The effect of prostaglandin F2α on the activator calcium of the uterus

The mechanism through which PGF2_α exerts its oxytocic action was studied in 150 uterine strips, 48 excised from 25 days pregnant and 102 from post partum rabbits. PGF2_α delayed the loss of excitability upon exposure of the uterus to Ca-free Krebs Ringer Bicarbonate (KRB) solution and accelerated recovery when half (1.25 mM) of the normal CaCl₂ was replaced in the KRB. This effect of PGF2_α was more pronounced in the post partum than the pregnant uterus, which resisted Ca-deficiency by high affinity binding in its plasma membrane of the activator-Ca (A-Ca).The Ca-ionophore A23187 simulated the action of PGF2_α but only during stimulation with 12 V/5 cm and not with 60 V/5 cm. This finding suggests that while the effect of PGF2_α is limited to a control of the movement of the extracellular and membrane-bound Ca, A23187 affects the distribution of Ca liberated by the strong (60 V/5 cm) electric field from internal stores of the myometrial cells. In the presence of Ruthenium-red (a Ca-influx inhibitor) and only 1.25 mM CaCl₂, PGF2_α restored excitability slowly, indicating that PGF2_α is an oxytocic agent because it promotes Ca-influx. However, knowing that Ca-efflux is in part dependent upon Ca-influx the most informative present finding was the observation that PGF2_α did not promote ⁴⁵Ca-efflux when ⁴⁰Ca-influx was inhibited by Verapamil. This demonstration provided more direct evidence that PGF2_α promotes the influx of the A-Ca and thus explained the mechanism of action of this key regulator at the molecular level.     

The effects of progesterone, prostaglandin F2α and oxytocin on the calcium-activation of the uterus

The relationship between “activator-calcium” (A-Ca), progesterone (P), prostaglandin F2α (PGF2α) and oxytocin (Oxy) has been examined in 100 uterine strips of 34 pregnant and 100 strips of 34 post partum rabbits. At the 25th day of gestation, uterine P was 13.9±1.3 ng/g, while within 3–12 hours post partum 3.3±0.3 ng/g tissue (P<0.001). Uterine strips, mounted isometrically in Krebs' solution, sustained maximum excitability in a steady state when exposed every 30 seconds for 4 seconds to an electric field of 12 V/5 cm (a.c.). The maximally contracting muscles were then rinsed at intervals of 6 minutes with Ca-free Krebs.In Ca-free Krebs, the post partum uterus lost 31% of its Ca and 96% of its excitability in a short 25 minutes, while the pregnant uterus lost 30% of its Ca and 93% of its excitability in 50 minutes (P<0.001). Since the extracellular space is 30% in the uterus, this 30% Ca, lost by both muscles, most probably was extracellular Ca and the small A-Ca fraction which is presumably “bound” more strongly at the membrane systems of the P-dominated pregnant, than the non-dominated post partum uterus. The significantly faster and more complete recovery from Ca-deficiency and inexcitability of the pregnant than the post partum uterus (P<0.001), at different levels of external Ca, further substantiates this premise. So does the demonstration that exposure to Ca-free Krebs increases ⁴⁵Ca-efflux 400% in the post partum and only 110% in the pregnant uterus (P<0.001). Exposure to 100 ng/ml PGF2α in normal Krebs has a similar effect on the ⁴⁵Ca-efflux of the post partum uterus, while the response of the pregnant uterus is indistinct (P<0.001).These highly significant differences between the post partum and the pregnant uteri in their Ca-efflux explain the higher threshold (P<0.001) and lower “sensitivity” to PGF2α and Oxy (P<0.001) of the pregnant than the post partum uterus. The already very highly significant differences between the two muscles, in threshold and sensitivity to these two most potent oxytocics, were increased still further by rendering the uterine strips Ca-deficient. All together, these findings substantiate the early contention (1–7,18,19) that uterine function at the cellular level is regulated by opposing actions of the suppressor P and the intrinsic stimulant PG or other oxytocic agents on threshold, excitability and the Ca-activation of the contractile process.     

The effect of Ca-transport inhibitors on the excitability of the pregnant and post partum rabbit uterus

To continue an earlier study, a total of 54 uterine strips were excised from 25 days pregnant and post partum rabbits. These muscles were examined under a variety of experimental conditions to further define functionally their “activator-Ca” (A-Ca) which couples excitation with contraction under these two extreme regulatory states of the uterus. As in earlier studies, increasing the stimulus strength in graded steps from 2 to 80 V/5 cm revealed two distinct “peaks” of excitability, one at ～12 V/5 cm and another at ～60 V/6 cm, separated by a zone of inhibition. Knowing that effective “first peak” stimulation demands intact membrane function and the availability of A-Ca, while the “second peak” stimulation probably bypasses the membrane process and activates the contractile system directly by mobilizing Ca bound to intracellular structures, the effects on the strength-tension curve of normal and Ca-free KRB, excess KCl, Verapamil, Ruthenium-red and MnCl₂ were examined. These studies exposed again the different states of the A-Ca in the uteri of pregnant and post partum rabbits, which partly explain the functional differences of the myometrium under these two extreme regulatory conditions.     

The “activator-Ca” of the pregnant and post partum rabbit uterus

From 25 days pregnant and post partum rabbits, a total of 133 uterine strips were excised and studied $\text{in}$$\text{vitro}$ under a variety of experimental conditions, to define functionally the regulatory state of the “activator-Ca” (ACa) which couples excitation with contraction in these two extreme regulatory conditions of the uterus. Using Verapamil and Ruthenium-red (both of which inhibit Ca-influx), MnCl₂ (which, in addition, displaces Ca from binding sites), or EGTA (a chelating agent), excitability and ⁴⁵Ca-efflux, were measured. The studies showed that in the progesterone-dominated pregnant uterus the ACa is more strongly bound than in the progesterone-deficient post partum uterus and that extracellular-Ca plays a greater role in the activation of the latter than in that of the former. The effects on excitability of Ca-deficiency and inhibition of Ca-transport indicate that the site of strong binding of the ACa in the pregnant uterus is in or near the plasma membrane, partly accounting for the physiological character of this membrane under progesterone dominance.     

Effect of using calcium-free Krebs' solution on basal and A23187-stimulated prostaglandin output from the day 15 guinea-pig uterus superfused in vitro

The basal outputs of prostaglandin (PG) F_2α and PGE₂ from the Day 15 guinea-pig uterus superfused in vitro were unaffected by omitting Ca²⁺ from the Krebs' solution. In contrast, this omission of Ca²⁺ reduced the basal output of 6-oxo-PGF_1α (which reflects PGI₂ production) from the uterus by an average of 50%. Spontaneous and A23187-stimulated contractions of, and the stimulation by A23187 of PGF_2α, PGE₂ and 6-oxoPGF_1α outputs from the Day 15 guinea-pig uterus were all abolished by superfusing the tissue with Krebs' solution lacking Ca²⁺. It is concluded that the basal output of 6-oxo-PGF_1α, the occurrence of spontaneous contractions, and the effects of A23187 on PG output and contractility of the Day 15 guinea-pig uterus are dependent on extracellular Ca²⁺. However, the increase in PGF_2α output from the guinea-pig uterus on Day 15 compared to days much earlier in the cycle is apparently not dependent upon extracellular Cat+. The implications of these findings regarding the biochemical mechanisms involved in the increased synthesis of PGF_2α (the uterine luteolytic hormone) by the guinea-pig endometrium during the last one-third of the cycle are discussed.     

Effects of pregnancy, oxytocin, ovine trophoblast protein-1 and their interactions on endometrial production of prostaglandin F2 alpha in vitro in perifusion chambers

Pregnancy and intrauterine infusion of ovine trophoblast protein one (oTP-1) decrease oxytocin-induced secretion of prostaglandin F2 alpha (PGF) from the uterus. In the present study, effects of oTP-1 and pregnancy on endometrial secretion of PGF were examined in an in vitro perifusion system. In Experiment 1, endometrium from day 14 pregnant and cyclic ewes was perifused sequentially on both the lumenal and myometrial sides with Krebs Ringers Bicorbonate solution (KRB), KRB plus oxytocin (1 IU/ml) and KRB alone. Endometrium from pregnant ewes secreted more PGF from both lumenal and myometrial sides than endometrium from cyclic ewes (P less than 0.05). Oxytocin stimulated secretion of PGF from both sides of endometrium regardless of status. Secretion of PGF was greater from the lumenal surface of endometrium compared to myometrium (P less than 0.05) for pregnant and cyclic ewes. For Experiment 2, endometrium was collected from day 15 cyclic ewes and perifused sequentially with KRB, KRB plus 300 ng/ml of either Bovine Serum Albumin (BSA) or oTP-1, KRB with or without BSA or oTP-1 plus oxytocin (1 IU/ml) and then KRB alone. Oxytocin stimulated greater release of PGF from oTP-1-treated than BSA-treated endometrium. Pretreatment of endometrium with oTP-1 had the same effect on oxytocin-induced PGF secretion as cotreatment with oTP-1 and oxytocin. In Experiment 3, uterine horns of cyclic ewes were catheterized on day 10 of the estrous cycle, and infused with either oTP-1 or day 16 pregnant sheep serum proteins on days 12, 13 and 14. Endometrium was collected on day 15 and perifused sequentially with KRB, KRB plus oxytocin (1 IU/ml) and then KRB alone. Treatment of ewes with oTP-1 attenuated endometrial secretion of PGF in response to oxytocin. Results of this study indicate that: (1) pregnancy stimulates basal secretion of PGF from endometrium and has no effect on oxytocin-induced secretion of PGF in vitro; (2) short-term oTP-1 treatment enhances oxytocin-induced PGF secretion from day 15 cyclic endometrium and (3) long-term oTP-1 treatment in vivo inhibits oxytocin-induced PGF secretion in ewes.     

Effects of pregnancy, oxytocin, ovine trophoblast protein-1 and their interactions on endometrial production of prostaglandin F2α in vitro in perifusion chambers

Pregnancy and intrauterine infusion of ovine trophoblast protein one (oTP-1) decrease oxytocin-induced secretion of prostaglandin F2α (PGF) from the uterus. In the present study, effects of oTP-1 and pregnancy on endometrial secretion of PHF were examined in an in vitro perifusion system. In Experiment 1, endometrium from day 14 pregnant and cyclic ewes was perifused sequentially on both the lumenal and myometrial sides with Krebs Ringers Bicorbonate solution (KRB), KRB plus oxytocin (1 IU/ml) and KRB alone. Endormetrium pregnant ewes secreted more PGF fro both lumenal and myotrial sides than endometrium from cyclic ewes (P<0.05). Oxytocin stimulated secretion of PGF was greater from the lumenal surface of endometrium compared to myometrium was collected from day 15 cyclic ewes and perifused sequentially with KRB, KRB plus 300 ng/ml of either Bovine Serum Albumin (BSA) or oTP-1, KRB with or without BSA or oTP-1 plus oxytocin (1 IU/ml) and then KRB alon. Oxytocin stimulated greater release of PGF from oTP-1-treated than BSA-treated endometrium. Pretreament of endometrium with oTP-1 has the same effect on oxytocin-induced PGF section was cotreatment with oTP-1 and oxytocin. In Experiment 3, uterine horns of cyclic ewes were catheterized on day 10 of the estrous cycle, and infused with either oTP-1 or day 16 pregnant sheep serum proteins on days 12, 13 and 14. Endometrium was collected on day 15 and perifused sequentially with KRB, KRB plus oxytocin (1 IU/ml) and then KRB alone. Treatment of ewes with oTP-1 attenuated endometrial secretion of PGF in response to oxytocin. Results of this study indicate that: (1) preganancy stimulates basal secretion of PGF from endometrium and has no effect on oxytocin-induced secretion of PGF in vitro; (2) short-term oTP-1 treatment enhances oxytocin-induced PGF secretion from day 15 cyclic endometrium and (3) long-term oTP-1 treatment in vivo inhibits oxytocin-induced PGF secretion in ewes.     

The mechanism of prostaglandin action on the pregnant human uterus

The concentrations of 15 methyl PGF2α, progesterone and estradiol in the peripheral plasma were assayed sequentially and the resting and active pressures of the uterus were quantitated in 10 first trimester pregnant patients, treated with a vaginal suppository containing 3 mg U-36,384. The purpose of the study was to determine the sequence of the prostaglandin induced changes in regulatory profile and uterine function and thus expose further the mechanism of prostaglandin action.The temporal relationships of the changes revealed that the primary action of exogenous prostaglandin is the disruption of the normal endocrine function of the conceptus and that the delayed oxytocic effect of this compound is secondary, a consequence of the primary action. Apparently prostaglandins are only effective as postconceptional agents if they convert the refractory normal pregnant uterus into a reactive organ. The academic and therapeutic significance of this finding is discussed.     

Prostaglandin F2α back transfer from the mesometrium vasculature into the uterus of the gilt during early pregnancy and estrogen-induced pseudopregnancy

Isolated reproductive tracts from gilts on the days of luteal regression (13–17 day of estrous cycle), pregnant gilts (14–18 days of pregnancy) and estrogen-induced pseudopregnant gilts (15–18 day of estrous cycle) were supplied with autologous, oxygenated blood. ³H-PGF_2α (10⁸ dpm) was infused at a constant rate into three different sites of the most superficial layer of the myometrium along the length of the horn close to the broad ligament. During infusion (60 min) and 60 min after the infusion had been stopped, arterial blood was collected continuously in 5-min samples from a small branch of the uterine artery in the mesometrium area about 10 cm from the uterine horn. A significantly higher concentration of ³H-PGF_2α in the uterine-artery blood plasma was found in pseudopregnant and pregnant gilts than in the control group. The total ³H-PGF_2α back transfer with arterial blood from the broad ligament vasculature into the uterus was 2.4 × 10⁶ dpm, 6.0 × 10⁶ dpm and 19.8 × 10⁶ dpm of infused ³H-PGF_2α in the control, pregnant and pseudopregnant gilts, respectively. We suggest that ability for PGF_2α binding and back transfer from the broad ligament vasculature into the uterus, as observed in pseudopregnant and pregnant gilts, may strongly reduce the peak concentration during the pulsatile release of PGF_2α from the uterus and may protect the corpus luteum against luteolysis.     

The mechanism of prostaglandin action on the pregnant human uterus.

The concentrations of 15 methyl PGF2 alpha, progesterone and estradiol in the peripheral plasma were assayed sequentially and the resting and active pressures of the uterus were quantitated in 10 first trimester pregnant patients, treated with a vaginal suppository containing 3 mg U-36,384. The purpose of the study was to determine the sequence of the prostaglandin induced changes in regulatory profile and uterine function and thus expose further the mechanism of prostaglandin action. The temporal relationships of the changes revealed that the primary action of exogenous prostaglandin is the disruption of the normal endocrine function of the conceptus and that the delayed oxytocic effect of this compound is secondary, a consequence of the primary action. Apparently prostaglandins are only effective as postconceptional agents if they convert the refractory normal pregnant uterus into a reactive organ. The academic and therapeutic significance of this finding is discussed.     

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-PGF_1α but less active than PGE₂ or PGF_2α. Apart from a direct stimulant effect, prostacyclin also exhibited an indirect potentiating action. In threshould concentrations prostacyclin caused a 3-fold potentiation of threshold doses of oxytocin. A lesser 1.5-fold potentiation of PGF_2α was also observed. The implications of these findings in relation to prostacyclin playing a role in parturition are discussed.     

Localization of prostaglandin F in the ovine uterus during early pregnancy

As part of a study on the anti-luteolytic action of the sheep conceptus, the distribution of prostaglandin F (PGF) within the uteri of 19 pregnant and 14 non-pregnant ewes was studied using three antisera to PGF_2α and fluorescent antibody tracing. In the uteri of seven non-pregnant ewes up to Day 11 after estrus (Day 0) and in uteri of $\text{18}\text{19}$ pregnant ewes up to Day 50, PGF was localized mainly in the lamina propria, with very little on epithelial cells lining the uterine lumen. After Day 11, in the uteri of seven non-pregnant ewes, PGF was localized on the surface of luminal epithelial cells and throughout their cytoplasm, and to a lesser extent in the lamina propria. The distribution of PGF on Days 14 and 17 in the gravid and non-gravid horns of the uteri of 13 ewes made unilaterally pregnant (conceptus confined to one uterine horn) was similar to that described above for normal pregnant and non-pregnant ewes after Day 11 respectively.These results are interpreted to indicate a change in the distribution of PGF in sheep uteri due to the presence of a conceptus.     

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.     

The effects of ovariectomy and strech on the regulatory profile and activity of the uterus

Following ovariectomy of five New Zealand white rabbits at day 25 of pregnancy, the intrauterine pressure (IUP) and uterine progesterone (P) and prostaglandin (PG) levels were measured sequentially at days 25, 26 and 27. At day 25, when the uterine P and PGE and PGF were high, massive intrauterine treatment with 500 μg PGF2α provoked only a sustained contracture on which only low level oscillation in IUP was superimposed. At day 26, when the P levels had decreased significantly (P<0.001) and the PG levels had not changed significantly, 50 μg PGF2α significantly increased cyclic IUP as compared with the day 25 value (P<0.001). At day 27, when the P levels decreased further, as little as 5 μg PGF2α provoked still higher cyclic IUP, in spite of a significant reduction in PG levels (P<0.05).Stretching the uterus of six post partum and six 26 days pregnant rabbits (after removing the uterine contents) significantly increased the uterine PGF levels (P<0.001). However, stretch increased only cyclic IUP of the post partum uterus and was without effect on the pregnant uterus, which still had high P levels. These results indicate that the myometrium activated by exogenous PG or stretch, regardless of whether the uterine PG levels increase, remain unchanged or even moderately decrease, provided that the uterine P levels are reduced to a critical value.     

Wall appositions induced by ionophore A 23187, CaCl2, LaCl3, and nifedipine in characean cells

Summary The formation of wall appositions (plugs) by ionophore A 23187, CaCl₂, LaCl₃, and nifedipine was studied in mature internodal cells of characeaen algae. CaCl₂ at concentrations above 10^–2M induces thick fibrillar plugs without callose inNitella flexilis. InChara corallina andNitella flexilis ionophore A 23187 (1.25×10^–5 to 5×10^–5M) and LaCl₃ (7.5×10^–5 to 2.5×10^–4M) cause flat appositions which contain callose and have a more granular structure. Plug formation by ionophore A 23187, CaCl₂, and LaCl₃ is pH-dependent and occurs beneath the alkaline regions of the cell. Nifedipine (10^–4 to 10^–5M) induces plugs inNitella flexilis after previous injury. These callose-containing wall appositions consist of a heterogeneous granular core which is covered by a fibrillar layer. The results of this work are compared with previous studies on wound wall formation and chlortetracycline (CTC)-induced plug formation which reveal that abundant coated vesicles occur only when a thick fibrillar wall layer is formed. Neither LaCl₃ nor nifedipine inhibit the formation of CaCl₂- or CTC-plugs. The unusual effects of these substances, which normally act as Ca²⁺ antagonists and therefore should prevent and not induce plug formation, are discussed. It is suggested that La³⁺ mimicks the effects of calcium and that nifedipine binding to the Ca²⁺ channels is altered in the alkaline regions of characean internodes and allows an influx of Ca²⁺.Abbreviations AFW artificial fresh water - CTC chlortetracycline - DCMU dichlorphenyldimethylurea - DMSO dimethylsulfoxide - EGTA ethyleneglycoltetraacetic acid - MES 2-(N-morpholino) ethanesulfonic acid - HEPES N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid - TAPS N-tris[hydroxymethyl]methyl-3-aminopropanesulfonic acid     

Effects of hydrocortisone, oestradiol and progesterone on A23187-stimulated prostaglandin output from the guinea-pig uterus superfused in vitro

Hydrocortisone (10 micrograms/ml) had no effect on the basal outputs and A23187-stimulated outputs of PGF2 alpha, PGE2 and 6-keto-PGF1 alpha from the Day 15 guinea-pig uterus superfused in vitro. These findings indicate that the high output of PGF2 alpha from the guinea-pig uterus during the last one-third of the oestrous cycle is not modulated by the adrenal glucocorticoid hormones. Progesterone (10 micrograms/ml) had no effect on the A23187-induced increases in PG output from the Day 15 guinea-pig uterus. However, oestradiol (10 micrograms/ml but not 1 microgram/ml) significantly reduced the increases in outputs of PGF2 alpha, PGE2 and 6-keto-PGF1 alpha induced by A23187 from the Day 15 guinea-pig uterus, without affecting basal PG outputs. The increase in uterine tone induced by A23187 in the Day 15 guinea-pig uterus was reduced by 20-50% by oestradiol (10 micrograms/ml). The addition of oestradiol (10 micrograms/ml) and progesterone together (10 micrograms/ml) produced the same effects on the Day 15 guinea-pig uterus as oestradiol alone. Oestradiol (10 micrograms/ml) also reduced the A23187-induced increases in PG output from the Day 7 guinea-pig uterus, but did not reduce the increase in uterine tone. Oestradiol (10 micrograms/ml) reduced the increases in outputs of PGF2 alpha, PGE2 and 6-keto-PGF1 alpha induced by exogenous arachidonic acid from the Day 7 and Day 15 guinea-pig uterus. Previous studies have shown that oestradiol is not a cyclo-oxygenase inhibitor. The present findings suggest that oestradiol, at a relatively high concentration, may interfere with the access of arachidonic acid to the cyclo-oxygenase enzyme. This action of oestradiol may explain its anti-luteolytic action when administered to guinea-pigs in large doses after Day 9 of the cycle.     

Prolactin Release induced by Prostaglandin F2α in Pregnant Rats

PROSTAGLANDIN (PGF2α) induces lactogenesis and advanced parturition when injected into pregnant rats¹. A luteolytic effect of PGF2α has also been demonstrated in pseudopregnant and pregnant rats^3,4, normal guinea-pig⁵, autotransplanted sheep ovary⁶ and pregnant hamster⁷. The decrease of plasma progesterone concentration before parturition^8–10 will trigger the release of prolactin and other hormones from the hypophysis and induce lactogenesis^11–13 which in the normal pregnant rat occurs around 12 h before parturition¹³.     

Effects of hydrocortisone, oestradiol and progesterone on A23187-stimulated prostaglandin output from the guinea-pig uterus superfused in vitro

Hydrocortisone (10 μg/ml) had no effect on the basal outputs and A23187-stimulated outputs of PGF_2α, PGE₂ and 6-keto-PGF_1α from the Day 15 guinea-pig uterus superfused . These findings indicate that the high output of PGF_2α from the guinea-pig uterus during the last one-third of the oestrous cycle is not modulated by the adrenal glucocorticoid hormones. Progesterone (10 gmg/ml) had no effect on the A23187-induced increases in PG output from the Day 15 guinea-pig uterus. However, oestradiol (10 gmg/ml but not 1 μg/ml) significantly reduced the increases in outputs of PGF_2α, PGE₂ and 6-keto-PGF_1α induced by A23187 from the Day 15 guinea-pig uterus, without affecting basal PG outputs. The increase in uterine tone induced by A23187 in the Day 15 guinea-pig uterus was reduced by 20–50% by oestradiol (10 μg/ml). The addition of oestradiol (10 μg/ml) and progesterone together (10 gmg/ml) produced the same effects on the Day 15 guinea-pig uterus as oestradiol alone. Oestradiol (10 μg/ml) also reduced the A23187-induced increases in PG output from the Day 7 guinea-pig uterus, but did not reduce the increase in uterine tone. Oestradiol (10 gmg/ml) reduced the increases in outputs of PGF_2α, PGE₂ and 6-keto-PGF_1α induced by exogenous arachidonic acid from the Day 7 and Day 15 guinea-pig uterus. Previous studies have shown that oestradiol is not a cyclo-oxygenase inhibitor. The present findings suggest that oestradiol, at a relatively high concentration, may interfere with the access of arachidonic acid to the cyclo-oxygenase enzyme. This action of oestradiol may explain its anti-luteolytic action when administered to guinea-pigs in large doses after Day 9 of the cycle.     

Effects of epinephrine and oxytocin on the release of prostaglandin F from the rat uterus in vitro

Isolated uteri from rats with regular 4-day cycles were incubated in Krebs-Ringer bicarbonate buffer and the release of PGF into the medium was measured by radio-immunoassay after extraction of the incubation medium with ethyl acetate at pH 3.0–3.5. PGF was produced from endogenous precursors and accumulated in equal amounts in the medium during two successive 60 min periods on each day of cycle, but the magnitude of the production varied significantly during the cycle, being greatest in estrus. Oxytocin in doses up to 500 mU/ml had no effect on PGF accumulation in the incubation period at any stage of the cycle, while epinephrine (10⁻³) greatly stimulated PGF release from the estrous uterus but had no effect on PGF release from the diestrous uterus. Phentolamine, an α-blocking agent, had no effect on the epinephrine-induced release of PGF, while propranolol, a β-blocking agent, not only prevented the increase in PGF production induced by epinephrine but also reduced the basal release of PGF by the estrous uterus. Since oxytocin contracts and epinephrine relaxes the nonpregnant rat uterus both in vivo and in vitro, it is unlikely that the effects of these two compounds on uterine contractility are mediated by the release of PGF_2α.     

Role of calcium ion in the excitability and electrogenic pump activity of theChara corallina membrane: I. Effects of La3+, verapamil,EGTA, W-7, and TFP on the action potential

Summary The cytoplasmic streaming of the normal internodal cell of giant algaChara stops transiently at about the peak of action potential. Application of La³⁺ or verapamil (a calcium channel blocker) or removal of external Ca²⁺ by EGTA caused a partial depolarization of the resting potential, partial decrease of the membrane conductance and a marked decrease of the amplitude of action potential. Under these conditions, the conductance at the peak of action potential reduced markedly and the streaming of cytoplasm did not cease during action potential (excitation-cessation (EC) uncoupling). The effects of Ca²⁺ channel blockers could not be removed by addition of CaCl₂ to the external medium. In contrast, the effect of EGTA on the excitability could be removed to a greater extent and the cytoplasmic streaming ceased at about the peak of action potential by the addition of Ca²⁺ externally. Application of calmodulin antagonists W-7 or TFP caused similar effects on the action potential and on the cytoplasmic streaming.