The action of ketonic prostaglandins on the guinea-pig myometrium

Hyperpolarization and slowing of the rate of contraction in human uterus in pregnancy by prostaglandins E2 and f2alpha: involvement of the Na+ pump

1. The effects of prostaglandins E2 (PGE) and F2alpha (PGF) on membrane potential and isometric tension and cytoplasmic free calcium concentration ([Ca2+]i) and tension were studied in strips of uterine smooth muscle obtained from women undergoing Caesarean delivery at term and during established labour. 2. Prostaglandins (PGs) evoked a biphasic response. The excitatory component consisted of depolarization of the membrane, which initiated spike action potentials, an increase in [Ca2+]i and tension development. The membrane remained depolarized at -19 +/- 1 mV for about 2 min, then repolarized abruptly, [Ca2+]i promptly returned to basal levels, and tension development ceased. 3. This component of the response to PGE or PGF was followed by a slow hyperpolarization which reached -85 +/- 2 mV (n = 22) at term and -70 +/- 2 mV (n = 9) during labour, and during which spontaneous action potentials and tension development did not occur. 4. Nifedipine (10-6 M) abolished spontaneous activity, abolished PG-induced action potentials and reduced the increase in [Ca2+]i (9 +/- 3 %, n = 6), the depolarization (10 +/- 1 mV, n = 14), the tension (2 +/- 1 %, n = 14) and the hyperpolarization (9 +/- 1 mV, n = 14, at term). 5. A variety of K+ channel blockers were without effect on the peak amplitude of the PG-induced hyperpolarization but the latter did not occur in the presence of ouabain (10-6 M) or in K+-free or low-Na+ solutions, suggesting an involvement of the Na+-K+-ATPase pump. 6. In conclusion, a substantial dependence on Ca2+ influx through voltage-operated Ca2+ channels accounts for the importance of membrane potential in regulating contractions in human uterine smooth muscle. The classical excitatory effect of PGE and PGF is followed by hyperpolarization involving the Na+-K+-ATPase pump. The hyperpolarization restricts the response to a single contraction and decreases the frequency of subsequent contractions. The amplitude of the hyperpolarization decreases during labour, allowing contraction frequency to increase. Its persistence at this time ensures complete relaxation between each single robust contraction, preventing spasm of the uterus that would restrict blood flow to the fetus during delivery.

Oxytocin receptors and prostaglandin release in rabbit amnion

Besides stimulating uterine myometrial and mammary myoepithelial cell contraction, oxytocin (OT) causes the release of prostaglandins (PGs) from uterine endometrium/decidua and amnion cells. Lacking information about OT receptors eliciting PG release, we don't know how they are related to OT receptors involved in smooth muscle contraction. The amnion offers great potential for characterizing OT receptors associated with PG release, as the amount of iodinated OT antagonist ([125I]OTA) bound to rabbit amnion membranes during labor is among the greatest of any tissue yet studied, reaching about 10 pmol/mg membrane protein. The relative affinities of several OT analogues for binding sites on amnion membranes are the same as those on decidual membranes. There are differences in the ligand profile between amnion and myometrium, but they could be due to the additional presence of vasopressin receptors on myometrial membranes. An increase in the sensitivity of PGE2 release from amnion cells in culture to OT and analogues accompanies the rise in OT receptor concentration at the end of gestation. Increases in [125I]OTA binding in vivo can be mimicked with cultured amnion cells by addition of agents that elevate intracellular cAMP levels. Based on the time course and inhibition of the increase with cycloheximide, cAMP might induce OT receptor gene expression. The increase also is reflected by a marked elevation in the covalent labeling of a 50-kDa electrophoretic band with a photoactivated derivative of [125I]OTA. Because of the homogeneity of cell types in the amnion, the ease of culturing amnion cells, and the high concentration of OT receptors that can be induced, this tissue should be very useful in characterizing OT receptors associated with PG synthesis.

Mast cells mediators evoke contractility and potentiate each other in mouse uterine horns

1. The ability of prostaglandin F2 alpha (PGF2 alpha), histamine and serotonin to induce and/or potentiate contractions in mouse uterine horns was studied in an attempt to analyze whether uterine mast cells degranulation could favor contractions during labor. 2. PGF2 alpha was the most potent of the three compounds (EC50 = 0.7 microM), being followed by serotonin (EC50 = 1.2 microM) and histamine. Between 10 and 100 microM histamine only evoked weak contractions, not higher than 50% of maximal amplitude of contractions. 3. Serotonin (0.1 microM) potentiated the contractile effect of both histamine and PGF2 alpha when added simultaneously with the corresponding compound to the isolated organ bath. Also, histamine at threshold contractile concentration (3 microM) was able to potentiate the contractile effect of both serotonin and PGF2 alpha. 4. The potentiating effect of PGF2 alpha on both histamine and serotonin evoked contractions was recorded after treating the preparation with 10 microM of the compound for at least 10 min before the corresponding concentration-effect curves were performed. 5. It is postulated that the simultaneous presence of these mast cells mediators in myometrium could be an important stimulus for both to trigger and/or to maintain contractions during labor.

Induction of prolonged excitability in myometrium of pregnant guinea-pigs by prostaglandin F2 alpha

1. Electrical and mechanical responses to prostaglandin F2 alpha (PGF) were studied in circular myometrium, with or without endometrium, during the first 3 weeks of gestation of the guinea-pig. 2. Muscle strips from which endometrium had been removed became inexcitable within 30-40 min of isolation from the animal such that action potentials and contraction could not be initiated by depolarizing current steps. Raising the concentration of potassium in the perfusing solution resulted in a small contraction. 3. In these preparations PGF induced complex action potentials that consisted of spikes and a prolonged plateau of depolarization. Each action potential was associated with a large phasic contraction. 4. Contractions induced by PGF are unlikely to result predominantly from release of calcium from intracellular stores since the ability of the agonist to evoke a response was reduced by some 97% in the absence of external calcium or in the presence of calcium channel blockers. 5. When preceded by a brief exposure to PGF, the contractile response to high potassium was enhanced to equal that in response to PGF. Enhancement persisted for approximately 30 min after removal of PGF and was not dependent upon the presence of external calcium. 6. Muscle strips with intact endometrium contracted spontaneously for hours. Each contraction was associated with a complex action potential, both of which were abolished by indomethacin. 7. It is concluded that PGF transforms inexcitable calcium channels in the membrane of the smooth muscle cells of the circular myometrium into excitable ones. The study also suggests that endogenous prostaglandin of endometrial origin may prevent the 'run-down' of channels in vivo.

In vitro contractile effect of platelet-activating factor on guinea-pig myometrium

Platelet-activating factor (PAF) evoked myometrial contractions in two different patterns, depending on whether spontaneous activity was present. In spontaneously active myometrial strips (58%), both PAF and oxytocin enhanced the amplitude of myometrial contractions. In quiescent myometrial strips, PAF induced contractions characterized by a prompt development of tension, a plateau, and a final, rapid relaxation. In 54% of these strips, PAF-induced contraction was followed by rhythmic activity. PAF contractile response was dependent upon the concentration (0.1-100 nM); the minimal effective concentration of PAF was 0.1 nM and the EC50 was 1 nM. The response to oxytocin (0.01-10 mU/ml), assumed as reference stimulus, was characterized by a prompt development of tension, which was followed by a sustained, slow contraction and relaxation. PAF response was almost completely dependent on cyclooxygenase and partially on lipoxygenase pathways, as inferred from studies with indomethacin and FPL 55712, respectively. A receptor mediated mechanism of PAF action was suggested by specific desensitization of the myometrium to a second challenge with an equimolar concentration of PAF (but not with oxytocin) and the blocking effect of CV 3988, a specific PAF receptor antagonist.

Nanomolar concentrations of prostaglandin F2 alpha potentiate cholinergic contractions of rabbit isolated tracheal muscle

The effects of prostaglandin F2 alpha (PGF2 alpha) on stimulation- or acetylcholine-evoked contractions were studied in isolated airway muscle preparations from rabbits, guinea-pigs and humans. Low concentrations of PGF2 alpha (10(-9) to 9 X 10(-8) mol/1) produced a dose-related (10-300%) increase in the contractile responses of the rabbit trachealis muscle to electrical stimulation at 2 Hz. This effect was inversely related to the rate of stimulation. In seven out of forty two preparations the resting muscle tone was also increased by 1.1 X 10(-8) mol/1 or higher concentrations of PGF2 alpha. This substance enhanced the contractile responses to acetylcholine (1-2.7 X 10(-8) mol/1) to the same extent as those to electrical stimulation. The potentiation produced by PGF2 alpha was not affected by indomethacin, mepyramine, methysergide or phenoxybenzamine. Electrically evoked contractions of isolated tracheal strips of guinea-pig or segments of human bronchial muscles were not changed significantly in the presence of 0.1-5 X 10(-8) mol/1 of PGF2 alpha. These results suggest that PGF2 alpha may modulate airway muscle tone by enhancing the postsynaptic stimulatory effect of acetylcholine released from the pulmonary cholinergic nerve endings. This modulation seems to be species-dependent.

Effects of prostaglandin E1 on contractility and 45Ca release in rabbit aortic smooth muscle

Concentrations of prostaglandin E1 (PGE1; 10(-7) M) that do not elicit tension responses in aortic strips potentiate contractions induced by submaximal concentrations (10(-8)-10(-7) M) of norepinephrine (NE) or angiotensin III (Ang III) but not those of high K+ depolarization or maximal NE or Ang III concentrations. Higher concentrations of PGE1 (10(-6) M and above) initiate contractions which are additive with submaximal responses to NE and Ang III but not to K+. These same concentrations of PGE1 also decrease 45Ca retention at high affinity La+++-resistant sites in a manner similar to but not additive with NE and Ang III. Uptake of 45Ca at low affinity La+++-resistant sites (which is increased by high K+-depolarization) is not altered by 10(-6) M PGE1. The effects of PGE1 are not altered by decreased extracellular Ca++ (0.1 mM), decreased temperature, phentolamine or meclofenamate. Thus, PGE1 does not appear to increase uptake of extracellular Ca++ in this smooth muscle tissue. Instead, PGE1 increases mobilization of Ca++ from the same high affinity La+++-resistant sites affected by Ang III and NE and, in this manner, may increase responses to these two stimulatory agents.

Interaction of vasopressin and prostaglandin on myometrial activity in vivo in the first trimester of human pregnancy

The interaction of vasopressin (VP) and prostaglandin (PG) F2 alpha on myometrial activity was studied by intrauterine pressure recording in 40 women due to undergo therapeutic abortion in the first trimester. In a control group the uterine activity did not change during eight hours of recording without drug administration. In a second group, who had intravenous injections of 8-lysine vasopressin (LVP, 0.1-0.5 IU), it was found that 0.3 IU was the smallest dose that consistently gave an effect on myometrial activity, and that repeated injections gave responses of practically the same magnitude. In the remaining women in the study, LVP injections, 0.3 IU, were given before, during and two hours after an intravenous infusion of PGF2 alpha at 4 or 20 micrograms/minute. Although there were great individual variations, potentiation and enhancement of the LVP effect were the most common reactions, especially with the lower dose of PG. An interaction of PG and other endogenous myometrial stimulants such as VP might be important in the aetiology of spontaneous abortion and the combination of PG with VP-like substances might be of value in therapeutic abortion during the first trimester.

Dual actions of prostacyclin (PGI2) on the rat pregnant uterus

Using strips of rat pregnant uterus, treated with indomethacin to suppress spontaneous contractility, the oxytocic activity of prostacyclin was compared with other prostaglandins. A prostacyclin concentration of 32 ng/ml elicited uterine contractions in all experiments. In this respect prostacyclin was 80 times more active than 6-oxo-PGF1 alpha but less active than PGE2 or PGF2 alpha. Apart from a direct stimulant effect, prostacyclin also exhibited an indirect potentiating action. In threshold concentrations prostacyclin caused a 3-fold potentiation of threshold doses of oxytocin. A lesser 1.5-fold potentiation of PGE2 alpha was also observed. The implications of these findings in relation to prostacyclin playing a role in parturition are discussed.

Effects of estradiol and oxytocin on the release of prostaglandin-like substance from isolated rat uterus

Effects of estradiol on the release of prostaglandin-like substance (PG) from rat uterus was investigated. The amount of PG released into a medium that did not contain oxytocin was decreased by estradiol, but the release of PG was remarkably increased by the addition of oxytocin. No significant correlation was found in the relationship between PG release and uterine contraction in uterus of either the ovariectomized rat and that of the estradiol-injected rat. The amount of PG released from the uterus of ovariectomized rat and the uterus of 6 hours after estradiol injection was decreased by indomethacin. The PG release promoting action of oxytocin observed in estradiol-injected rat uterus disappeared completely with treatment of phospholipase C.

The effect of PGE1 on peristalsis and on perivascular nerve inhibition of peristaltic activity in guinea-pig isolated ileum

The effect of PGE1 on peristalsis of guinea-pig isolated ileum was examined using a modified Trendelenburg method to evoke and record peristaltic activity. PGE1 (14 nM, 0.11 microM and 0.56 microM) increased peristaltic activity of both longitudinal and circular muscle, mainly by increasing the amplitude of contraction. Preparations of ileum subjected to a 'minimal' peristaltic stimulus were more sensitive to the effects of PGE1 than were preparations subjected to a 'just-maximal' peristaltic stimulus. The inhibition of peristaltic activity caused by perivascular nerve stimulation was antagonized by 0.56 microM PGE1 but slightly increased by 14 nM PGE1.

The action of bradykinin and oxytocin on the isolated whole uterus and myometrium of the rat in oestrus

1. A technique is described for obtaining a myometrial preparation devoid of endometrium, from the uterus of the rat in oestrus. 2. Acetylcholine and prostaglandin F2alpha (PGF2alpha) produced concentration-effect curves with the same maximal tensions and slope on the whole uterus and myometrial preparations. Concentration-effect curves to bradykinin and oxytocin on the myometrial preparation were altered, resulting in a shift to the right and a decreased maximum response compared with those produced by the whole uterus. 3. Indomethacin produced greater antagonism of the responses of the whole uterus to bradykinin and oxytocin than to acetylcholine and PGF2alpha, whereas responses of the myometrium to all four agonists were similarly depressed. 4. Responses of the myometrial preparation to a range of concentrations of bradykinin and oxytocin were significantly enhanced by prior sensitization of the myometrium to PGF2alpha. This significant enhancing effect of PGF2alpha was only seen with the threshold dose of acetylcholine. 5. It appears that the mechanism of action of bradykinin and oxytocin on the rat uterus involves both a direct action and an indirect action. The indirect action possibly involves release of prostaglandin(s) from the endometrium.

Prostaglandins restore the hyoscine-induced inhibition of the guinea-pig ileum

Prostaglandins E1, E2 and F2alpha as well as arachidonic acid partially increase the hyoscine inhibited twitch and tetanic responses of the guinea-pig ileum to electrical stimulation. They also produce a shift to the left of the dose-effect curve for acetylcholine and histamine previously displaced to the right by the administration of hyoscine or mepyramine. These results indicate a postsynaptic sensitization effect of the prostaglandins rather than a presynaptic facilitatory activity.

Potentiation of itch and flare responses in human skin by prostaglandins E2 and H2 and a prostaglandin endoperoxide analog

The itch and erythematous responses induced by intradermal injection of prostaglandin E2 (PGE2), the unstable prostaglandin endoperoxide PGH2 (t1/2 approximately 5 min at 37 degrees C) and the stable endoperoxide analog (15S)-hydroxy-9alpha, 11alpha-(epoxymethano)prosta-5,13-dienoic acid (EPA) were studied in volunteers. The compounds were given alone or in combination with histamine. All the compounds produced flare reaction in the skin; the order of potency was PGE2 greater than PGH2 greater than EPA. PGE2 and PGH2 evoked a sensation of itch in about half of the subjects whereas the same doses of EPA gave no itch response. In combination with histamine all compounds elicited itch of longer duration and flare of larger area than could be accounted for by simple additive effects of any released histamine. The results indicate that the PGs and PG intermediates formed in skin may potentiate the pruritogenic and flare-inducing effects of inflammagens in man.

Spasmogenic and potentiating actions of some amino acids on the guinea-pig myometrium

1 Thirty-three amino acids were applied separately in concentrations of 2 to 10 mM to guinea-pig uterine horns in vitro at pH 7.4. About half the acids regularly produced contractions.2 Glycine and the straight-chain L-alpha-amino acids up to norleucine were active (longer ones not tested); D-isomers were less potent or inactive in these concentrations. The omega-amino acids gamma-aminobutyric acid (GABA) and delta-aminovaleric, and the alpha,omega-diamino acids L-alpha,beta-diaminopropionic and L-alpha,gamma-diaminobutyric were active, whereas others of similar chain-length such as beta-alanine and lysine were not. The diacidic acids, glutamic and homocysteic, were more active than the amido-amino acids, glutamine and asparagine. Histidine and phenylalanine showed little or no activity.3 The use of appropriate blocking agents indicated that the responses to representative acids were not mediated by histamine, 5-hydroxytryptamine, acetylcholine, noradrenaline or by prostaglandins. Attempts to block the actions of glycine and GABA with strychnine, thebaine, picrotoxin, bicuculline or tetramethylenedisulphotetramine (TETS) were unsuccessful.4 When some of the acids that were spasmogenic at 2 to 10 mM were applied at sub-spasmogenic doses, they transiently potentiated other spasmogens such as oxytocin or acetylcholine. This effect was also shown by a mixture of amino acids at approximately the normal plasma concentrations.5 There is some similarity between the spasmogenic activities of different amino acids and their known abilities to depolarize neurones.

Clinical and experimental studies on the action of prostaglandins and their synthesis inhibitors on detrusor muscle in vitro and in vivo

Experiments on muscle strips have shown that prostaglandin is naturally produced by the detrusor, and acts to increase the tone and spontaneous activity. An intimate relation between acetylcholine and prostaglandin has been demonstrated. Therapeutic application of prostaglandins has been successful in the treatment of chronic retention in women.

Effect of indomethacin and prostaglandin on the smooth muscle contracting activity of angiotensin and other agonists

Indomethacin had an equal inhibitory effect on the response of the guinea-pig isolated ileum to angiotensin II (angiotensin), bradykinin, histamine and acetylcholine. This effect did not seem to result from inhibition of prostaglandin synthesis, as it did not depend on the time of treatment with indomethacin. Prostaglandin E2 (prostaglandin) potentiated the responses of the guinea-pig ileum to angiotensin, bradykinin, histamine and acetylcholine without significant differences in the effects observed. In the rabbit isolated mesenteric and coeliac arteries, indomethacin had an equal potentiating effect on the responses to angiotensin and to adrenaline. In these organs pre-incubation with indomethacin was necessary for the effect to be observed, and this effect lasted for 2 h or more after that drug was removed from the medium. No cross-tachyphylaxis between angiotensin and adrenaline was observed in the rabbit mesenteric and coeliac arteries. It is concluded that the effects of indomethacin and prostaglandin on the response of the guinea-pig ileum to the four agonists result from an action on the smooth muscle contractile mechanism per se rather than from an inhibitory action on the release of endogenous prostaglandin produced by the four agonists. The results with the rabbit isolated arteries indicate that tachyphylaxis to angiotensin in these organs is not caused by prostaglandin release.

Effects of prostaglandin E1 on the positive inotropic actions of noradrenaline, nerve stimulation and calcium in the isolated blood-perfused papillary muscle of the dog

The effects of PGE1 on the dog heart were studied using the blood-perfused sinus node and papillary muscle preparations isolated separately from the same animal. PGE1 administered into the papillary muscle artery as bolus injections in doses of 1-1000 ng caused a dose-dependent increase of the developed tension and dT/dt of the papillary muscle. The effect was not inhibited by the beta-adrenoceptor blocking agent pindolol. PGE1 injected into the sinus node artery in doses of 3-300 ng did not change the rate of contraction of the sinus node preparation. PGE1 in the blood concentrations of 4.4 X 10(-9) to 1.7 X 10(-7) M enhanced the positive inotropic responses to noradrenalin and field stimulation as well as to calcium. The influence of PGE1 on the positive inotropic effect of perivascular nerve stimulation was not consistent: the action of perivascular nerve stimulation was enhanced by PGE1 in the majority of preparations but was reduced in one third of preparations. PGE1 in the same blood concentrations as used in the papillary muscle significantly depressed the positive chronotropic responses to noradrenaline and dopamine. The present results indicate that PGE1 induces multiple actions on the dog heart. Its predominant effect on the ventricular myocardium appears to be enhancement of the adrenergic stimuli probably via the facilitation of calcium movement through the myocardial cell membrane. In addition, PGE1 may decrease the sensitivity of beta-adrenoceptors to adrenergic stimuli in the sinus node.

The effects of hormones and prostaglandins on the calcium pools in cultured myocardial cells

Determinations of the calcium pools in myocardial cells in vitro have shown the existence of at least three pools of exchangeable calcium. Epinephrine and glucagon were found to produce significant increases in the size of the two slower exchanging pools. Prostaglandins E1 and F1alpha also increased significantly calcium pool size whereas E2 and F2alpha did not; results which correlate well with the effects of the two former prostaglandins on intracellular cAMP levels. The results imply that these agents cause small, but significant, changes in the transmembrane exchange of calcium and large increases in the intracellular calcium pool. Effects which may involve the direct or indirect action of cAMP.

Terbutaline inhibition of midtrimester uterine activity induced by prostaglandin F2alpha and hypertonic saline

The effect of the beta receptor stimulator, terbutaline, on uterine activity induced by intra-amniotic injection of prostaglandin F2alpha or hypertonic saline was investigated in 20 patients undergoing therapeutic abortion in the second trimester. Intrauterine pressure was recorded, and the basal tone, intensity and frequency of contractions and "total" activity (expressed in Montevideo Units or MU), were measured. Uterine activity was higher in the prostaglandin group (mean value 393 MU) than in the hypertonic saline group (mean value 238 MU). However, terbutaline, infused at a rate of 5 to 20 mug/minute, decreased the activity by approximately the same absolute amount in the two groups to a mean value of 214 MU in the prostaglandin group and to 68 MU in the saline group. In some patients of the saline group, terbutaline completely inhibited uterine contractions. Basal tone, which was high in the prostaglandin group (mean value 28 mm Hg) decreased to a mean value of 17 mm Hg during terbutaline infusion. In the saline group the mean value for basal tone decreased from 10-5 to 7-5 mm Hg. It is concluded that uterine activity, induced by intra-amniotic injection of prostaglandin F2alpha or hypertonic saline, can be inhibited by terbutaline. The mechanisms of action for prostaglandins and hypertonic saline are discussed.

Potentiation of antinociceptive action of morphine by prostaglandin E1 in albino rats

1. Prostaglandin E1 (0.4 ml/kg, i.p.) significantly potentiated the antinociceptive action of morphine in albino rats. 2. This potentiation was significantly inhibited by pretreatment with reserpine, p-chlorophenylalanine or methysergide but not by alpha-methyl-p-tyrosine, phentolamine or propranolol. 3. The results suggest that the potentiation of morphine by prostaglandin E1 is mediated by serotonin, since it is inhibited by pharmacological agents known to reduce central turnover of serotonin receptors.

A modulating role of prostaglandins in contractions of the guinea-pig ileum

1 The role of prostaglandins in contractions of the guinea-pig ileum evoked either directly by acetylcholine or indirectly by angiotensin and by coaxial stimulation has been investigated.2 Prostaglandin E(2) in low concentration (6 nM) slightly augmented both types of contraction. Indomethacin, an inhibitor of prostaglandin synthesis, markedly reduced the indirectly evoked contractions but did not affect contractions in response to acetylcholine. The addition of prostaglandin E(2) to the preparation treated previously with indomethacin restored the effect of indirect stimulation.3 The pretreatment of the preparation with guanethidine or alpha-methyl-p-tyrosine prevented the inhibitory effect of indomethacin on indirectly evoked contractions. Prostaglandin E(2) addition to such preparations considerably augmented both types of contraction.4 The stimulation of non-cholinergic, non-adrenergic inhibitory nerves in the taenia coli and ileum preparations evoked hyperpolarization and relaxation of the preparations followed by action potentials and contraction. These responses were not changed by indomethacin pretreatment and prostaglandin E(2), but rebound contraction was sometimes augmented by the prostaglandin.5 Two mechanisms for the effects of prostaglandin E(2) are suggested: a direct effect on smooth muscle, and an indirect action through the sympathetic nerves which by release of noradrenaline affect the acetylcholine release from parasympathetic nerve endings.

Effect of calcium and vasopressin on the response of frog skin to prostaglandin E1

1. Prostaglandin E(1) increases sodium transport as measured by short circuit current (SCC) across isolated frog skin whereas calcium, added to the external Ringer fluid, decreases sodium transport. To help establish the site of action of prostaglandin the possible interaction of these two agents on sodium transport has been examined.2. The effect of a standard dose of prostaglandin (0.5 x 10(-6)M) on the short circuit current was tested on paired skins with either zero or high calcium (22.4 mM) in the external Ringer fluid. In ten experiments the responses to prostaglandin (expressed in muA/cm(2)) were not significantly affected by external calcium.3. In another series of experiments the chelating agent, EGTA, was included in calcium-free external Ringer in order to promote greater depletion of skin calcium. The response of these skins to the standard dose of prostaglandin was of the same order of magnitude as that of control skins. The response was not sustained in contrast to that of normal skins and skins in high-calcium fluids.4. In a further series of experiments the reverse procedure was adopted whereby the response of the skin to low and high doses of calcium in the external Ringer was recorded in control conditions and when the skin had responded fully to twice the standard dose of prostaglandin. In addition, the calcium-sensitive current was calculated for each skin in both circumstances. The latter was unchanged on addition of prostaglandin, and graded doses of calcium caused the same degree of inhibition of the short circuit current.5. The results show no interaction between external calcium and prostaglandin and also no need for external calcium in prostaglandin stimulation of sodium transport.6. The findings do not support the concept of chelation by prostaglandin of calcium from critical sites on the skin as the primary mechanism of its action on sodium transport. The results closely parallel those of a similar type of study into the relationship between vasopressin and external calcium on frog skin also.7. When frog skin has responded fully to either prostaglandin E(1) or vasopressin, it shows no response to the other, although removal of calcium from the external Ringer fluid causes a further increase in short circuit current.8. Vasopressin causes a further increase in short circuit current in skins treated with prostaglandin F(1alpha). Prostaglandin F(1alpha) may be a weaker agonist on frog skin than either vasopressin or prostaglandin E(1).

The contribution of prostaglandin production to contractions of the isolated uterus of the rat

1. Indomethacin and meclofenamate, both of which are potent inhibitors of prostaglandin synthetase, antagonized the contractor effects of oxytocin on the isolated uterus from the non-pregnant rat. Contractions induced by acetylcholine or prostaglandin F(2a) were not antagonized.2. Uteri from rats 17-22 days pregnant exhibited intermittent spontaneous contractions when used as isolated preparations. They also released prostaglandin-like activity (mainly similar to F(2a)) into the bathing fluid. Both the prostaglandin release and the uterine activity were abolished by indomethacin. Activity could be restored by addition of low concentrations of prostaglandin E(2) or F(2a).3. The release of prostaglandin F(2a)-like activity by the uteri increased dramatically on the expected day of delivery (day 22).4. The results add force to the hypothesis that the spontaneous activity of some isolated organs is due to an intramural prostaglandin generation, and that increased uterine prostaglandin production contributes to the expulsion of the foetus.

Interaction of prostaglandin E 1 and calcium in the guinea-pig myometrium

1. Prostaglandin E(1) (PGE(1)) increased the responses of guinea-pig myometrium in a low calcium medium to added Ca(2+), acetylcholine, vasopressin, Ba(2+) and Sr(2+). The concentration of PGE(1) used (50 pg/ml) was clearly below the threshold for direct spasmogenesis. In the presence of PGE(1) the doses necessary for half-maximal contractions were decreased by factors of 2.6 for Ca(2+), 2.4 for acetylcholine, and 3.7 for vasopressin. The responses to Ba(2+) or Sr(2+), though studied less extensively, were found to be affected in much the same manner.2. The K(+) depolarized myometrium in a low Ca(2+) medium contracts in response to added Ca(2+). These responses also were increased by low concentrations of PGE(1), but the effective concentration of PGE(1) was indistinguishable from that for direct spasmogenesis.3. Possible mechanisms for the interaction of PGE(1) and Ca(2+) in the myometrium are discussed. It is tentatively suggested that these findings may be relevant to the physiological control of human myometrium.

Prostaglandin-oxytocin enhancement and potentiation and their clinical applications

The pharmacological phenomena of enhancement and potentiation of uterine response occur respectively when combinations of some prostaglandins and oxytocin are given serially and simultaneously to a patient. Employing these phenomena allows small doses of the drugs to achieve the same effects as a large dose given alone. In a pilot study of the use of the combination of prostaglandin and oxytocin for the induction of mid-trimester abortion seven of nine women were aborted within 48 hours. Side effects attributable to prostaglandin were eliminated or reduced in severity.