Temporal changes in serum estradiol and prolactin in immature female rats given a single injection of estradiol benzoate

Serum concentrations of estradiol 17β(E₂) measured by immunoassay reached peak levels (100pg/ml) within one hour in immature female rats given a subcutaneous injection of lug 17βestradiol-3-benzoate (EB). Hypophysectomy did not alter the E₂ concentration, but lower peak levels were found in ovariectomized females. In animals with ovaries a secondary rise in serum E₂ was apparent 12 hours after the injection of EB; from 12 to 48 hours E₂ decreased linearly. A dose of Bug EB caused an abrupt rise in E₂ (180pg/ml) within 30 minutes, but during the next 24 hours rather wide fluctuations in serum levels were found. In animals acclimated to a reversed light schedule and given 5ug EB early in the dark period, E₂ decreased linearly over a period of 72 hours. Prolactin increased in response to the E₂: a rhythm was suggested by the occurrence of increases during the dark periods. The results indicate that statistically significant fluctuations in both E₂ and prolactin occur after a single injection of EB and that measurements at a single point in time are inadequate to determine the true pattern of hormonal changes.     

Effects of estrogen deprivation on brain estrogen and progestin receptor levels and the activation of female sexual behavior

Ovariectomy (OVX) in rats is followed by a decline in behavioral sensitivity to combined estrogen and progesterone therapy. The purpose of this study was to further characterize this behavioral change, and to explore its biochemical basis in terms of estrogen and progesterone receptor concentrations in the brain. Sexually inexperienced female rats were used 5 (short-term) or 35 (long-term) days after OVX. Short- and long-term OVX animals were injected with estradiol-17β (E₂; 36 μg/kg body wt, iv) then subjected to one of the following three treatment schedules. (1) Animals were treated with progesterone (1 mg, sc in oil) 20–21 hr after E₂ injection, then tested at 24 hr for female sexual behavior. (2) One or twelve hours after the E₂, cell nuclear estrogen receptors (ER_n) were measured in the pituitary (PIT) and pooled preoptic area and mediobasal hypothalamus (POA-MBH). (3) Twenty-four hours after E₂, progestin receptor (PR_c) concentrations were measured in cytoplasmic fractions prepared from PIT and POA-MBH. Long-term OVX animals showed a reduced capacity to exhibit proceptive and receptive sexual behavior, and a lower PR_c level in the PIT and POA-MBH 24 hr after E₂ injection than animals that had been OVX for only 5 days. However, no differences were observed between long- and short-term OVX rats with respect to ER_n concentrations in PIT and POA-MBH cell nuclei 1 or 12 hr after E₂. Thus, it appears that the decline in behavioral responsiveness to E₂ which occurs after ovariectomy cannot be attributed to a decrease in the ability of E₂ to translocate estrogen receptors into POA-MBH cell nuclei, but is more probably associated with a change in the biochemical processes subsequent to ER_n binding. One of these processes may well be the induction of cytoplasmic progestin receptors.     

Follicle stimulating hormone and prolactin release induced by prostaglandins in rat

Ten to 60 minutes following a single i.v. injection of PGE₂ (500 μg/rat) into male rats of 30 to 35 days of age FSH concentration in the serum was raised significantly. The rise in FSH was maintained from 10 to 60 minutes after treatment, then at 90 minutes FSH had declined and was not significantly different from that of the control before treatment. Prostaglandin E₁, E₂ or F_2α (670μg/rat) significantly increased the serum prolactin level 10 to 60 minutes after a single i.v. injection in spayed rats primed with estrogen and progesterone. And, rats primed with estrogen and progesterone. And, increases in prolactin in the serum were observed with as little as 2μg of PGE₁ or E₂, and 20μg of PGF_2α. Twenty μg of PGE₂, and 200μg of PGE₁ or F_2α gave the maximum stimulation. These results indicate that release of pituitary hormones is affected by prostaglandins.Prostaglandins (PGs) are widely distributed in mammalian tissues, and they have been reported to have an almost equally wide variety of endocrine and metabolic effects. It was recently postulated that PGs may be involved in the process of ovulation because ovulation was blocked by inhibitors of PG synthesis (1–5).     

Effect of diethylstilbesterol and prolactin on the induction of follicle stimulating hormone receptors in immature and cycling rats

Induction of follicle stimulating hormone receptor in the granulosa cells of intact immature rat ovary by diethylstilbesterol, an estrogen, has been studied. A single injection of 4 mg of diethylstilbesterol produced 72 h later a 3-fold increase in follicle stimulating hormone receptor concentration as monitored by [¹²⁵I]-_oFSH binding to isolated cells. The newly induced receptors were kinetically indistinguishable from the preexisting ones, as determined by Lineweaver-Burk plot of the binding data. The induced receptors were functional as evidenced by increased ability of the granulosa cells to incorporate [³H]-leucine into cellular proteins. Neutralization of endogenous follicle stimulating hormone and luteinizing hormone by administering specific antisera had no effect on the ability of diethylstilbesterol to induce follicle stimulating hormone receptors, whereas blockade of endogenous prolactin secretion by ergobromocryptin administration significantly inhibited (∼ 30 %) the response to diethylstilbesterol; this inhibition could be completely relieved by ovine prolactin treatment. However, ovine prolactin at the dose tried did not by itself enhance follicle stimulating hormone receptor level. Administration of ergobromocryptin to adult cycling rats at noon of proestrus brought about as measured on diestrusII, (a) a reduction of both follicle stimulating hormone (∼ 30 %) and luteinizing hormone (∼ 45 %) receptor concentration in granulosa cells, (b) a drastic reduction in the ovarian tissue estradiol with no change in tissue progesterone and (c) reduction in the ability of isolated granulosa cells to convert testosterone to estradiol in response to follicle stimulating hormone. Ergobromocryptin treatment affected only prolactin and not follicle stimulating hormone or luteinizing hormone surges on the proestrus evening. Treatment of rats with ergobromocryptin at proestrus noon followed by an injection of ovine prolactin (1 mg) at 1700 h of the same day completely reversed the ergobromocryptin induced reduction in ovarian tissue estradiol as well as the aromatase activity of the granulosa cells on diestrus II, thus suggesting a role for proestrus prolactin surge in the follicular maturation process     

Estrogen and n-3 polyunsaturated fatty acid supplementation have a synergistic hypotriglyceridemic effect in ovariectomized rats

The n-3 polyunsaturated fatty acids (PUFAs), EPA and DHA, as well as estrogen have been shown to decrease circulating levels of triglyceride (TG), but their underlying mode of action is unclear. The purpose of this study was to determine the effects of n-3 PUFA consumption and estrogen injection on TG metabolism. Rats (n = 48) were fed a modified AIN-93G diet with 0, 1, or 2 % EPA + DHA relative to the total energy intake during 12 weeks. At 8 weeks, rats were ovariectomized (OVX), and after a 1-week recovery, rats were injected with either 17β-estradiol-3-benzoate (E₂) or corn oil for the last 3 weeks. The n-3 PUFA consumption and E₂ injection independently decreased the hepatic expressions of sterol regulatory element-binding protein 1, acetyl-CoA carboxylase 1, fatty acid synthase (FAS), and diacylglycerol acyltransferase 2 (DGAT2) (P < 0.05). There were interactions between n-3 PUFA consumption and E₂ injection on hepatic expression of FAS and DGAT2. In addition, n-3 PUFA consumption and E₂ injection up-regulated the expression of AMP-activated protein kinase (AMPK), phosphorylated AMPK, peroxisomal proliferator-activated receptor α, and carnitine palmitoyltransferase 1 in liver and skeletal muscle. E₂ injection increased the expression of estrogen receptor α and β in skeletal muscle and liver, but n-3 PUFA consumption increased the expression of both receptors only in skeletal muscle. The present study suggests that the hypotriglyceridemic effects of n-3 PUFA consumption and E₂ injection could be due to the down-regulation of hepatic TG synthesis and up-regulation of TG oxidation in liver and skeletal muscle in OVX rats.     

Subclones of normal clonal prolactin cells of the rat

Summary We recently established a clone (2B8) of normal rat prolactin cells that secretes only prolactin into the medium. When grown in the presence of thyrotrophin-releasing hormone (TRH), estradiol (E₂) or arginine vasotocin (AVT), the cells show increased production of prolactin. Subclones of single cell origin were developed from 2B8 cells exposed for 1 week to TRH, E₂ or TRH plus E₂. These subclones differ in their response to TRH, E₂ or AVT and therefore may possess different receptors for these hormones. Presented in preliminary from at the meeting of the Endocrine Society in Chicago, June 9, 1977. This work was supported by USPHS Grant AM12583.     

The effects of long-term estrogen exposure on the induction of sexual behavior and measurements of brain estrogen and progestin receptors in the female rat

The purpose of this study was to investigate long-term effects of estradiol-17β (E₂) on sexual receptivity and to study the molecular basis for estrogen potentiated changes in receptivity. We therefore examined the long-term effects of E₂ on E₂ and progestin receptors in the hypothalamus-preoptic area-septum (HPS) and pituitary (PIT) of the female rat. Twenty-one days following ovariectomy, females received a 5-mm Silastic capsule of E₂ or cholesterol (C) for 1 week (pretreatment). Some animals were sacrificed for chemical analyses (i). The remainder had their capsules removed and 5 days later these animals either were sacrificed for chemical analyses (ii), or received E₂ (reimplantation). Forty-six hours after reimplantation, females either were sacrificed for chemical analyses (iii), or were tested for receptivity. When tested with sexually active males or by a manual stimulation method, animals pretreated with E₂ showed significantly better lordosis scores than animals pretreated with C. The tests for lordosis were carried out after administration of E₂ or E₂ + P to all subjects tested. During E₂ pretreatment, HPS and PIT cytosol progestin receptors increased significantly, while available estrogen receptor levels decreased significantly, as compared with C controls. Five days after E₂ pretreatment, HPS progestin receptor levels had decreased to the level observed in C controls, while PIT progestin receptors were slightly elevated. In HPS and PIT, levels of available E₂ receptor in E₂ and C pretreated animals were indistinguishable from each other. Neither the saturation capacity of the estrogen receptor nor the dissociation constant for binding [³H]E₂ was altered by E₂ pretreatment, as shown by Scatchard equilibrium analysis. Forty-six hours following E₂ reimplantation, progestin HPS and PIT receptor levels in E₂ and C pretreated animals were identical. Long-term potentiation of lordosis by E₂ does not result from a change in estrogen or progestin receptor dynamics in HPS of female rats.     

Asynchrony in the Diurnal Rhythms of Dopaminergic D2 Receptors in Different Brain Regions of the Rat

Diurnal variations of dopaminergic D₂ receptors have been described in the striatum of rats, while other dopaminergic regions remain unstudied. Diurnal variations of dopamine D₂ receptors in the striatum, frontal cortex, and amygdala of the rat, were characterized by the stereospecific binding of [³H]-spiperone. Clear rhythms were found in all these areas, but asynchronous to each other. Striatal receptors had diurnal variations with a single peak at 00:00 hours. Frontal cortex receptors showed two peaks at 00:00 and 12:00 hours. Amygdaline complex receptors had two peaks at 18:00 and 06:00 hours. Saturation binding curves and their Scatchard analysis indicated that the diurnal variations in [³H]-spiperone binding are related to changes in receptor density rather than its affinity. The diurnal variations asynchrony in [³H]-spiperone binding to dopaminergic D₂ receptors from different neural regions, suggest different regulation in each area. Other functional implications of these rhythms remains to be established.     

In vitro liver clearance of tritiated estradiol-17β in the female rat after retrochiasmatic transection and ovariectomy

It was reported in a previous study that serum estradiol-17β (E₂) was elevated in rats after retrochiasmatic transection (FC). Serum E₂ was also higher in estradiol cypionate treated, ovariectomized (ovx) rats that had been subjected to FC than in those that had not. This suggested that increased secretion of E₂ was not the only factor responsible for elevated serum E₂ after FC. To ascertain the contribution of decreased metabolism of E₂ to this response, liver tissue slices were incubated with 3H-estradiol-17β, and the rates of ³h uptake and conversion to water-soluble conjugates were measured.The rate of uptake of ³H by the tissue was not indicative of the rate of conjugate formation. Livers of rats with high serum E₂ exhibited lower rates of ³h uptake than those of rats with low serum E₂. Even so, the formation of ³H conjugates was greater in liver of rats with high serum E₂. As hypothesized, livers of rats with FC formed conjugates at a significantly lower rate than those of similarly treated rats without FC. Thus, FC of an intact rat leads to an increase in serum E₂ by increasing the secretion of E₂, and apparently also by decreasing the rate of E₂ metabolism by the liver.     

Suppression of Receptors for Prolactin and Estrogen in Rat Liver Due to Treatment with the Growth Hormone Analogue Produced by the Tapeworm Spirometra Mansonoides

Abstract

Somatogenic hormones play an important role in regulation of receptors for prolactin (PRL) and estrogen. Plerocercoids of the tapeworm, S. mansonoides produce a factor which mimics some, but not all of the actions reported for GH. Intact female rats were subjected to a constant infusion of plerocercoid growth factor (PGF) via a subcutaneous infection for two weeks to determine if PGF influences receptors for PRL, GH or estradiol. The rate of weight gain in the PGF-treated rats was accelerated in spite of a marked reduction in serum GH. Partially-purified PGF specifically displaced [¹²⁵I]hGH from rat liver receptors but microsomes prepared from rats treated with PGF specifically bound significantly less [¹²⁵I]hGH than microsomes from control rats. The reduction in [¹²⁵I]hGH binding was not due to occupancy or to a change in affinity but to a suppression in receptor concentration. Scatchard analysis of [³H]estradiol binding in rat liver cytosols shows a 50% reduction in receptor concentration in the PGF-treated group. Specific binding of [³H]estradiol in anterior pituitary was also suppressed by PGF treatment.     

Sex-related differences in cadmium-induced lipid peroxidation in the rat

Male and female rats were dosed once a day for 2 days injection with 1.5 mg of Cd/kg as CdCl₂. 24 hr after administration of cadmium, lipid peroxidation determined by estimation of malondialdehyde (MDA) was greatly increased in male rat liver, but was not in female rats. Cadmium in a larger dose of 4.5 mg/kg, subcutaneous single injection, significantly increased content of MDA in female rat liver. These results suggest that sex-related differences exist in the ability of cadmium to induce MDA formation in rat liver, although administration of cadmium causes the enhancement of MDA formation in both male and female rats. The reason why sex-related differences exist in lipid peroxidation of rat liver is discussed.     

Regulation of formation of covalently bound flavins in liver and cerebrum by thyroid hormones.

The effects of thyroxine (T₄) and triiodothyronine (T₃) treatment upon the formation of [2-¹⁴C]flavins bound covalently to tissue proteins in liver and cerebrum were measured 1 h after a subcutaneous injection of [2-¹⁴C]riboflavin in male rats of different ages. In livers of rats of ages 2, 3, and 12 months, T₄ (100 μg/100 g body wt) and T₃ (25 μg/100 g body wt) in daily intraperitoneal doses for 7 days each increased incorporation into covalently bound flavins 50% above that in saline-treated controls. In newborn rats, T₄ in doses of 10 μg/rat for 7 days increased incorporation similarly to that in adults. In adult rats doses of T₃ from 2.5 to 25 μg/100 g body wt were nearly as effective as larger doses of T₃ and T₄ in increasing the formation of covalently bound flavins in liver. In cerebra of newborn rats, T₄ was ineffective in increasing the formation of covalently bound flavins. However, in cerebra of rats of ages 2, 3, and 12 months, both T₃ and T₄ consistently increased the formation of covalently bound flavins. Doses of T₃ from 2.5 to 25 μg/100 g body wt produced significant increases. These findings are of interest in view of our previous demonstration that the formation of flavin adenine dinucleotide, the major tissue flavin, is not increased in rat brain even by massive doses of thyroid hormones. The present results indicate that the formation of the fraction of flavins bound covalently to tissue proteins differs from the usual pattern of brain metabolism of adult rats in being subject to control by thyroid hormones.     

Increases in Tumor Necrosis Factor-α in Response to Thyroid Hormone-induced Liver Oxidative Stress in the Rat

Thyroid hormone-induced calorigenesis contributes to liver oxidative stress and promotes an increased respiratory burst activity in Kupffer cells, which could conceivably increase the expression of redox-sensitive genes, including those coding for cytokines. Our aim was to test the hypothesis that l -3,3',5-triiodothyronine (T₃)-induced liver oxidative stress would markedly increase the production of TNF- α by Kupffer cells and its release into the circulation. Sprague-Dawley rats received a single dose of 0.1 mg T₃/kg or vehicle (controls) and determinations of liver O₂ consumption, serum TNF-α, rectal temperature, and serum T₃ levels, were carried out at different times after treatment. Hepatic content of total reduced glutathione (GSH) and biliary glutathione disulfide (GSSG) efflux were measured as indices of oxidative stress. In some studies, prior to T₃ injection animals were administered either (i) the Kupffer cell inactivator gadolinium chloride (GdCl₃), (ii) the antioxidants α-tocopherol and N-acetyl-L-cysteine (NAC), or (iii) an antisense oligonucleotide against TNF-α (ASO TJU-2755). T₃ elicited an 80-fold increase in the serum levels of TNF-α at 22h after treatment, which coincided with the onset of thyroid calorigenesis. Pretreatment with GdCl₃ , α-tocopherol, NAC, and ASO TJU-2755 virtually abolished this effect and markedly reduced T₃-induced liver GSH depletion and the increases in biliary GSSG efflux. It is concluded that the hyperthyroid state in the rat increases the circulating levels of TNF-α by actions exerted at the Kupffer cell level and these are related to the oxidative stress status established in the liver by thyroid calorigenesis.     

Effect of selenite and selenate on rat liver nuclear 3,5,3′-triiodothyronine (T3) receptor

The present study was undertaken to investigate the effects of selenite (Se^IV) or selenate (Se^VI) on nuclear T₃ receptors of rat liver. Selenite at 0.1 μM (p<0.01) inhibited the T₃ specific binding to rat liver nuclear receptors. The specific binding of the T₃ receptor was fully restored when even 1.0 μM selenite was separated from the T₃ receptor by gel filtration. No inhibitory effect of selenite (up to 100 μM) on the T₃ binding to nuclear receptor was found in the presence of 1.0 mM dithiothreitol. The rate of dissociation of the T₃-nuclear receptor complex was effectively increased by 0.1 μM selenite. Selenate up to 1 mM as well as sulfite or sulfate up to 0.1 mM did not exert an inhibitory effect on T₃ receptors. The results based on the in vitro experiments suggest that the selenium in the form of selenite may reversibly affect the T₃ binding on the receptor molecule.     

Involvement of dopamine D2 receptor in the diurnal changes of tuberoinfundibular dopaminergic neuron activity and prolactin secretion in female rats

Background

An endogenous dopaminergic (DA) tone acting on D₃ receptors has been shown to inhibit tuberoinfundibular (TI) DA neuron activity and stimulate prolactin (PRL) surge in the afternoon of estrogen-primed ovariectomized (OVX+E₂) rats. Whether D₂ receptor (D₂R) is also involved in the regulation of TIDA and PRL rhythms was determined in this study.

Results

Intracerebroventricular (icv) injection of PHNO, a D₂R agonist, in the morning inhibited TIDA and midbrain DA neurons’ activities, and stimulated PRL secretion. The effects of PHNO were significantly reversed by co-administration of raclopride, a D₂R antagonist. A single injection of raclopride at 1200 h significantly reversed the lowered TIDA neuron activity and the increased serum PRL level at 1500 h. Dopamine D₂R mRNA expression in medial basal hypothalamus (MBH) exhibited a diurnal rhythm, i.e., low in the morning and high in the afternoon, which was opposite to that of TIDA neuron activity. The D₂R rhythm was abolished in OVX+E₂ rats kept under constant lighting but not in OVX rats with regular lighting exposures. Pretreatment with an antisense oligodeoxynucleotides (AODN, 10 μg/3 μl/day, icv) against D₂R mRNA for 2 days significantly reduced D₂R mRNAs in central DA neurons, and reversed both lowered TIDA neuron activity and increased serum PRL level in the afternoon on day 3. A diurnal rhythm of D₂R mRNA expression was also observed in midbrain DA neurons and the rhythm was significantly knocked down by the AODN pretreatment.

Conclusions

We conclude that a diurnal change of D₂R mRNA expression in MBH may underlie the diurnal rhythms of TIDA neuron activity and PRL secretion in OVX+E₂ rats.     

Prevention of liver ischemia reperfusion injury by a combined thyroid hormone and fish oil protocol

Several preconditioning strategies are used to prevent ischemia–reperfusion (IR) liver injury, a deleterious condition associated with tissue resection, transplantation or trauma. Although thyroid hormone (T₃) administration exerts significant protection against liver IR injury in the rat, its clinical application is controversial due to possible adverse effects. Considering that prevention of liver IR injury has also been achieved by n-3 polyunsaturated fatty acid (n-3 PUFA) supplementation to rats, we studied the effect of n-3 PUFA dietary supplementation plus a lower dose of T₃ against IR injury. Male Sprague-Dawley rats receiving fish oil (300 mg/kg) for 3 days followed by a single intraperitoneal dose of 0.05 mg T₃/kg were subjected to 1 h of ischemia followed by 20 h of reperfusion. Parameters of liver injury (serum transaminases, histology) and oxidative stress (liver contents of GSH and oxidized proteins) were correlated with fatty acid composition, NF-κB activity, and tumor necrosis factor-α (TNF-α) and haptoglobin expression. IR significantly modified liver histology; enhanced serum transaminases, TNF-α response or liver oxidative stress; and decreased liver NF-κB activity and haptoglobin expression. Although IR injury was not prevented by either n-3 PUFA supplementation or T₃ administration, substantial decrease in liver injury and oxidative stress was achieved by the combined protocol, which also led to increased liver n-3 PUFA content and decreased n-6/n-3 PUFA ratios, with recovery of NF-κB activity and TNF-α and haptoglobin expression. Prevention of liver IR injury achieved by a combined protocol of T₃ and n-3 PUFA supplementation may represent a novel noninvasive preconditioning strategy with potential clinical application.     

Increased ability of apomorphine to reduce serum concentrations of prolactin in rats treated chronically with alpha-methyltyrosine

Serum concentrations of prolactin were significantly increased in rats for up to 9 hours after a single i.p. injection of α-methyltyrosine (50 mg/kg); apomorphine caused a dose-dependent reduction of the elevated prolactin concentrations. Doses of apomorphine required to reduce serum prolactin concentrations.were lower in animals previously injected with α-methyltyrosine three times a day for 10 days than in animals which received a single injection of α-methyltyrosine. These results suggest that chronic disruption of the normal release of dopamine from tuberoinfundibular neurons leads to the development of increased sensitivity of dopamine receptors involved with the inhibition of prolactin release from the anterior pituitary.     

Roles of leukotrienes in two rat allergic inflammatory models; IgE-mediated and IgG-antigen complex-induced pleurisies

Rat IgE pleurisy was induced by the injection of dinitrophenol-conjugated bovine serum albumin (DNP-BSA) 48 hours after the intrapleural injection of rat anti-DNP-IgE serum. IgG-BSA complex pleurisy was also induced by the intrapleural injection of IgG-BSA complexes produced at the optimum ratio . Plasma exudation was markedly increased in the first 20 minutes, but not observed thereafter, in IgE pleurisy, whereas marked plasma exudation in the first 20 minutes was followed by weak exudation at three and five hours in IgG-BSA complex pleurisy. Leukotrienes (LTs) E₄ (100 ng/rat), D₄ (32) and B₄ (16) were detected on HPLC in the pleural exudate in the first 20 minutes of IgG-BSA complex pleurisy, but less (9 ng/rat) LTE₄ alone was detected in the five-hour exudate. The first 20-minute pleural exudate contained 13 ng/rat of LTE₄ in IgE pleurisy. The plasma exudation for the initial 20 minutes in IgE pleurisy was completely inhibited by simultaneous treatment of rats with pyrilamine (2.5 mg/kg, i.p.) and methysergide (3 mg/kg, i.p.), as it was in compound 48/80-induced pleurisy. In IgG-BSA complex pleurisy, 90% of the pleural exudate for the first 20 minutes was inhibited by the same treatment, and the rest was completely suppressed by simultaneous treatment with an intrapleural injection of AA-1777, a selective 5-lipoxygenase inhibitor. AA-1777 alone did not reduce the plasma exudation significantly. The 5-lipoxygenase inhibitor was also very effective in reducing the migrating numbers of polymorphonuclear and mononuclear leukocytes to half, without affecting the eosinophils or mast cells.     

Characterization and localization of prostaglandin E1 receptors in rat liver plasma membranes

Methodology for measurement and characterization of prostaglandin binding to membranes has been developed. The binding assay was used to study the presence of prostaglandin receptors in high purified cell fractions derived from rat liver. High affinity binding receptors which have a saturation value of 1.0 pmole/mg protein and a dissociation constant of 1.2 nM were found exclusively in the plasma membrane. High affinity receptors were not found in cell fractions containing nuclei, rough microsomes. Golgi complex or mitochondria. The binding by other prostaglandins was competitive with prostaglandin E₁. Competitive binding studies were used to obtain dissociation constants for prostaglandins F_1α, F_2α, B₁, B₂, A₁, A₂, and 15-keto prostaglandin E₂ which were 1100, 100, 300, 180, 16. 16 and 700 nM, respectively. Eicosa-5.8.11.19-tetraynoic acid, an inhibitor of prostaglandin synthesis did not bind appreciably to the prostaglandin E receptor, whereas two prostaglandin analogues, which have high physiological activity compete effectively with prostaglandin E₁ for the receptor. Thus, the binding receptor for the E-type prostaglandins is highly specific both with respect to cell localization as well as the type of substrate. Numerical routines for the fitting of the data and a procedure for the determination of the specific activity of the labelled prostaglandin are provided.