Effects of a single administration of prostaglandin F2alpha, or a combination of prostaglandin F2alpha and prostaglandin E2, or placebo on fertility variables in dairy cows 3–5 weeks post partum, a randomized, double-blind clinical trial

Background  

Delayed uterine involution has negative effects on the fertility of cows; use of prostaglandin F2alpha alone as a single treatment has not been shown to consistently improve fertility. Combined administration of PGF2alpha and PGE2 increased uterine pressure in healthy cows. We hypothesized, that the combination of both prostaglandins would accelerate uterine involution and have, therefore, a positive effect on fertility variables. In commercial dairy farming, the benefit of a single post partum combined prostaglandin treatment should be demonstrated.     

Developmental sensitivity of the bovine corpus luteum to prostaglandin F2alpha (PGF2alpha) and endothelin-1 (ET-1): is ET-1 a mediator of the luteolytic actions of PGF2alpha or a tonic inhibitor of progesterone secretion?

We examined the responsiveness of large luteal cells (LLC), small luteal cells (SLC), and endothelial cells of the Day 4 and Day 10 bovine corpus luteum (CL) to prostaglandin (PG) F2alpha and endothelin (ET)-1. Using a single-cell approach, we tested the ability of each agonist to increase the cytoplasmic concentration of calcium ions ([Ca2+]i) as function of luteal development. All tested concentrations of agonists significantly (P = 0.05) increased [Ca2+]i in all cell populations isolated from Day 4 and Day 10 CL. Day 10 steroidogenic cells were more responsive than Day 4 cells to PGF2alpha and ET-1. Response amplitudes and number of responding cells were affected significantly by agonist concentration, luteal development, and cell type. Response amplitudes were greater in LLC than in SLC; responses of maximal amplitude were elicited with lower agonist concentrations in Day 10 cells than in Day 4 cells. Furthermore, on Day 10, as the concentration of PGF2alpha increased, larger percentages of SLC responded. Endothelial cells responded maximally, regardless of agonist concentration and luteal development. In experiment 2, we tested the developmental responsiveness of total dispersed and steroidogenic-enriched cells to the inhibitory actions of PGF2alpha and ET-1 on basal and LH-stimulated progesterone accumulation. The potency of PGF2alpha steroidogenic-enriched cells on Day 4 was lower than on Day 10; in contrast, the potency of ET-1 was not different. Therefore, ET-1 was a tonic inhibitor of progesterone accumulation rather than a mediator of PGF2alpha action. The lower efficacy of PGF2alpha in the early CL more likely is related to signal transduction differences associated with its receptor at these two developmental stages than to the inability of PGF2alpha to up-regulate ET-1.     

Is tumor necrosis factor alpha a trigger for the initiation of endometrial prostaglandin F(2alpha) release at luteolysis in cattle?

To determine the physiological significance of tumor necrosis factor alpha (TNFalpha) in the regulation of luteolytic prostaglandin (PG) F(2alpha) release by the bovine endometrium, the effect of TNF-alpha on PGF(2alpha) output by the endometrial tissues in vitro was investigated and compared with the effect of oxytocin (OT). Furthermore, the presence of specific receptors for TNFalpha in the bovine endometrium during the estrous cycle was determined. Endometrial slices (20-30 mg) taken from six stages of the estrous cycle (estrus: Day 0; early I: Days 2-3; early II: Days 5-6; mid-: Days 8-12; late: Days 15-17; and follicular: Days 19-21), as determined by macroscopic examination of the ovaries and uterus, were exposed to TNFalpha (0.06-6 nM) and/or OT (100 nM). OT stimulated PGF(2alpha) output at the follicular stage and at estrus (P < 0.001), but not at the late luteal stage. On the other hand, the stimulatory effects of TNFalpha on PGF(2alpha) output were observed not only at the follicular stage but also at the late luteal stage (P < 0.001). When the endometrial tissues at late luteal stage were simultaneously exposed to TNFalpha (0.6 nM) and OT (100 nM), the stimulatory effect on PGF(2alpha) output was higher than the effect of TNFalpha or OT alone (P < 0.05). Specific binding of TNFalpha to the bovine endometrial membranes was observed throughout the estrous cycle. The concentration of TNF-alpha receptor at the early I luteal stage was less than the concentrations at other luteal stages (P < 0.01). The dissociation constant (K(d)) values of the endometrial membranes were constant during the estrous cycle. The overall results lead us to hypothesize that TNFalpha may be a trigger for the output of PGF(2alpha) by the endometrium at the initiation of luteolysis in cattle.     

Does injection of prostaglandin F(2alpha) (PGF2alpha) cause ovulation in anestrous Western White Face ewes?

In a previous study in our laboratory, treatment of non-prolific Western White Face (WWF) ewes with PGF(2 alpha) and intravaginal sponges containing medroxyprogesterone acetate (MAP) on approximately Day 8 of a cycle (Day 0 = first ovulation of the interovulatory interval) resulted in ovulations during the subsequent 6 days when MAP sponges were in place. Two experiments were performed on WWF ewes during anestrus to allow us to independently examine if such ovulations were due to the direct effects of PGF(2 alpha) on the ovary or to the effects of a rapid decrease in serum concentrations of progesterone at PGF(2 alpha)-induced luteolysis. Experiment 1: ewes fitted with MAP sponges for 6 days (n = 12) were injected with PGF(2 alpha) (n = 6; 15 mg im), or saline (n = 6) on the day of sponge insertion. Experiment 2: ewes received progesterone-releasing subcutaneous implants (n = 6) or empty implants (n = 5) for 5 days. Six hours prior to implant removal, all ewes received a MAP sponge, which remained in place for 6 days. Ewes from both experiments underwent ovarian ultrasonography and blood sampling once daily for 6 days before and twice daily for 6 days after sponge insertion. Additional blood samples were collected every 4 h during sponge treatment. Experiment 1: 4-6 (67%) PGF(2 alpha)-treated ewes ovulated approximately 1.5 days after PGF(2 alpha) injection; these ovulations were not preceded by estrus or a preovulatory surge release of LH, and resulted in transient corpora hemorrhagica (CH). The growth phase was longer (P < 0.05) and the growth rate slower (P < 0.05) in ovulating versus non-ovulating follicles in PGF(2 alpha)-treated ewes. Experiment 2: in ewes given progesterone implants, serum progesterone concentrations reached a peak (1.7 2 ng/mL; P < 0.001) on the day of implant removal and decreased to basal concentrations (<0.17 ng/mL; P < 0.001) within 24 h of implant removal. No ovulations occurred in either the treated or the control ewes. We concluded that ovulations occurring after PGF(2 alpha) injection, in the presence of a MAP sponge, could be due to a direct effect of PGF(2 alpha) at the ovarian level, rather than a sudden decline in circulating progesterone concentrations.     

Coupling between cyclooxygenases and prostaglandin F2α synthase: Detection of an inducible,glutathione-activated,membrane-bound prostaglandin F2α-synthetic activity

Distinct functional coupling between cyclooxygenases (COXs) and specific terminal prostanoid synthases leads to phase-specific production of particular prostaglandins (PGs). In this study, we examined the coupling between COX isozymes and PGF synthase (PGFS). Co-transfection of COXs with PGFS-I belonging to the aldo-keto reductase family into HEK293 cells resulted in increased production of PGF_2α only when a high concentration of exogenous arachidonic acid (AA) was supplied. However, this enzyme failed to produce PGF_2α from endogenous AA, even though significant increase in PGF_2α production occurred in cells transfected with COX-2 alone. This poor COX/PGFS-I coupling was likely to arise from their distinct subcellular localization. Measurement of PGF_2α-synthetic enzyme activity in homogenates of several cells revealed another type of PGFS activity that was membrane-bound, glutathione (GSH)-activated, and stimulus-inducible. In vivo, membrane-bound PGFS activity was elevated in the lung of lipopolysaccharide-treated mice. Taken together, our results suggest the presence of a novel, membrane-associated form of PGFS that is stimulus-inducible and is likely to be preferentially coupled with COX-2.     

Colocalization of prostaglandin F2�� receptor FP and prostaglandin F synthase-I in the spinal cord

Prostaglandin F_2α is synthesized by prostaglandin F synthase, which exists in two types, prostaglandin F synthase I (PGFS I) and prostaglandin F synthase II (PGFS II). Prostaglandin F_2α binds to its specific receptor, FP. Our previous immunohistochemical study showed the distinct localization of prostaglandin F synthases in rat spinal cord. PGFS I exists in neuronal somata and dendrites in the gray substance, and PGFS II exists in ependymal cells and tanycytes surrounding the central canal. Both enzymes are also present in endothelial cells of blood vessels in the white and gray substances of the spinal cord. In this study, we found that FP localizes in neuronal somata and dendrites but not in ependymal cells, tanycytes, or endothelial cells. Immunohistochemical analysis of serial sections showed the colocalization of FP and PGFS I. FP immunoreactivity was intense in spinal laminae I and II of the dorsal horn, a connection site of pain transmission, and was similar to that of PGFS I in neuronal elements. These findings suggest that prostaglandin F_2α synthesized in the neuronal somata and dendrites exert an autocrine action there.—Suzuki-Yamamoto, T., K. Toida, Y. Sugimoto, and K. Ishimura. Colocalization of prostaglandin F_2α receptor FP and prostaglandin F synthase-I in the spinal cord.     

Induction of therapeutic abortion with a single dose of intra-amniotically administered prostaglandin F2α

To determine the abortifacient effectiveness and complications of a single intra-amniotic injection of 50 mg of Prostaglandin F_2α (PGF_2α), 40 gravidas were studied. While all subjects received a 50 mg dose of PGF_2α at the initiation of the trial with no additional oxytocics or surgical intervention until they had aborted or until the end of the 48-hour trial period, they received intramuscularly administered prochlorperazine by one of two dose schedules. Twenty-five Group I subjects received 10 mg of prochlorperazine whenever they requested medication for alleviation of nausea or vomiting, while 15 Group II subjects received 10 mg one-half hour prior to the administration of PGF_2α and at 6-hour intervals until they had aborted. Within the initial 24 hours, 77% of the subjects aborted while within the 48-hour trial period, 95% of the subjects aborted with a mean induction-to-abortion time of 19.1 hours for those aborting. Sixty-eight percent aborted completely, 28 percent aborted incompletely, and 5 percent failed to abort within the trial period. No serious complications were observed. The proportion of patients having no vomiting and the mean number of episodes of vomiting were significantly less in the Group II subjects than in the Group I subjects. No significant differences in the mean abortion times, cumulative abortion rates, or intra-amniotic pressures were noted between the two groups of subjects. It appears that the single intra-amniotic administration of 50 mg of PGF_2α results in practicable rates of abortion and the associated vomiting can be significantly attenuated with prochlorperazine without significantly altering the abortifacient or oxytocic effect of PGF_2α.     

Serum selenium predicts levels of F2-isoprostanes and prostaglandin F2α in a 27 year follow-up study of Swedish men

Low concentrations of selenium (Se) predict mortality and cardiovascular diseases in some populations. The effect of Se on in vivo indicators of oxidative stress and inflammation, two important features of atherosclerosis, in human populations is largely unexplored. This study investigated the longitudinal association between serum selenium (s–Se) and a golden standard indicator of oxidative stress in vivo (8-iso-prostaglandin F_2α, a major F₂-isoprostane), an indicator of cyclooxygenase (COX)-mediated inflammation (prostaglandin F_2α), high sensitive C-reactive protein (hsCRP), interleukin-6 (IL-6) and serum amyloid A protein (SAA) in a follow-up study of 27 years. The s–Se was measured in 615 Swedish men at 50 years of age in a health investigation. The status of oxidative stress and inflammation was evaluated in a re-investigation 27 years later by quantification of urinary 8-iso-PGF_2α and 15-keto-dihydro- PGF_2α (a major metabolite of PGF_2α) and serum hsCRP, SAA and IL-6. Men in the highest quartile of s–Se at age 50 had decreased levels of 8-iso-PGF_2α compared to all lower quartiles and decreased levels of PGF_2α compared to all lower quartiles at follow-up. These associations were independent of BMI, diabetes, hyperlipidemia, hypertension, smoking, α-tocopherol and β-carotene at baseline. The s–Se was not associated with hsCRP, SAA or IL-6 at follow-up. In conclusion, high concentrations of s–Se predict reduced levels of oxidative stress and subclinical COX-mediated (but not cytokine-mediated) inflammation in a male population. The associations between Se, oxidative stress and inflammation, respectively, might be related to the proposed cardiovascular protective property of Se.     

Is the inhibitory effect of progesterone on endometrial prostaglandin F2 alpha production due to an inhibition of protein synthesis?

Progesterone and a high concentration of oestradiol (i) reduced the outputs of prostaglandin (PG) F2 alpha and, to a lesser extent, PGE2 from Day-7 and Day-15 guinea-pig endometrium in culture, but had little or no effect on the output of 6-keto-PGF1 alpha, (ii) prevented the increase in PGH synthase concentrations which normally occur in Day-7 and Day-15 guinea-pig endometrium during culture, and (iii) reduced the synthesis of secreted proteins by Day-15 guinea-pig endometrium in culture. These findings suggest that the inhibitory effect of progesterone and of high concentrations of oestradiol on endometrium PGF2 alpha synthesis is due to an inhibition of the syntheses of proteins involved in PGF2 alpha production.     

Enzymatic formation of prostaglandin F2α in human brain

Prostaglandin (PG)E₂ 9-ketoreductase, which catalyzes the conversion of PGE₂ to PGF₂, was purified from human brain to apparent homogeneity. The molecular weight, isoelectric point, optimum pH, Km value for PGE₂, and turnover number were 34,000, 8.2, 6.5–7.5, 1.0 mM, and 7.6 min^–1, respectively. Among PGs tested, the enzyme also catalyzed the reduction of other PGs such as PGA₂, PGE₁, and 13,14-dihydro-15-keto PGF₂, but not that of PGD₂, 11-PGE₂, PGH₂, PGJ₂, or ¹²-PGJ₂. The reaction product formed from PGE₂ was identified as PGF₂, by TLC combined with HPLC. This enzyme, as is the case for carbonyl reductase, was NADPH-dependent, preferred carbonyl compounds such as 9,10-phenanthrenequinone and menadione as substrates, and was sensitive to indomethacin, ethacrynic acid, and Cibacron blue 3G-A. The reduction of PGE₂ was competitively inhibited by 9,10-phenanthrenequinone, which is a good substrate of this enzyme, indicating that the enzyme catalyzed the reduction of both substrates at the same active site. These results suggest that PGE₂ 9-ketoreductase, which belongs to the family of carbonyl reductases, contributes to the enzymatic formation of PGF₂ in human brain.Special issue dedicated to Dr. Sidney Udenfriend.     

Pulses of prolactin before, during, and after luteolysis and synchrony with pulses of a metabolite of prostaglandin F2α in heifers

Pulses of prolactin (PRL) and a metabolite of prostaglandin F2α (PGFM) were determined from hourly blood samples collected before, during, and after luteolysis (n=7 heifers). Progesterone concentrations were used to partition the results into six 12-h sets from 12h before to 36h after luteolysis. Pulses of PRL with a nadir-to-nadir interval of 4.4±0.2h were detected in each 12-h set. Pulses were rhythmic (P<0.05) in six heifers, beginning 12h before the end of luteolysis. The peak of a PRL pulse was greater (P<0.05) for the 12h after the end of luteolysis than for other 12-h sets, except for the last set of luteolysis. Area under the curve of a pulse was greater (P<0.05) for the 24h that encompassed the end of luteolysis than for two previous 12-h sets. Synchrony between the peaks of PRL and PGFM pulses was greater (P<0.03) during and after luteolysis (same hour for 29/39 pairs) than before luteolysis (0/12). Concentration of PRL centralized to the peak (Hour 0) of PGFM pulses was greater (P<0.05) at Hours 0 and 1 than at Hours -2, -1, and 3. Results supported the hypothesis that PRL is secreted in pulses in heifers. The pulses were most prominent and rhythmic during the last 12h of luteolysis and thereafter. The pulse peaks of PRL and PGFM were synchronized for most PRL pulses during and after luteolysis.     

Induction of parturition in the mare with prostaglandin F2α

Thirty-one mares of Quarter Horse and Thoroughbred breeding were utilized in two experiments to evaluate the efficacy of prostaglandin F₂α (PGF₂α)_for induction of equine parturition and to monitor the effects of this treatment on viability of the resulting foals.Three of five mares given 5 mg PGF₂α (im) on day 338 of gestation foaled 19.6 ± 8.2 hr postinjection. In the second experiment immediately following 3 daily injections of 10 mg estradiol cypionate (ECP) given on days 326, 327 and 328 of gestation, seven mares were infused (iv) with PGF₂α at the rate of 1.3 mg/hr for 24 hr or until parturition occurred. Four of the seven mares foaled in 8.8 ± 1.8 hr after the start of infusion. Side effects including sweating, hypothermia, increased respiration rate and diarrhea were evident in both injected and infused mares, but effects were transient. Neither the injection, nor infusion route of administration of PGF₁α adversely affected the viability of foals. However, some mares induced to foal 12 days prior to expected parturition had foals with slightly weaker pasterns than those of control mares.     

Doses of prostaglandin F2α effective for induction of parturition in goats

Twelve mixed breed does were injected with different doses of prostaglandin F₂α (PGF₂α) or saline on day 144 of gestation. Four each received single intramuscular injections of 5.0 or 2.5 mg PGF_2α, or 1.0 ml saline (controls). Systemic progesterone (P₄) concentrations were determined daily from day 144 until the day of kidding. Does receiving 5.0 mg PGF₂α, 2.5 mg PGF₂α, or saline kidded within mean (± SD) hours and range (hours) of 35 ± 8.6 and 28–48, 43 ± 11.8 and 29–57, and 111 ± 79.1 and 41–200, respectively. Mean (± SD) concentrations of P₄ (ng/ml) on the day of injection and on day 1 postinjection were 5.2 ± 2.6 and 0.7 ± 0.9, 5.3 ± 2.2 and 1.1 ± 1.0, and 6.4 ± 3.9 and 4.1 ± 2.6 for does receiving 5.0 mg PGF₂α, 2.5 mg PGF₂α, or saline, respectively. It was concluded that 5.0 mg and 2.5 mg PGF₂α effectively shortened the interval from injection to parturition, but that this interval was not as predictable as that previously reported with 20 mg PGF₂α.     

Temporal interrelationships at 15-min intervals among oxytocin, LH, and progesterone during a pulse of a prostaglandin F2α metabolite in heifers

A pulse of a PGF2α metabolite (PGFM) was induced by treatment with 0.1 mg of estradiol-17β on Day 15 (Day 0=ovulation; n=9 heifers). Blood samples were taken every 15 min for 9h beginning at treatment (Hour 0). For PGFM and LH, an intraassay-CV method was used to detect fluctuations in the 15-min samples and pulses in the hourly samples. A mean of 6.9 ± 0.4 PGFM fluctuations/9 h were superimposed on the hourly PGFM concentrations, compared to 2.1 ± 0.5 LH fluctuations/9 h (P<0.02). An increase (P<0.02) in oxytocin began 15 min before the beginning nadir of the PGFM pulse. A transient increase in progesterone did not occur at the beginning nadir of the PGFM pulse. Progesterone decreased (P<0.02) during the ascending portion and increased (P<0.03) as a rebound during the descending portion of the PGFM pulse. The peak of an LH pulse occurred 1.5 ± 0.4 h (range, 0.25-2.75 h) after the peak of the PGFM pulse. The wide range in the interval from a PGFM peak to an LH peak obscured the contribution of increasing LH to the rebound. The results did not support the hypothesis that oxytocin and PGFM increase concurrently. Results supported the hypothesis that the immediate transient progesterone increase that has been demonstrated with exogenous PGF2α does not occur during the ascending portion of an endogenous PGFM pulse. The hypothesis that the progesterone rebound after the peak of a PGFM pulse is temporally related to an LH pulse was supported.     

Effect of prostaglandin F2α on anterior pituitary function in man

Five healthy adult men received iv PGF_2α at dosages of 0.05, 0.20 and 2.0 μg/kg/min for 30 min. There were no significant changes in serum FSH, LH or TSH levels. Serum GH and cortisol levels were slightly increased at the highest dosage. These responses were associated with, and presumably a result of, stressful side effects. Thus, PGF_2α cannot be used as a provocative test of pituitary hormone reserve.Prostaglandins (PG's) have recently been implicated in the release of a number of hormones from the anterior pituitary gland. The stimulation of GH release by PG's of the E series from incubated rat pituitary slices has been demonstrated. $\text{In vivo}$ stimulation by PGE₁ of ACTH in rats and of GH release in man has also been shown.The present study was undertaken in order to examine the efficacy of iv administration of PGF_2α as a provocative test of anterior pituitary hormone reserve in man. The responses in circulating levels of gonadotropins, TSH, GH, and cortisol (as an index of ACTH) were measured.     

Effects of prostaglandin F2α, cloprostenol and fenprostalene on uterine clearance of radiocolloid in the mare

Uterine clearance of radiocolloid was measured by scintigraphy in 5 reproductively normal mares and in 4 mares exhibiting a delay in uterine clearance (DUC) after administration of PGF_2α, cloprostenol or fenprostalene. Scintigraphy studies were performed on the second or third day of estrus during 3 consecutive estrous cycles. Drugs, PGF_2α (5 mg IM), cloprostenol (CLO; 2mg IM) or fenprostalene (FEN; 250 μg subcutaneously) were given in random order, with only 1 drug given each estrus. Treatment response curves were generated, and the effect of each drug on uterine clearance of radiocolloid was compared to the clearance of radiocolloid when no drug was given and between treatments. In reproductively normal mares, CLO and PGF caused a rapid clearance of radiocolloid within 60 min (P < 0.01), with <25% of the initial dose of radiocolloid (% IDR) remaining by 120 min. Mean percentage of IDR at 120 min when no drug was given was 39% ± 4. Response of reproductively normal mares to FEN varied, with 3 mares clearing > 85% and 2 mares clearing <35%. In mares exhibiting DUC, all 3 drugs (CLO, PGF and FEN) caused rapid clearance of radiocolloid from the uterus by 60 min (P < 0.0001). Mares cleared significantly more colloid after treatment with CLO at 60 and 120 min than after PGF (P < 0.001). In conclusion, CLO appears to be the best drug of the 3 tested for stimulating clearance of intrauterine fluid since variation in response was observed following treatment with PGF and FEN.     

Comparison of the pulmonary vascular response to prostaglandin A1, prostaglandin F2α and angiotensin II; Metabolism of PGA1 in lung

A rabbit lung preparation, perfused $\text{in vitro}$, was used to examine pulmonary metabolism of prostaglandin A₁ (PGA₁) and to compare the vasoconstrictor actions of PGA₁, prostaglandin F_2α (PGF_2α) and angiotensin II. PGF_2α caused significantly more, and angiotensin II significantly less, vasoconstriction than did an equimolar concentration of PGA₁. Of three likely PGA₁ metabolites only 15-keto-PGA₁ had any significant vasoconstrictor action. Furosemide and aminophylline (10^?3 M) reduced PGA₁, PGF_2α or angiotensin II-induced vasoconstruction. Diphloretin phosphate potentiated the vascular effect of angiotensin II. Furosemide (10^?3 M) and DPP (9.5 × 10^?6 M) significantly reduced pulmonary metabolism of PGA₁ while aminophylline (10^?3 M) had no effect on this process. Perfusion of the lungs with a hypoxic medium had no effect on PGA₁ metabolism.     

Utero-ovarian venous concentrations of prostaglandin E2 (PGE2 and prostaglandin F2α (PGF2α) following PGE2 intrauterine infusions

The uterine horns and utero-ovarian veins of nine crossbred mature gilts were bilaterally cannulated on day 9 of the estrous cycle (day 0 - first day of estrus). Each uterine horn in treated gilts (N=5) was infused with 150 μg PGE₂ in 3 ml of saline at 0900 h on day 12, 15 and 18 of the estrous cycle. Control gilts (N=4) received 3 ml saline intrauterine infusions on the corresponding day. Blood samples were collected from the utero-ovarian veins 15 min before each infusion and for the following 6 h with 15, 30 and 60 min intervals through the first, second and third two-hour periods, respectively. Venous concentrations of PGE₂ and PGF_2α were determined by radioimmunoassay procedures. Infusion of PGE₂ resulted in an immediate elevation in PGE₂ concentration in utero-ovarian venous drainage. Coincident elevations of PGF_2α utero-ovarian venous concentrations were observed after PGE₂ infusion. Plasma PGF_2α concentrations in the utero-ovarian veins were elevated (P<.01) in PGE₂ treated gilts for one hour post-treatment. The duration of PGE₂ and PGE₂α elevations as well as the peak values were influenced by day of the cycle.     

Desensitization of prostaglandin F2α receptor-mediated phosphoinositide hydrolysis in cultured rat astrocytes

Desensitization of prostaglandin (PG) F₂ receptor-mediated phosphoinositide (PI) hydrolysis was investigated in cultured rat astrocytes. Prolonged exposure of astrocytes differentiated by dibutyryl cyclic AMP-treatment to PGF₂ caused the desensitization of subsequent PGF₂-induced PI hydrolysis. The desensitization was time- and PGF₂ dose-dependent; maximal decrease in the PI hydrolysis was observed after exposure to 10 M PGF₂ for 4 h and the degree of the desensitization was 31.7±2.7% of control. Pretreatment with either PGD₂ or PGE₂ also induced the desensitization of subsequent PGF₂-stimulated PI hydrolysis and conversely pretreatment of PGF₂ decreased the PI responses to PGD₂ and PGE₂. The desensitization prevented by phloretin and was reversible upon removal of the agonist. Protein synthesis inhibitors blocked the recovery of the desensitization. Treatment of the cells with phorbol 12-myristate 13-acetate had no effect on the desensitization. These results suggest that prolonged exposure of the astrocytes to PGF₂ caused the desensitization of the receptors.