Interaction of prostaglandins and adenosine 5'-triphosphate in the noncholinergic neurotransmission in rabbit detrusor.

Part of the excitatory transmission in rabbit detrusor is noncholinergic and nonadrenergic, and prostaglandins (PGs) and adenosine 5'-triphosphate (ATP) have been implicated in this transmission. The present experiments investigate the possibility of an interaction between PGs and ATP in rabbit detrusor. Indomethacin (2.8 muM) depressed the contraction produced by ATP although it did not antagonize the contraction produced by carbachol. Treatment of detrusor strips with 1.5 mM ATP depressed the frequency response curve in field stimulated tissues. This depression was additive with that produced by atropine. In the present experiments indomethacin did not significantly augment the effect of desensitization with ATP. It is suggested that the atropine-resistant neurotransmission in rabbit detrusor may involve both ATP and PGs acting in cooperation.     

Prostaglandin involvement in contractions evoked in rabbit detrusor by field stimulation and by adenosine 5'-triphosphate

Previous reports suggest that in rabbit urinary bladder both noncholinergic nonadrenergic excitatory responses and the contraction produced by adenosine 5'-triphosphate (ATP) are antagonized by indomethacin. We have attempted by further indirect testing on isolated detrusor strips to determine what role prostaglandins (PGs) might play in these processes. The second part of the biphasic contractile response to ATP was reduced to about 30% of control by PG synthesis inhibitors but the initial phase of the ATP response and te contraction produced by the beta, gamma-methylene analogue of ATP were unaffected. At concentrations that did not affect the response to acetylcholine but greatly suppressed the response to arachidonic acid, indomethacin antagonized the contraction evoked by field stimulation by about 30% at 1-2 Hz (largely noncholinergic and nonadrenergic). SC 19220, a putative PG receptor blocker, also produced about 25% reduction in the response to field stimulation but with only about 50% reduction in the response to arachidonic acid, PGE2, or PGF2alpha, SC 19220 also antagonized the frequency-response curve in atropine-treated strips. These findings lead us to suggest that beside maintaining tone and spontaneous activity in the bladder PGs mediate the slow tonic phase of the ATP response and may contribute to facilitatory modulation of noncholinergic nonadrenergic excitatory transmission.     

Sow (Sus scrofa) follicular fluid: Prostaglandin content and effect on the motility of isolated oviducts

The present study provides information regarding the effects of the sow follicular fluid (FF) on the motility of isolated segments of swine and rabbit oviducts. In addition, the concentration of prostaglandins (PGs) F_{2 α}, E₂ and E₁ in the follicular fluid of sow ovaries isolated at different stages of the sex cycle as well as the generation of the same PGs by walls of ovarian follicles in early and late proestrus, in estrus, in metestrus and in diestrus, were explored. The stimulatory contractile effect of proestrous FF in isolated segments of sow fimbria was antagonized by polyphloretin phosphate (PPP), a PG receptor blocker and by indomethacin, an inhibitor of PG synthesis. The positive inotropism evoked by the FF was mimiked by bradykinin and the influences of both interventions were similarly antagonized by PPP. It appears plausible that the inotropic effect of the preovulatory FF on the sow fimbria could be not only by PGs already present in the fluid, but also by the stimulation of the synthesis of tubal PGs by follicular fluid bradykinin. The FF also stimulated the ampullary tubal segments isolated from proestrous sows whereas the same volume of FF depressed significantly the isometric developed tension of rabbit ampulla. The total concentration of the three PGs in the FF from late proestrous follicles was significantly greater than that of the same PGs in the other two stages of the sex cycle (early proestrus and diestrus), whereas the concentration of each PG (PGE₂, PGF_{2 α} or PGE₁), did not differ within any of the stages of the cycle. Furthermore, the total amount of the three PGs produced by the walls of follicles from late proestrous ovaries was also significantly greater than that generated by ovarian follicles from early proestrus, estrus, metestrus and diestrus.     

Interaction between phenylephrine and prostaglandins E1 and F1α on the contractile responses of vascular muscle

Prostaglandins (PGs) E₁ or F_1α (1.4−8.4 × 10⁻⁸ M) contracted strips of rabbit aorta and increased the contractions produced by 1−6 × 10⁻⁷ M phenylephrine (PE). The addition of the PGs simultaneously with PE or after a low concentration of PE (2 × 10⁻⁷ M) significantly increased the PE-induced contractions. However, when the PGs were added after a higher concentration of PE (6 × 10⁻⁷ M) an additional increase in the PE-induced contraction was produced with PGF_1α but not with PGE₁. Isobolic plots of the data obtained from the simultaneous addition of PE and the PGs indicate that both PGs interact with PE in a synergistic or potentiative manner, suggesting that their effects are mediated through different receptor mechanisms. Addition of the PGs after a high dose of PE indicates that there may also be either qualitative or quantitative differences between PGE₁ and PGF_1α.     

Accelerated ovum transport in rabbits induced by endotoxin II. Changes in oviductal smooth muscle activity

Oviductal motility, measured with open-ended perfused catheters in anesthetized animals injected with human Chorionic Gonadotropin (hCG), is depressed 2 h following endotoxin injection and returns to control levels by 3 h after endotoxin injection. This decrease in motility is prevented by indomethacin. Endotoxin did not affect spontaneous or phenylephrine (PE)-induced contractions of oviduct when it was added to the bathing medium of in vitro tissues. Oviductal segments removed 2 h after endotoxin (26 h after hCG) showed electrical activity confined to the ampullary-isthmic-junction (AIJ), where ova were located; the dose-response curve for PE was shifted to the right and the maximum contraction was depressed. Activity of tissues removed 4 h after endotoxin more closely resembled control tissues except that the maximum contraction to PE was depressed, ova had passed out of the oviduct and a proovarian bias in the isthmus was not present. The response of the oviduct to prostaglandins (PGs) in vivo is critically dependent on the previous exposure to PGs. In endotoxin-treated animals PGE then PGF levels increase and the decrease in motility coincides with increased PGE levels, but accelerated ovum transport with the return of motility and activation of the isthmus.     

Effects of prostaglandins and arachidonic acid on baboon cerebral and mesenteric arteries

Effects of prostaglandins (PGs) E₁, E₂, F_2α and I₂ in a wide range of concentration were examined in mesenteric and cerebral arteries isolated from mature baboons. PGs E₁, E₂ and F_2α at low concentrations (10⁻¹⁰ to 10⁻⁷ M) elicited relaxation in helically cut strips of cerebral arteries precontracted with phenylephrine. In contrast, the PGs did not cause relaxation in the mesentric artery. PGI₂ (10⁻⁹ to 10⁻⁶ M) produced marked relaxation in both arteries. The EC₂₅ for PGI₂ in the mesenteric artery was significantly lower than that in the cerebral artery. During baseline conditions, cerebral arteries contracted in response to high concentrations (greater than 10⁻⁷ M) of PGs E₁, E₂ and F_2α. In mesentric arteries, a large contraction was induced by PGs F_2α and E₂ but not by PGE₁. Arachidonic acid (10⁻⁶ M) produced an aspirin-inhibitable relaxation in both arteries to a similar extent, so that the vasodilator PG(s) formed in the two different arterial walls appear to exert a similar relaxant action. Thus, the baboon mesenteric artery was more sensitive to PGI₂ for the relaxant effect than was the cerebral artery, while PGs F_2α, E₁ and E₂ caused only a contraction in the mesenteric artery but both relaxation and contraction in the cerebral artery.     

Effect of indomethacin on atropine-resistant transmission in rabbit and monkey urinary bladder : Evidence for involvement of prostaglandins in transmission

The effects of 2.9 × 10⁻⁵M atropine and 2.8 × 10⁻⁶M indomethacin on responses of rabbit and monkey detrusor muscle to transmural stimulation were investigated. Responses to transmural stimulation were partially inhibited by atropine. Indomethacin caused further inhibition in the presence of atropine, but did not alter responses to acetylcholine. Prostaglandins E₂ and F_2α contracted rabbit and monkey detrusor. It is suggested that prostaglandins are liberated during stimulation of excitatory nerves to the rabbit and monkey detrusor, and contibute to the resultant contractile response.     

The influence of xanthines on the contractile responses of PGF2α and PGD2 in human parenchymal strips

Prostaglandins (PGs) F_2α and D₂ are bronchoconstrictor agents which are released under allergic conditions such as asthma. The efficacy and potency of PGF_2α and PGD₂ differ in some tissues. We compared the effects of these two PGs in sensitized human parenchymal strips. In six experiments, PGF_2α 0.1 and 0.3 μM produced greater contractions than PGD₂ at the same concentrations. There were no significant differences between the contractions from the two PGs at concentrations of 0.01, 0.03, 1.0–10 μM and the two PGs appeared to be equipotent.We studied the effects of the anti-asthmatic drug theophylline, and its analogue enprofylline, on the contraction caused by these PGs. Theophylline 100 μM caused no change to the cumulative concentration response curves. However, enprofylline 100 μM reduced the PGF_2α-induced contractions.     

Accelerated ovum transport in rabbits induced by endotoxin II. Changes in oviductal smooth muscle activity

Oviductal mortility, measured with open-ended perfused catheters in anesthetized animals injected with human Chorionic Gonadotropin (hCG), is depressed 2 h following endotoxin injection and returns to control levels by 3 h after endotoxin injection. This decrease in motility is prevented by indomethacin. Endotoxin did not affect spontaneous or phenylephrine (PE)-induced contractions of oviduct when it was added to the bathing medium of in vitro tissues. Oviductal segments removed 2 h after endotoxin (26 h after hCG) showed electrical activity confined to the ampullary-isthmic-junction (AIJ), where ova were located; the dose-response curve for PE was shifted to the right and the maximum contraction was depressed. Activity of tissues removed 4 h after endotoxin more closely resembled control tissues except that the maximum contraction to PE was depressed, ova had passed out of the oviduct and a proovarian bias in the isthmus was not present. The response of the oviduct to prostaglandins (PGs) in vivo is critically dependent on the previous exposure to PGs. In endotoxin-treated animals PGE then PGF levels increase and the decrease in motility coincides with increased PGE levels, but accelerated ovum transport with the return of motility and activation of the isthmus.     

Aalpha,beta-methylene ATP enhances P2Y4 contraction of rabbit basilar artery

Interactions between different selective P2 receptor agonists have been used as tools to identify different P2 receptor subtypes. In the present study, we examined the P2 receptor subtypes and the mechanisms of potentiation of UTP contraction (P2Y contraction) by alpha,beta-methylene ATP [(2-carboxypiperazin-4-yl)propyl-1-phosphanoic acid (CPP), a P2X agonist] using isometric tension in the denuded rabbit basilar artery. We made the following observations: 1). a predominant P2X receptor contraction was observed in the rabbit ear artery by the rank order of CPP > 2-methylthioATP > ATP > UTP; 2). functional P2Y receptors were observed in the rabbit basilar artery by the rank order of UTP > ATP = CPP = 2-methylthioATP; 3). CPP potentiated UTP-, ATP-, and ATPgammaS-induced contractions, possibly by activation of P2Y4 receptors because ATPgammaS does not activate P2Y6 receptors; and 4). ectonucleotidase did not play a predominant role in the potentiative effect of CPP because Evans blue, Ca(2+)-free medium, or divalent cation Ni(2+) did not affect the effect of CPP. Evans blue potentiated the contraction by UTP but not by ATP or ATPgammaS. We conclude that CPP enhanced P2Y4-mediated contraction in the rabbit basilar artery, and the influence by ectonucleotidases on CPP-potentiation remains unclear.     

External calcium dependence of the uterine contraction induced by prostaglandins E2 and F2α and its antagonism with natural progestins

Prostaglandins (PGs) E₂ and F2α are strong inducers of uterine contraction by promoting a Ca²⁺ increase into the cell through specific receptors coupled with the calcium channels. On the contrary, progesterone and 5β-reduced progestins promote smooth muscle relaxation by blocking the ion calcium influx. Thus, this study was designed to emphasize the importance of external calcium in the PGs-induced rat uterus contraction. Likewise, also studied was the antagonism and the interaction between PGs and progestins (progesterone and its 5α and 5β-reduced derivatives) in the myometrium. Results showed that uterine contraction induced by PGs depends on external calcium, since verapamil or extracellular calcium depletion abolished the PGs effect. Regarding the PGs-progestins antagonism, it was observed that pregnanedione, pregnanolone and epipregnanolone were quite effective for counteracting of PGs-induced contraction. However, progesterone was effective in a middle range, whereas 5α-reduced progestins (allopregnanedione and allopregnanolone) were almost ineffective. It has been concluded that the participation of PGs and progestins in the modulation of uterine contraction might be achieved through the control of calcium influx by opening (PGs) or blocking (progestins) receptor-operated calcium channels.     

Morphine diminishes the constancy of spontaneous uterine contractions, antagonizes the positive inotropic effects of prostaglandin E2, but not of prostaglandin F2α and inhibits prostaglandin E and F outputs from the uterus of ovariectomized rats

The effects of morphine on the constancy of spontaneous contractions (isometric developed TENSION = IDT and contractile FREQUENCY = CF), in uterine strips isolated from ovariectomized rats and the influence of naloxone, were explored. The inotropic responses to added prostaglandins (PGs) E₂ and F_2α and the influences of morphine and of morphine in the presence of naloxone on PG actions, were also determined. Moreover, the synthesis and outputs of PGs E and F from uteri and the effects of morphine alone and of morphine plus naloxone, were studied. Morphine (10⁻⁶ M) significantly depressed uterine constancy of IDT during the first hours following delivery, but its action on CF did not differ from controls. Naloxone, neither at 10⁻⁸ M nor at 10⁻⁶ M, altered the negative inotropoic influence of morphine on IDT. Exogenous PGs E₂ and F_2α, stimulated uterine inotropism in a concentration-dependent fashion. Morphine altered dose-response curves for exogenous PGE₂, evoking a parallel surmountable shift to the right, but did not affect the inotropic action of added PGF_2α. This antagonistic effect of the opioid was not altered by preincubation with naloxone. Basal synthesis and outputs of PGs E and F in uteri from ovariectomized rats were significantly depressed by morphine (10⁻⁶ M) but not altered by incubating tissues with morphine in presence of naloxone. Results are discussed in terms of a presumptive dual action of morphine on uterine motility, i.e., antagonizing PGE₂ receptors and inhibiting the synthesis of some PGs by the uterus. These influences of morphine do not appear to be subserved by the activation of μ opioid receptors. Moreover, the possibility that endogenous opioids could play a relevant role modulating uterine PG influences, is also discussed.     

Effects of prostaglandins E1, E2 and F2α on the cardiac chronotropism by affecting the cardiac ganglionic transmission in spinal dogs

The effects of several prostaglandins (PGs) injected through the subclavian artery toward the cardiac sympathetic ganglia of spinal dogs were studied by utilizing changes of the heart rate as indicator of ganglionic function. PGF_2α (10–270 μg) administered intra-arterially in the presence or absence of preganglionic stimulation produced weak positive chronotropic effects, which were increased by physostigmine. This positive chronotropic effect of F_2α after physostigmine was inhibited by hexamethonium plus atropine, and depressed after hemicholinium-3 except for the response elicited by the first dose of F_2α. PGE₁ and E₂ injected during preganglionic stimulation did not affect the heart rate. Intra-arterially administered epinephrine and dopamine depressed dose-dependently transmission in the cardiac ganglia, the effect being inhibited by E₁ and E₂ but not by F_2α. These results suggest that F_2α facilitates the release of acetylcholine from preganglionic nerve ending, whereas E₁ and E₂ antagonize the inhibitory actions of catecholamine in the cardiac ganglia.     

The influence of prostaglandins on neurotransmission in the rabbit isolated vas deferens

Arachidonic acid and PGs of the D, E, F and I series were examined for influences on neurogenic contractions of the rabbit isolated vas deferens. This preparation exhibits two pharmacologically distinct contractions in response to electrical stimulation. All of the PGs tested inhibited the neurogenic contractions but the pattern of inhibition differed. PGE1 and PGI2 inhibited the adrenergic contractile phase more potently than the nonadrenergic, and PGF2 alpha exhibited the opposite selectivity. Arachidonic acid, PGE2 and PGD2 produced equipotent effects on both contractile phases, although PGE2 was the most potent in producing these effects. None of the PGs altered the concentration-response curve to norepinephrine. Contractile responses to ATP, a putative neurotransmitter, were inhibited by PGF2 alpha but not by the other PGs. These results suggest that the PG effects are predominantly prejunctional. The differing potencies of the PGs on the two neural components are consistent with the hypothesis that neurotransmitters in the vas deferens are released by distinct types of nerves.     

Impairment of ATP hydrolysis decreases adenosine A1 receptor tonus favoring cholinergic nerve hyperactivity in the obstructed human urinary bladder

This study was designed to investigate whether reduced adenosine formation linked to deficits in extracellular ATP hydrolysis by NTPDases contributes to detrusor neuromodulatory changes associated with bladder outlet obstruction in men with benign prostatic hyperplasia (BPH). The kinetics of ATP catabolism and adenosine formation as well as the role of P1 receptor agonists on muscle tension and nerve-evoked [³H]ACh release were evaluated in mucosal-denuded detrusor strips from BPH patients (n = 31) and control organ donors (n = 23). The neurogenic release of ATP and [³H]ACh was higher (P < 0.05) in detrusor strips from BPH patients. The extracellular hydrolysis of ATP and, subsequent, adenosine formation was slower (t_1/2 73 vs. 36 min, P < 0.05) in BPH detrusor strips. The A₁ receptor-mediated inhibition of evoked [³H]ACh release by adenosine (100 μM), NECA (1 μM), and R-PIA (0.3 μM) was enhanced in BPH bladders. Relaxation of detrusor contractions induced by acetylcholine required 30-fold higher concentrations of adenosine. Despite VAChT-positive cholinergic nerves exhibiting higher A₁ immunoreactivity in BPH bladders, the endogenous adenosine tonus revealed by adenosine deaminase is missing. Restoration of A₁ inhibition was achieved by favoring (1) ATP hydrolysis with apyrase (2 U mL⁻¹) or (2) extracellular adenosine accumulation with dipyridamole or EHNA, as these drugs inhibit adenosine uptake and deamination, respectively. In conclusion, reduced ATP hydrolysis leads to deficient adenosine formation and A₁ receptor-mediated inhibition of cholinergic nerve activity in the obstructed human bladder. Thus, we propose that pharmacological manipulation of endogenous adenosine levels and/or A₁ receptor activation might be useful to control bladder overactivity in BPH patients.     

Interaction between phenylephrine and prostaglandins E1 and F1 alpha on the contractile responses of vascular muscle.

Prostaglandins (PGs) E1 or F1 alpha (1.4--8.4 x 10(-8) M) contracted strips of rabbit aorta and increased the contractions produced by 1--6 x 10(-7) M phenylephrine (PE). The addition of the PGs simultaneously with PE or after a low concentration of PE (2 x 10(-7) M) significantly increased the PE-induced contractions. However, when the PGs were added after a higher concentration of PE (6 x 10(-7) M) an additional increase in the PE-induced contraction was produced with PGF1 alpha but not with PGE1. Isobolic plots of the data obtained from the simultaneous addition of PE and the PGs indicate that both PGs interact with PE in a synergistic or potentiative manner, suggesting that their effects are mediated through different receptor mechanisms. Addition of the PGs after a high dose of PE indicates that there may also be either qualitative or quantitative differences between PGE1 and PGF1 alpha.     

Sow (Sus scrofa) follicular fluid: prostaglandin content and effect on the motility of isolated oviducts

The present study provides information regarding the effects of the sow follicular fluid (FF) on the motility of isolated segments of swine and rabbit oviducts. In addition, the concentration of prostaglandins (PGs) F2 alpha, E2 and E1 in the follicular fluid of sow ovaries isolated at different stages of the sex cycle as well as the generation of the same PGs by walls of ovarian follicles in early and late proestrus, in estrus, in metestrus and in diestrus, were explored. The stimulatory contractile effect of proestrous FF in isolated segments of sow fimbria was antagonized by polyphloretin phosphate (PPP), a PG receptor blocker and by indomethacin, an inhibitor of PG synthesis. The positive inotropism evoked by the FF was mimiked by bradykinin and the influences of both interventions were similarly antagonized by PPP. It appears plausible that the inotropic effect of the preovulatory FF on the sow fimbria could be not only by PGs already present in the fluid, but also by the stimulation of the synthesis of tubal PGs by follicular fluid bradykinin. The FF also stimulated the ampullary tubal segments isolated from proestrous sows whereas the same volume of FF depressed significantly the isometric developed tension of rabbit ampulla. The total concentration of the three PGs in the FF from late proestrous follicles was significantly greater than that of the same PGs in the other two stages of the sex cycle (early proestrus and diestrus), whereas the concentration of each PG (PGE2, PGF2 alpha or PGE1), did not differ within any of the stages of the cycle. Furthermore, the total amount of the three PGs produced by the walls of follicles from late proestrous ovaries was also significantly greater than that generated by ovarian follicles from early proestrus, estrus, metestrus and diestrus. In summary the results document that the concentration of each one of the PGs measured (E2, E1 or F2 alpha) attained maximal values at the time of ovulation. The results regarding the effects of FF on the inotropic activity of fimbrial and ampullary segments of sow oviducts also suggest that the fluid might play a physiological role, favouring the capture and transfer of ova into the oviducts at the moment of ovulation.     

The role of prostaglandins in the excitatory innervation of the rat urinary bladder

The possible role of PGs in hyoscine-resistant nerve mediated responses of the rat urinary bladder was investigated. Responses to electrical stimulation were inhibited by cinchocaine (30 μmol/l) but were only partially inhibited by a high concentration of hyoscine (25 μmol/l) or by the choline uptake inhibitors, hemicholinium-3 (500 μmol/l) and troxypyrrolidinium (500 μmol/l). Indomethacin (50 μmol/l) produced partial blockade (30%) of responses to electrical stimulation without markedly affecting responses to acetylcholine and the degree of blockade was of a similar order in the presence of hyoscine or troxypyrrolidinium. PGE₂ (0.028 – 2.8 μmol/l) or F_2α (0.029 – 2.9 μmol/l) produced a slowly developing increase in tone and spontaneous activity. Responses to electrical stimulation were at most only slightly increased in the presence of either PG. However, the PGs always increased the responses to electrical stimulation after indomethacin, indomethacin plus hyoscine or indomethacin plus troxypyrrolidinium. Responses to acetylcholine in the presence of indomethacin were not increased by PGE₂. It is concluded that PGE₂ and F_2α do not function as transmitters responsible for resistance to anti-muscarinic drugs in the bladder but may exert a modulating effect on nervous transmission.     

Serum and plasma stimulate prostaglandin production by alveolar macrophages

Fetal bovine serum (FBS) stimulated rabbit alveolar macrophages to synthesize prostaglandins (PG) and release lysosomal enzymes. This stimulatory actions was not entirely due to the effect of foreign protein in FBS, since rabbit serum and plasma, both homologous and autologous, also induced release of PGs and lysosomal enzymes. Rabbit serum and plasma are less effective than FBS as a stimulus for PG release, with rabbit serum being more potent than plasma at the same concentration. Bovine serum albumin elicited a dose-dependent increase of arachidonic acid release by macrophages, but not of PG production. Hence, the fatty acid “trapping” effect of albumin in serum and plasma is not responsible for the PG stimulation. The PG stimulating factors were stable at 56°C for 30 min., but lost half the activity after heating at 100°C for 10 min. Gel permeation chromatography of FBS showed several peaks of PG stimulating and arachidonic acid releasing activity. The molecular weight of the major one (150,000 daltons) is similar to that of immunoglobulin G. Rabbit IgG, when added to the macrophage culture, stimulated release of arachidonic acid and PGs. However, the major stimulatory effect in serum or plasma is not all due to IgG, since removal of IgG by a Protein A-agarose column did not remove the stimulatory effect of FBS and rabbit serum. The possibility of other factors, such as complement fragments, is discussed.     

Triphasic effect of prostaglandins E1, E2 and F2α on the fluid transport of isolated gall-bladder of guinea-pigs

Prostaglandins (PGs) F_2α, E₁ and E₂ exerted a triphasic influence on the fluid transport of isolated guinea-pig gall-bladders, when applied to the serosal side. PGE₁ and PGE₂ produced these effects in lower concentrations than F_2α. Directly after PG addition to the serosal side a short stimulation of fluid transport to between 200 and 400% was observed. The stimulatory effect of PGs was most distinct in gall-bladders from female guinea-pigs, less pronounced in male and nearly absent in pregnant animals. Since PGs increased intraluminal hydrostatic pressure in gall-bladders by contraction of the smooth muscle, experiments were performed in which hydrostatic pressure was increased by different procedures. These included the addition of imidazole (10⁻² M), raising of K⁺ in the bathing solution and an increase in intraluminal pressure by addition of Ringer's solution into the lumen. All three procedures stimulated fluid reabsorption temporarily in the same way as PGs, hence increase of intraluminal pressure is thought to be the reason for the observed temporary stimulation of fluid transport. Direct evidence for this thesis was obtained when the gall-bladder was mounted as a flat sheet over a chamber; in this preparation no stimulation of fluid transport was obtained. The second phase of the PG influence was characterized by a concentration-related inhibition of fluid reabsorption followed by a significant but small reverse of fluid transport (secretion of fluid). When PGs were applied to the mucosal side, only an inhibition of fluid transport was observed, which was much weaker compared to the addition to the serosal side.