea throughout pregnancy

These studies revealed that the cell membranes prepared from bovine corpora lutea contain receptors for both human chorionic gonadotropin and prostaglandin E throughout pregnancy. The presence of these receptors correlated well with the earlier findings that the corpus luteum remains functional and responds to gonadotropic stimulation throughout the bovine pregnancy. This contrasts with our earlier findings that human corpora lutea of pregnancy of more than 6 weeks contain very few or no available receptors for human chorionic gonadotropin or prostaglandin E.

Prostaglandin E1 specific binding in rhesus myometrium

Myometrial low speed supernatant prepared from non-pregnant rhesus uteri was incubated with 3H-Prostaglandin (PG)E1 with or without addition of unlabelled prostaglandins. The uptake of 3H-PGE1 was inhibited in a dose dependent fashion by PGE2greater thanPGE1greater thanPGA1greater thanpgf2alpha=PGA1greater thanPGB1=PGB2greater than or equal toPGD2. PGE1 metabolites inhibited 3H-PGE1 binding in the following order: 13, 14-dihydro-PGE1greater than13,14-dihydro-15-keto-PGE1=15-keto-PGE1. The specific binding of 3H-PGE1 and 3H-PGF2alpha was similarly affected by the temperature and time of incubation. Equilibrium binding constants determined using rhesus uteri obtained during the luteal phase of the menstrual cycle indicate the presence of high affinity PGE1 binding sites with an average (n=3) apparent dissociation constant of 2.2 X 10(-9)M and a lower affinity PGE1 binding site with a Kd approximately equal to 1 X 10(-8)M. No high affinity - low capacity 3H-PGF2alpha sites could be demonstrated. Relative uterine stimulating potencies of some natural prostaglandins and prostaglandin analogs tested after acute intravenous administration in mid-pregnant rhesus monkeys corresponded with the PGE1 binding inhibition of the respective compound. The uterine stimulating potencies of the prostaglandin analogs tested were: (15S)-15-methyl-PGE2=16,16-dimethyl-PGE2greater than17-phenyl-18,19,20-trinor-PGE2greater than16 phenoxy-17,18,19,20-tetranor-PGF2alpha=PGE2=PGE1=(15S)-15-methyl-PGF2alpha greater than PGF2alpha.

Mechanism of nucleotide inhibition of gonadotropin binding to cell membranes of bovine corpus luteum

ATP, CTP, ADP, AMP, cyclic 3',5'-AMP (cAMP) and cyclic 3',5'-CMP (cCMP) effectively inhibited the specific binding of 125I-labelled human chorionic gonadotropin ([125I]HCG) to bovine corpus luteum cell membranes. This inhibition was observed with 2.5 X 10(-4) M to 1.0 X 10(-3) M nucleotide concentrations, regardless of the presence of a nucleotide regenerating system. Submaximal concentrations of combinations of the nucleotides were additive in inhibiting binding. The inhibition of [125I]HCG binding was observed when the nucleotides were added at the beginning of or during incubation or preincubation of the membranes with nucleotides. Preincubation of membranes with CTP and cAMP, subsequent washing and reincubation with hormone, showed time-dependent inhibition of [125I]HCG binding when the preincubation temperature was 38 degrees C but not at 4 degrees C. The concentrated supernates from nucleotides preincubated with membranes had no inhibitory effect on [125I]HCG binding to fresh membranes. In the absence of added nucleotides, [125I]HCG-membrane interaction had the following apparent binding constants: a Kd of 1.5 X 10(-10) M, 46.3 fmoles of binding sites per mg membrane protein, and rate constants for association and dissociation 4.0 X 10(6) M-1 sec-1 and 1.0 X 10(-3) sec-1, respectively. At steady state conditions of [125I]HCG binding, CTP inhibited [125I]HCG at lower concentrations of added hormone (less than 3 X 10(-9) M) whereas at higher concentrations, this nucleotide enhanced [125I]HCG binding. Scatchard analysis of the data revealed that inhibition and enhancement of [125I]HCG binding in the presence of CTP were due to lowered affinity of gonadotropin receptors (32-37) fold) and to exposure of new low-affinity binding sites for [125I]HCG, respectively. At non-steady-state conditions, nucleotides increased dissociation rates (80 to 100%) and decreased association rates (30 to 38%). The data appear to be compatible with the suggestion that the nucleotides may bind to sites in the membranes and subsequently induce conformational changes in membrane components, resulting in a decreased affinity of gonadotropin receptors. The physiological significance of these findings needs to be determined.

Prostaglandin E1 and F2alpha specific binding in bovine corpora lutea: comparison with luteolytic effects

Preliminary characterization indicated the presence of separate prostaglandin (PG)E1 and (PG)F2alpha binding sites in membrane fractions prepared from bovine corpora lutea. These differ in the rate and temperature dependence of the specific binding. Equilibrium binding data indicate the apparent dissociation constants as 1.32 x 10(-9)M and 1.1 x 10(-8)M for PGE1 and PGF2alpha, respectively. Competition of several natural prostaglandins for the PGE1 and PGF2alpha bovine luteal specific binding sites indicates specificity for the 9-keto or 9alpha-hydroxyl moiety, respectively. Differences in relative ability to inhibit 3H-PG binding were found due to sensitivity to the absence or presence of the 5, 6-cis-double bond as well. Bovine luteal function was affected following treatment of heifers with 25 mg PGF2alpha as measured by reduced estrous cycle length, decreased corpus luteum size and significantly decreased plasma progesterone levels. In contract, treatment with 25 mg PGE1 resulted in cycle lengths comparable to those of non-treated herdmates with no apparent modification in corpus luteum size. However, plasma progesterone levels were increased significantly following PGE1 treatment compared to pretreatment values. In so far as data obtained in vitro on PGF2alpha relative binding affinity to the bovine CL can be compared to data obtained independently in vitro on PGF2alpha induced luteolysis in the bovine, PGF2alpha relative binding to the CL and luteolysis appeared to be associated. By similar reasoning, there was no apparent relationship between PGE1 relative binding affinity in the luteal fractions and luteolysis in estrous cyclic cattle.

Structural requirements for the binding of prostaglandins

The binding of prostaglandins and analogs to the lipocyte PGE receptor was shown to exhibit a high degree of structural specificity. Small changes, particulary at the 9-keto or 15-hydroxyl position, were found to drastically diminish interaction with the receptor. Studies of a rather substantial number of compounds revealed a close relationshio between affinity for the lipocyte PGE receptor and the ability to stimulate cyclic AMP synthesis in the isolated mouse ovary. In general, activities in these two parameters follow the biological potencies generally recognized for the compounds.

Steroid hormone modulation of 3H-prostaglanding E1 binding to bovine corpus leteum cell membranes

The specific binding of 3H-prostaglandin E1 (3H-PGE1) to bovine corpus luteum cell membranes was not affected by cholesterol or various progestins at concentrations of up to 9.0x10-minus-6M. At concentrations above 2.5 x 10-minus-6M; estrone, 17beta-estradiol (but not 17alpha-estradiol or 17beta-estradiol glucuronide), estroil, equilin, D-equilenin, 17-ethynyl estradiol, diethylstilbestrol, cortisol, corticosterone, deoxycorticosterone and aldosterone inhibited specific binding of 3H-PGE1. On the other hand, testosterone and dihydrotestosterone (DHT) (but not androstenedione) significantly enhanced 3H-PGE1 binding. These findings permitted the following correlations between steroid structure and modulation of 3H-PGE1 binding: steroids with a free phenolic ring and a 17beta-hydroxyl or 17-keto group or C-21 steroids with a C-20 ketone and a C-21 hydroxy group decrease, whereas C-19 steroids with a C-17 hydroxy group enhance specific binding of 3H-PGE1. PGE receptors are heterogeneous with respect to affinity for 3H-PGE1. The steroids that decreased 3H-PGE1 binding caused a lowering to a complete loss of low affinity PGE receptors. Steroids that increased 3H-PGE1 binding caused appearance of new low affinity PGE receptors. Association rate constants for 3H-PGE1 binding were decreased by 17beta-estradiol (61%) and increased by DHT (59%).