PGF2α-induced loss of corpus luteum gonadotrophin receptors

In experiments and we have previously shown that PGF_2α directly antagonized the action of gonadotrophins on the corpus luteum. To determine if this action of PGF_2α may occur as a consequence of an induced loss of gonadotrophin receptors, binding of hCG to rat luteal tissue was measured following PGF_2α treatment . In immature rats which were treated with exogenous gonadotrophin to luteinize the gonads, PGF_2α produced a marked and highly significant decrease in circulating progesterone when administered 24 hours before sacrifice. Although the affinity constant (Ka; 1.2-2 × 10¹⁰ L/M) of the luteal receptor to hCG was not affected, PGF_2α treatment produced a marked fall in the binding capacity of the luteal tissue to hCG. This response was absent, however, when PGF_2α was incubated directly with luteal receptor or administered during early pseudopregnancy when corpora lutea are more resistant to luteolysis. Experiments are in progress to determine if the decrease in capacity of luteal receptors to bind hCG is the mechanism or a consequence of luteolysis produced by PGF_2α.     

Activation of phospholipase D by prostaglandin F2α in rat luteal cells and effects of inhibitors of arachidonic acid metabolism

In rat luteal cells labeled with (³H]oleic acid, PGF_2α-stimulated phospholipase D (PLD) activation was investigated. The PLD activity was detected by measuring the accumulation of [³H]phosphatidylethanol (PtdEt) in the presence of ethanol. PGF_2α stimulated PtdEt accumulation at concentrations of more than 100 nM in the presence of ethanol. However, PtdEt accumulation did not change in the absence of ethanol. PGF_2α (1 μM) increased PtdEt accumulation after 1 min, and the accumulation reached a plateau by 2–3 min. These results indicate that PGF_2α activates PLD in rat luteal cells. U-73122, a phospholipase C (PLC) inhibitor, and staurosporine, a protein kinase C (PKC) inhibitor, did not inhibit PGF_2α-stimulated [³H]PtdEt accumulation. These results suggest that PGF_2α-induced PLD activation is different from PLC-PKC systems. We reported previously that PGF_2α stimulated the release of arachidonic acid. The effects of indomethacin, nordihydroguaiaretic acid (NDGA), and 5,8,11,14-eicosatetraynoic acid (ETYA), inhibitors of arachidonic acid metabolism, on PGF_2α-stimulated PtdEt accumulation were examined. Pretreatment with indomethacin enhanced PGF_2α-induced PtdEt accumulation. In contrast, pretreatment with NDGA and ETYA inhibited PGF_2α-induced PtdEt accumulation. It is suggested that PGF_2α-stimulated PLD activation is mediated via lipoxygenase products.     

PGE2 attenuates PGF2α-induced increases in free intracellular calcium in ovine large luteal cells

When ovine large luteal cells are placed in culture and exposed to PGF_2α, there is a rapid and sustained increase in the concentration of free intracellular calcium which is believed to play a major role in the luteolytic and cytotoxic effects of PGF_2α. Since administration of exogenous PGE₂ can prevent spontaneous and PGF_2α-induced luteolysis in vivo, and the cytotoxic effects of PGF_2α on large luteal cells in vitro, the objective of this study was to determine if one mechanism by which PGE₂ acts is to attenuate increases in free intracellular calcium induced by PGF_2α. At concentrations of 10 nM or greater, PGF_2α caused a significant and sustained increase in free intracellular calcium in large luteal cells. Similarly, PGE₂ also induced increases in free intracellular calcium but required doses 20-fold greater than PGF_2α. When PGE₂ (1, 10 or 100 nM) was incubated with PGF_2α (100 nM) increases in free intracellular calcium induced by PGF_2α were attenuated (P<0.05) when measured 5 min, but not at 30 min, after initiation of treatment. The observed decrease in the concentration of free intracellular calcium at 5 min in response to PGF_2α was the result of fewer cells responding to PGF_2α. In addition, the concentrations of free intracellular calcium attained in the cells that did respond was reduced 25% compared to cells treated with PGF_2α alone. Thus, part of the luteal protective actions of PGE₂ appears to involve an inhibition of the early (5 min) increase in free intracellular calcium induced by PGF_2α.     

Corpus luteum susceptibility to prostaglandin F2α (PGF2α) luteolysis in hysterectomized prepuberal and mature gilts

The susceptibility of induced corpora lutea (CL) of prepuberal gilts and spontaneously formed CL of mature gilts to prostaglandin F_2α (PGF_2α) luteolysis was studied. Prepuberal gilts (120 to 130 days of age) were induced to ovulate with Pregnant Mare Serum Gonadotropin and Human Chorionic Gonadotropin (HCG). The day following HCG was designated as Day 0. Mature gilts which had displayed two or more estrous cycles of 18 to 22 days were used (onset of estrus = Day 0). Gilts were laparotomized on Day 6 to 9, their CL marked with sterile charcoal and totally hysterectomized. On Day 20, gilts were injected IM with either distilled water (DW), 2.5 mg PGF_2α or 5.0 mg PGF_2α. An additional group of prepuberal gilts was injected with 1.25 mg PGF_2α, a dose of PGF_2α equivalent, on a per kilogram body weight basis, to the 2.5 mg PGF_2α dose given to the mature gilts. The percentages of luteal regression on Day 27 to 30 for mature and prepuberal gilts given DW, 2.5 mg PGF_2α and 5.0 mg PGF_2α were 0.0 vs 4.4, 43.5 vs 96.8 and 47.7 vs 91.6, respectively; the percentage of luteal regression for the prepuberal gilts given 1.25 mg PGF_2α was 75.1. These results indicate that induced CL of the prepuberal gilt were more susceptible to PGF_2α luteolysis than spontaneously formed CL of the mature gilt and that pregnancy failure in the prepuberal gilt could be due to increased susceptibility of induced CL to the natural luteolysin.     

Effect of prostaglandin F2α on endocrine-ovarian function in the domestic cat

The effect of prostaglandin F_2α(PGF_2α) on endocrine and ovarian function during the early luteal phase of the domestic cat was investigated. Queens were induced to ovulate and then injected subcutaneously with 0.5–5.0 mg PGF_2α/kg body weight. The greatest dose was found to approach toxicity. Concentrations of progesterone were similar in cats following treatment with PGF_2α compared to values of controls. Development and regression of corpora lutea as determined by serial laparoscopy were similar in all groups. These data indicate that PGF_2α at the tested dosages, given during the early luteal phase is not luteolytic in this species and suggest that these regimens would be ineffective for the premature termination of pseudopregnancy.     

Reduction by human chorionic gonadotropin of the luteolytic effect of prostaglandin F2 in ewes

The ability of human chorionic gonadotropin (HCG) to reduce the luteolytic effect of prostaglandin (PGF_2^α) was demonstrated in cycling ewes. As expected, treatment with 10 mg of PGF_2^α alone on Day 10 of the estrous cycle exerted a potent negative effect on the function and structure of corpus luteum (CL) as indicated by reduced plasma progesterone, CL progesterone, and CL weight. However, the identical PGF_2^α treatment failed to significantly reduce either luteal function or luteal weight when administered to ewes that were also treated with HCG on Days 9 and 10 of the estrous cycle. Treatment with HCG alone had a positive effect on CL as indicated by increased plasma progesterone, CL progesterone, and CL weight. Treatment with HCG did not render the CL totally insensitive to the negative effects of PGF_2^α because plasma progesterone was reduced when the dose of PGF_2^α was doubled. Whether CL regressed or continued to function after treatment with both HCG and PGF_2^α appeared to depend upon a balance between the positive and negative effects of the two hormones.     

Dose and time dependent luteotropic and luteolytic action of prostaglandin F2α in the luteinized rabbit ovary

The mechanism of stimulatory and inhibitory action of PGF_2α on ovarian steroidogenesis both under and conditions has been studied in the pseudo-pregnant rabbits. Short term incubation of the ovaries with PGF_2α (2.82 × 10⁻⁵M) resulted in an increased synthesis of progesterone and 20α-OH P. The addition of PGF_2α in the medium and further incubation of the ovaries obtained from rabbits that had been constantly infused with PGF_2α (0.5 μg/min.) for two hours resulted in increased synthesis of these progestins. The ratio of progesterone to 20α -OH P was also enhanced under these conditions and thus supported the luteotropic action of small doses of PGF_2α under short term incubations. However, as the amount of PGF_2α infused was increased to 5 μg/min., the addition of PGF_2α under conditions strikingly decreased the production of these progestins. The ratio of progesterone to 20α -OH P was also decreased and thus was indicative of luteolytic action of higher doses of PGF_2α. High doses of PGF_2α (5.64 × 10⁻⁴M) failed to I cause any significant change in the progestin synthesis under short term incubation. These results thus suggest that the luteotropic and luteolytic action of PGF_2α in the luteinized rabbit ovary is dose and time dependent.     

Prostaglandin E1 and F2α specific binding in bovine corpora lutea: Comparison with luteolytic effects

Preliminary characterization indicated the presence of separate prostaglandin (PG)E₁ and (PG)F_2α 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 × 10⁻⁹M and 2.1 × 10⁻⁸M for PGE₁ and PGF_2α, respectively. Competition of several natural prostaglandins for the PGE₁ and PGF_2α bovine luteal specific binding sites indicates specificity for the 9-keto or 9α-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 PGF_2α as measured by reduced estrous cycle length, decreased corpus luteum size and significantly decreased plasma progesterone levels. In contrast, 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 on PGF_2α relative binding affinity to the bovine CL can be compared to data obtained independently on PGF_2α induced luteolysis in the bovine, PGF_2α 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.     

Prostaglandin E1 and F2α specific binding in bovine corpora lutea: Comparison with luteolytic effects

Preliminary characterization indicated the presence of separate prostaglandin (PG)E₁ and (PG)F_2α 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 × 10⁻⁹M and 2.1 × 10⁻⁸M for PGE₁ and PGF_2α, respectively. Competition of several natural prostaglandins for the PGE₁ and PGF_2α bovine luteal specific binding sites indicates specificity for the 9-keto or 9α-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 PGF_2α as measured by reduced estrous cycle length, decreased corpus luteum size and significantly decreased plasma progesterone levels. In contrast, 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 PGF_2α relative binding affinity to the bovine CL can be compared to data obtained independently in vitro on PGF_2α induced luteolysis in the bovine, PGF_2α 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.     

An intra-corpus luteum site for the luteolytic action of prostaglandin F2α in the rhesus monkey

It has not been possible to demonstrate prostaglandin F₂α (PGF₂α) participation in primate luteolysis under conditions of systemic administration or of acute intraluteal injection. These study designs were hampered by the short biological half-life in the first instance and brevity of administration in the latter. In this study, luteolysis has resulted from chronic, intraluteal delivery of PGF₂ α. Using the Alzet osmotic pump-cannula system, normally cycling rhesus monkeys were continuously infused, until menses occurred, with PGF₂ α (10 ng/1/hr) directly into the corpus luteum (CL, n=6), into the stroma of the ovary bot bearing the corpus luteum (NCL, n=3), or subcutaneously (SC, n=5). An additional 5 monkeys received vehicle (V) into the corpus luteum. All experiments commenced 5–7 days after the preovulatory estradiol surge. Luteal function was assessed by the daily measurements of plasma progesterone, estradiol, and LH. Intraluteal PGF₂α caused premature functional luteolysis in all monkeys, as reflected by a highly significant decline in circulating progesterone and estradiol and the early onset of menstruation, when compared to the other groups. V, NCL, and SC infusions had no effect on either circulating steroid levels or luteal phase lengths. None of the experimental groups showed any change in plasma LH concentrations. These are the first data to indicate that PGF₂α can induce functional luteolysis in the primate, and the site of action appears to be the corpus luteum.     

Induction of 20α-hydroxysteroid dehydrogenase in rat corpora lutea of pregnancy by prostaglandin F2α

20α-OH-SDH is a marker of luteolysis in rat corpora lutea and appearance of this enzyme is inhibited by prolactin but stimulated by LH or hCG. PGF₂α induced 20 α-OH-SDH activity in corpora lutea of pregnant rats and a significant fall in peripheral plasma progesterone concentrations when administered i.m. for two consecutive days. Rats treated with PGF₂ α on days 8 and 9 of pregnancy were resorbing implants by day 10. Exogenous progesterone, but not estrogen, prevented implant resorption, yet 20 α-OH-SDH appeared in the corpora marking luteolysis. HCG, LH and prolactin, but not FSH, prevented pregnancy termination and inhibited induction of 20 α-OH-SDH in rats treated with PGF₂ α in early pregnancy. PGF₂α also induced 20α-OH-SDH in luteal tissue of intact and hypophysectomized rats treated on days 14 and 15 of pregnancy, but neither exogenous steroids or gonadotrophins blocked the induction of the enzyme in rats treated at this time. The increase in lutein 20α-OH-SDH activity during the peripartal period was partially blocked by administration of the prostaglandin biosynthesis inhibitor, indomethacin, suggesting a role for endogenous prostaglandins in the induction of 20α-OH-SDH at term. It appears that PGF₂α acts directly on the ovary to induce 20α-OH-SDH activity by preventing the luteotrophic action of prolactin. Other luteal NADPH-dependent dehydrogenase activities are not markedly stimulated following PGF₂α administration.     

Radioimmunoassays of prostaglandin F2α and prostaglandin F2α-main urinary metabolite with prostaglandin-I-tyrosine methylester amide

Radioimmunoassays for measuring prostaglandin F_2α (PGF_2α) and 5α, 7α-dihydroxy-11-keto tetranorprosta-1,16-dioic acid, PGF_2α-main urinary metabolite (PGF_2α-MUM), with ¹²⁵I-tyrosine methylester amide (TMA) of PGF_2α and PGF_2α-MUM were developed.Antibody to PGF_2α was produced in rabbits immunized with conjugates of PGF_2α coupled to bovine serum albumine. Antibody to PGF_2α-MUM was also produced in rabbits immunized with conjugates of PGF_2α-MUM coupled to bovine serum albumin.PGF_2α-¹²⁵I-TMA had an affinity to antiserum to PGF_2α. PGF_2α-MUM-¹²⁵I-TMA also responded to antiserum to PGF_2α-MUM.     

Arachidonic acid and its metabolites increase cytosolic free calcium in bovine luteal cells

We studied the effects of arachidonic acid and its metabolites on intracellular free calcium concentrations ([Ca²⁺]i) in highly purified bovine luteal cell preparations. Corpora lutea were collected from Holstein heifers between days 10 and 12 of the estrous cycle. The cells were dispersed and small and large cells were separated by unit gravity sedimentation and flow cytometry. The [Ca²⁺]i was determined by spectrofluorometry in luteal cells loaded with the fluorescent Ca²⁺ probe. Fura-2. Arachidonic acid elicited a dose-dependent increase in [Ca²⁺]i in both small and large luteal cells, having an effect at concentrations as low as 5μM; and was maximally effective at 50μM. Several other fatty acids failed to exert a similar response. Addition of nordihydroguaiaretic acid (NDGA) or indomethacin failed to suppress the effects of arachidonic acid. In fact, the presence of both inhibitors resulted in increases of [Ca²⁺]i, with NDGA exerting a greater stimulation of [Ca²⁺i than indomethacin. Prostaglandin F_2α (PGF_2α) as well as prostaglandin E₂ (PGE₂) increased [Ca²⁺ in the small luteal cells. These results support the idea that arachidonic acid exerts a direct action in mobilizing [Ca²⁺]i, in the luteal cells. Furthermore, they demonstrate that the cyclooxygenase (PGF_2α and PGE₂) and lipoxygenase products of arachidonic acid metabolism also play a role in increasing [Ca²⁺]i in bovine luteal cells. Since the bovine corpus luteum contains large quantities of arachidonic acid, these findings suggest that this compound may regulate calcium-dependent functions of the corpus luteum, including steroid and peptide hormone production and secretion.     

Effect of PGF2α on progesterone secretion and adenylate cyclase activity in ovine luteal tissue

Ovine luteal slices were used to study the effects of prostaglandins (PG) F₂α on luteinizing hormone (LH)-stimulated secretion of progesterone and adenylate cyclase activity. The accumulation of progesterone in incubation medium and adenylate cyclase activity was similar after incubation of luteal slices with Medium 199 alone or Medium 199 containing PGF₂α (250 ng/ml) for 3 hr. Addition of luteinizing hormone (LH; 100 ng/ml) resulted in a 2–3 fold increase in both the rate of progesterone accumulation and adenylate eyclase activity by 3 hr. When luteal slices were incubated in the presence of both LH and PGF₂α the rates of progesterone accumulation and adenylate cyclase activity were identical to those in flasks containing LH alone after 1 hr; however, after 3 hr both LH stimulated progesterone accumulation and adenylate cyclase activity were inhibited to levels similar to those observed in control slices.In a second experiment, after 60–120 min of exposure to PGF₂α the rate of progesterone accumulation in the medium was not different from that in untreated control slices. In addition, after this experiment the luteal slices were homogenized and the basal, sodium fluoride, LH, isoproterenol (ISO) and PGE₂ sensitive adenylate cyclase activities were determined to evaluate the hormonal specificity of the negative effect of the pretreatment with PGF₂α. Both LH and ISO stimulated adenylate cyclase activities were reduced after PGF₂α pretreatment. However, fluoride ion stimulated adenylate cyclase activity was not significantly effected by PGF₂α pretreatment and PGE₂ sensitive adenylate cyclase was effected only slightly.     

Effect of prostaglandin F2α on the hCG-stimulated progesterone production by human corpora lutea

The effect of prostaglandin PGF_2α on the hCG stimulated and basal progesterone production by human corpora lutea was examined . hCG (40 i.u./ml) stimulated progesterone formation in corpora lutea of early (days 16–19 of a normal 28 day cycle), mid (days 20–22) and late (days 23–27) luteal phases. This stimulation was inhibited by PGF_2α (10 μg/ml) in corpora lutea of mid and late luteal phases. PGF_2α alone did not show a consistent effect on basal progesterone production. The inhibition of hCG stimulated progesterone production by PGF_2α at times corresponding to luteolysis indicates a role for that prostaglandin in the process of luteolysis in the human corpus luteum.     

Aldo Keto Reductase 1B7 and Prostaglandin F2α Are Regulators of Adrenal Endocrine Functions

Prostaglandin F_2α (PGF_2α), represses ovarian steroidogenesis and initiates parturition in mammals but its impact on adrenal gland is unknown. Prostaglandins biosynthesis depends on the sequential action of upstream cyclooxygenases (COX) and terminal synthases but no PGF_2α synthases (PGFS) were functionally identified in mammalian cells. In vitro, the most efficient mammalian PGFS belong to aldo-keto reductase 1B (AKR1B) family. The adrenal gland is a major site of AKR1B expression in both human (AKR1B1) and mouse (AKR1B3, AKR1B7). Thus, we examined the PGF_2α biosynthetic pathway and its functional impact on both cortical and medullary zones. Both compartments produced PGF_2α but expressed different biosynthetic isozymes. In chromaffin cells, PGF_2α secretion appeared constitutive and correlated to continuous expression of COX1 and AKR1B3. In steroidogenic cells, PGF_2α secretion was stimulated by adrenocorticotropic hormone (ACTH) and correlated to ACTH-responsiveness of both COX2 and AKR1B7/B1. The pivotal role of AKR1B7 in ACTH-induced PGF_2α release and functional coupling with COX2 was demonstrated using over- and down-expression in cell lines. PGF_2α receptor was only detected in chromaffin cells, making medulla the primary target of PGF_2α action. By comparing PGF_2α-responsiveness of isolated cells and whole adrenal cultures, we demonstrated that PGF_2α repressed glucocorticoid secretion by an indirect mechanism involving a decrease in catecholamine release which in turn decreased adrenal steroidogenesis. PGF_2α may be regarded as a negative autocrine/paracrine regulator within a novel intra-adrenal feedback loop. The coordinated cell-specific regulation of COX2 and AKR1B7 ensures the generation of this stress-induced corticostatic signal.     

Changes in corpus luteum content of prostaglandin F2α and E in the adult pseudopregnant rat

Conflicting reports exist regarding the source of luteolytic PGF_2α in the rat ovary. To assess the quantities of different PGs, measurements of PGF_2α, PGE and PGB were performed by radioimmunoassay in the adult pseudopregnant rat ovary throughout the luteal lifespan. Ovaries of 84 rats were separated by dissection into two compartments, corpora lutea of pseudopregnancy and remainder of ovary. Tissue samples were homogenized and prostaglandins extracted and determined by radioimmunoassay. During the mid-luteal and late-luteal phases, levels of PGs were significantly higher in the corpora lutea of pseudopregnancy than in the remainder of ovary. An increase of PGF_2α-content in the corpus luteum was registered with peak-levels of 53.9 ± 8.5 (mean ± SEM, N=18) ng/g tissue wet weight at day 13 of pseudopregnancy. PGE-levels reached peak-values at day 11 of pseudopregnancy (271.6 ± 28.4 ng/g w w, mean ± SEM, N=12). PGB-levels were below detection limits in all compartments for all ages studied. The present study demonstrates increased availability of PGF_2α in the corpus luteum during the luteolytic period, and points toward either increased luteal synthesis or luteal binding of PGF_2α during the luteolytic period.     

A biochemical hypothesis to explain the mechanism of luteal regression.

It seems likely that luteal regression may involve a direct biochemical action of prostaglandin F2alpha (PGF2alpha) on the luteal cell since there are now several reports that PGF2alpha can directly inhibit steroidogenesis in vitro. However, the mechanism of such an action of PGF2alpha remains obscure. This article initially reviews the central role of adenosine 3,I5I-mono-phosphate (c-AMP) in initiating and maintaining the structural and functional changes occurring on luteinisation. A mechanism is suggested, supported by results obtained using granulosa cells in tissue culture, in which PGF2alpha initiates functional luteolysis by inhibiting further synthesis of c-AMP. This mechanism is then used in conjunction with further in vitro observations to provide a possible explanation for the inability of PGF2alpha to regress newly formed corpora lutea. Finally, the possible mechanisms of structural regression are discussed.     

The effects of synthetic prostaglandin analogs on canine hepatic bile flow

The synthetic prostaglandin analogs 16, 16-dimethyl PGF_2α and 16, 16-dimethyl PGE₂ were administered to dogs with chronic biliary and gastric fistulas. The effects of 16, 16 diMePGF_2α and 16, 16 diMePGE₂ were evaluated on bile flow and composition and bile adenosine 3′, 5′ monophosphate (cyclic AMP) secretion. 16, 16 diMePGF_2α in doses of 0.125 and 0.25 μg-kg-min significantly increased hepatic bile flow. The choleresis was characterized by increased cloride and bicarbonate secretion. Measurement by radioimmunoassay of bile cyclic AMP concentration demonstrated no evident increase in bile cyclic AMP secretion associated with the choleresis produced by 16, 16 diMePGF_2α. The administration of 16, 16 diMePGE₂ in a dose range 0.01 to 1.0 μg-kg-min did not significantly alter bile flow rates or composition. Bile erythritol-¹⁴C clearance, a measure of canalicular bile flow, was significantly increased by PGF_2α but not by 16, 16-dimethyl PGF_2α, suggesting that the mechanism of action of PGF_2α in stimulating hepatic bile flow may be different from that involved in 16, 16-dimethyl PGF_2α choleresis. The results of this study indicate that the synthetic PGF_2α analog produces a choleretic response not mediated by adenylate cyclase and associated with increased chloride and bicarbonate secretion.