Inhibition of vasoconstrictor responses by 6-keto-PGE1 in the feline mesenteric vascular bed

The effects of 6-keto-PGE₁ on vascular resistance and vascular responses to sympathetic nerve stimulation and vasoconstrictor hormones were investigated in the feline mesenteric vascular bed. Infusions of 6-keto-PGE₁ into the superior mesenteric artery dilated the mesenteric vascular bed and markedly inhibited vasoconstrictor responses to sympathetic nerve stimulation, norepinephrine and angiotensin II. The effects of 6-keto-PGE₁ and PGE₁ on vascular resistance and vasoconstrictor responses were quite similar and both substances inhibited responses to nerve stimulation and pressor hormones in a reversible manner. Responses to nerve stimulation, norepinephrine and angiotensin II were inhibited to a similar extent during infusion of 6-keto-PGE₁ and PGE₁. Results of these studies suggest that 6-keto-PGE₁, a newly identified prostaglandin metabolite, and PGE₁ possess the ability to inhibit the vasconstrictor effects of sympathetic nerve stimulation and pressor hormones by a nonspecific action on vascular smooth muscle in the feline small intestine.     

Release of endogenous prostaglandins by mild hyperosmotic saline inhibits tetragastrin-stimulated gastric acid secretion in rats

Filling of the gastric lumen of rats with 1.0 M NaCl solution (5 ml) for 10 min under urethane anesthesia caused an increase in the gastric fluid concentrations of prostaglandin (PG) E₂, 13, 14-dihydro-15-keto-PGE₂ and 6-keto-PGF_1α as determined by radioimmunoassay. PGE₂ was the major PG generated. The levels of PGE₂ in the gastric fluid were increased dose-dependently after filling the lumen with 0.3, 0.5, 0.7 or 1.0 M NaCl solutions. The pH of the gastric fluid increased similarly after 0.5 to 1.0 M NaCl solutions. Indomethacin (10 mg/kg, i.p.) suppressed the PGE₂ increase caused by 1.0 M NaCl solution, but did not prevent the increase of the pH of the gastric fluid induced by intragastric 1.0 M NaCl. Infusion of tetragastrin (62.5 μg/kg/hr, i.v., for 10 min) caused a marked increase of acid secretion without modifying intragastic concentration of PGE₂. The acid secretion due to tetragastrin was completely inhibited after intragastric administration of 1.0 M NaCl solution, while indomethacin restored the tetragastrin-induced acid secretion, with prevention of a rise of intragastric PGE₂ levels. These observations suggest that 1.0 M NaCl solutions suppress basal intragastric acid through a mechanism which is independent of prostaglandins. In contrast, the suppression of tetragastrin-induced acid secretion by intragastric 1.0 M NaCl solution appears to be mediated through a release of prostaglandins     

Analysis by high performance liquid chromatography of cyclooxygenase products of arachidonic acid metabolism in plasma of rabbits bearing the VX2 carcinoma

Cyclooxygenase products of arachidonic acid metabolism in the plasma of normal rabbits and animals bearing the VX₂ carcinoma were separated by high performance liquid chromatography and the effluent fractions assayed by serologic methods. The products measured were 6-keto-PGF_lα, thromboxane B₂, PGE₂, PGF_2α, 13,14-dihydro-PGE₂, 13,14-dihydro-15-keto-PGE₂, 15-keto-PGE₂, and 13,4-dihydro-15-keto-PGF_2α. In hypercalcemic, tumor-bearing rabbts, the plasma concentrations o 13,14-dihydro-15-keto-PGE₂ and 13,14-dihydro-15-keto-PGF_2α were markedly elevated (in the range of 0.5 to 16 ng/ml). Previously unmeasured 6-keto-PGF_1α, thromboxape B₂, 13,14-dihydro-PGE₂ and 15-keto-PGE₂ were not found in high concentrations in the plasma of tumor-bearing rabbits. These results add further support to our conclusion that the VX₂ tumor produces hypercalcemia in the host by a mechanism which utilizes PGE₂, rather than a subsequent metabolite of this prostaglandin, as the mediator between the neoplasm and bone.     

6-Keto-PGE1 is a potent relaxant of the fetal ductus arteriosus

6-Keto-PGE₁ is nearly as potent as PGE₂ in relaxing the ductus arteriosus of fetal lambs. This finding raises the possibility that 6-keto-PGE₁, if occuring naturally as a by-product of PGI₂ transformations, may contribute to prenatal patency of the vessel.     

Metabolism of N-acetyl PGE2 carboxamide in the rat

After intratracheal administration to rats, the bronchodilator N-acetyl PGE₂ carboxamide was converted rapidly to PGE₂ and 13,14-dihydro-15-keto-PGE₂, the major plasma metabolite. Oxidation of the N-acetyl carboxamide by prostaglandin dehydrogenase and hydrolysis of the imide bond were demonstrated in vitro.     

Salutary actions of nimodipine in traumatic shock

Nimodipine, a new calcium channel blocker, was found to prolong survival in a severely lethal form of traumatic shock in rats. Nimodipine, at infusion rates of 50 μg/kg/h also significantly limited the increase in circulating myocardial depressant factor (MDF) without significantly preventing the accumulation of the lysosomal protease, cathepsin D in the blood. Lower infusion rates were ineffective. Nimodipine did not stabilize lysosomal membranes directly in liver lysosomal suspensions, but was effective in preventing proteolysis in pancreatic homogenates. Nimodipine potentially prevents MDF formation by inhibiting proteases and probably by splanchnic vasodilation. Calcium channel blockers may be useful agents in the therapeutics of traumatic shock if given soon after the onset of the trauma.     

Accumulation of Cyclooxygenase Products of Arachidonic Acid Metabolism in Gerbil Brain During Reperfusion After Bilateral Common Carotid Artery Occlusion

: Several of the cyclooxygenase products of arachidonic acid were measured in the cerebral hemispheres of gerbils subjected to transient interruption of the cerebral circulation. The levels of PGD₂, PGF₂α, PGE₂, TXB₂, 13,14-H₂-15-keto-PGE₂, and the stable nonenzymic product of prostacyclin, 6-keto-PGF₁α, were not altered at the end of a 5-min period of ischemia. However, the onset of reperfusion was accompanied by a rapid accumulation of these products. Levels were highest during the initial period of reperfusion, then decreased to approach control levels after 120 min. PGD₂, PGF₂α, and PGE₂ were the predominant metabolites detected. This postischemic accumulation of arachidonic acid metabolites could be blocked by prior administration of inhibitors of cyclooxygenase activity.     

Opposition of the vasopressin-induced vasoconstriction in the isolated perfused rat kidney by some prostaglandins

PGE₂ can vasoconstrict or vasolidate the isolated Krebs-perfused rat kidney depending on the tone of the renal vasculature. Thus, it is weakly constrictor (threshold 5–10 ng bolus dose) in the perfused kidney whose perfusion pressure is 47 ± 2 SD mmHg (n = 6), but becomes a vasodilator (threshold ～ 10 pg) in the kidney whose perfusion pressure has been raised to 73 ± 6 SD mmHg (n = 6) or 121 ± 8 SD mmHg (n = 6) through constant infusion of Vasopressin (0.1 and 0.25 mU/ml respectively). PGE₁ was equally effective as PGE₂ while other PGs, I₂, I₁, and 6-keto E₁, were less effective in opposing vasoconstriction. PGF_2α was inactive up to a dose of 10 ng.     

Interaction between prostaglandin E2 and leukotriene D4 on the excretion of electrolytes by the dog kidney in vivo

Recent experiments indicate that prostaglandin E₂ potentiates the vasodilatory properties of leukotrienes in the skin microcirculation. The present experiments were undertaken to study the effect of leukotriene D₄ and prostaglandin E₂ on renal hemodynamics and urinary electrolytes in the dog. Experiments were performed in three groups of anesthetized Mongrel dogs: the first group was studied under hydropenia, whereas the two remaining groups were studied during water diuresis with (Group 3) or without indomethacin (Group 2). LTD₄ (100ng/min) and PGE₂ (3ug/min) were infused in the left renal artery to minimize systemic effects of these compounds. LTD₄ alone failed to influence urinary sodium excretion in all 3 groups. In Group 1, urinary sodium increased from 77 ± 6 to 393 ± 74uEq/min during PGE₂, and further increased to 511 ± 52uEq/min during LTD₄ + PGE₂. No change occured in the contralateral right kidney. In this group, glomerular filtration as well as renal plasma flow were not statistically influenced. In Group 2, the same phenomenon was observed for urinary sodium. The combined infusion of LTD₄ + PGE₂ increased urinary sodium without significant changes in glomerular filtration and renal plasma flow. Finally, in Group 3, indomethacin was shown to reduce the natriuretic effects of LTD₄ and PGE₂: during PGE₂ alone, urinary sodium increased from 90 ± 14 to 260 ± 66uEq/min, and only rose from 80 ± 10 to 175 ± 19uEq/min during the combined infusion of LTD₄ and PGE₂. In groups 2 and 3, free water clearance was utilized as an index of sodium chloride reabsorption in the thick ascending limb: this parameter increased from 2.35 ± 0.25 to 4.70 ± 0.30ml/min, while urinary volume was increasing from 3.55 ± 0.25 to 10.05 ± 0.65ml/min, during LTD₄ + PGE₂. Indomethacin, administered in Group 3, (3mg/kg/hr) again abolished the effect of combined PGE₂ + LTD₄. These results indicate a potentiating effect of leukotriene D₄ on the PGE₂-induced natriuresis in the anesthetized dog. These phenomena occured in the absence of significant changes in renal hemodynamics, therefore suggesting a direct tubular effect of these arachidonic acid metabolites. Finally, the water diuresis experiments suggest a proximal site of action of PGE₂ and LTD₄.     

Role of prostaglandis in the control of renin secretion in the dog (II)

Infusion of prostaglandin E₁ (PGE₁) into the renal artery of anesthetized dogs (1.03 μg/min) caused increases in urine flow rate (V), renal plasma flow (RPF) and renin secretion rate without any change in mean arterial blood pressure (MABP), whereas infusion of prostaglandin F₂α (PGF_2α), (1.03 μg/min) caused no consistent change in V, RPF, or renin secretion rate. Infusion of prostaglandin E₂ (PGE₂) (1.03 μg/min) into the renal artery of “non-filtering” kidneys caused renin secretion rate to rise from 567.7 ± 152.0 U/min(M ± SEM) during control periods to 1373.6 ± 358.5 U/min after 60 minutes of infusion of PGE₂ (P < 0.01), without significant change in MABP (P > 0.1). The data suggest that PGE₁ and PGE₂ play a role in the control of renin secretion. The data further suggest that PGE may control renin secretion through a direct effect on renin-secreting granular cells.     

Plasma prostaglandin levels and circulating fuel levels in rats with diabetic ketoacidosis: Effects of cyclooxygenase inhibitors and of alpha and beta adrenergic blockade

We studied the effects of two structurally unrelated inhibitors of the fatty acid cyclooxygenase and of alpha and beta adrenergic blockade on the elevated plasma levels of 13,14-dihydro-15-keto-prostaglandin (PG)E₂, 6-keto-PGF_1α and thromboxane(TX)B₂, the stable derivatives of PGE₂, PGI₂ (prostacyclin) and TXA₂, respectively, in rats with streptozotocin-induced diabetic ketoacidosis (DKA). Meclofenamic acid and indomethacin each produced a significant decrease in the elevated plasma levels of 13,14-dihydro-15-keto-PGE₂, 6-keto-PGF_1α and TXB₂. Phentolamine significantly reduced the plasma level of TXB₂ but had no effect on the elevated circulating levels of glucose, free fatty acids, total ketones, 13,14,-dihydro-15-keto-PGE₂ or 6-keto-PGF_1α. Propranolol significantly reduced the elevated circulating levels of glucose, free fatty acids and total ketones but had no effect on the levels of the three prostaglandin derivatives. The ability of meclofenamic acid and indomethacin to reduce the plasma levels of 13,14-dihydro-15-keto-PGE₂, 6-keto-PGF_1α and TXB₂ confirms that the plasma levels of these three derivatives are elevated in rats with DKA. Since abnormalities in the production of PGI₂ and perhaps other cyclooxygenase derivatives may contribute to the pathogenesis of certain important hemodynamic and gastrointestinal features of DKA, cyclooxygenase inhibitors may play a role in the management of selected patients with this disorder. Alpha adrenergic activity is essential for the maintenance of the elevated plasma TXB₂ level in rats with DKA. The fall in the plasma TXB₂ level during alpha adrenergic blockade appears to reflect inhibition of platelet aggregation and platelet TXA₂ production, but other sources of the elevated plasma TXB₂ level in DKA are not excluded. Beta adrenergic activity contributes to the maintenance of elevated circulating levels of glucose, free fatty acids and total ketones in experimental DKA but not to the elevated plasma levels of the prostaglandin derivatives.     

Characterization of biological properties of synthetic and biological leukotriene B3

The effect of micropuncture of the renal papilla through an intact ureter on urinary concetrating ability of rats was examined. Micropuncture of the renal papilla caused a fall in urine osmolality in the punctured kidney from 1718 ± 106 to 1035 ± 79 mosmol/kg·H₂O. In order to investigate the role of renal prostaglandins in this process, PGE₂ excretion was measured and found to increase from 63.4 ± 14.0 to 205.5 ± 57.1 pg/min. Urine osmolality and PGE₂ excretion from the contralateral kidney were not significantly altered. In animals given meclofenamate (2 mg/kg·hr), renal PGE₂ excretion was reduced to 22.3 ± 5.1 pg/min prior to micropuncture and it remained low at 8.9 ± 1.8pg/min after papillary micropuncture. Meclofenamate also blocked the fall in urine osmolality caused by micropuncture of the renal papilla, with urine osmolality averaging 1940 ± 122 before and 1782 ± 96 mosmol/kg·H₂O after the micropuncture. These results indicated that papillary micropuncture through an intact ureter increased renal PGE₂ excretion and that a rise in renal production of PGE₂ or some other prostanoid is associated with a fall in urine concentrating ability.     

Reversal of indomethacin-induced inhibition of tubuloglomerular feedback by prostaglandin infusion

Experiments were performed in rats to study the effect of infusion of PGI₂, PGE₂, and PGF_2α on tubuloglomerular feedback responses (i.e. the change of SNGFR in response to a change of loop of Henle flow rate) in the presence and absence of simultaneous inhibition of endogenous PG synthesis with indomethacin. Infusion of PGI₂ or PGE₂ at rates that did not alter arterial blood pressure did not significantly modify the magnitude of feedback responses (PGI₂) 8.5 μg/hr, PGE₂ 85 μg/hr). Some inhibition of feedback responses was seen when PGI₂ and PGE₂ were administered at higher rates were associated with a reduction of blood pressure (PGI₂ 20 μg/hr, PGE₂ 200 μg/hr). PGI₂ (8.5 μg/hr) and PGE₂ (85 μg/hr) largely prevented feedback inhibition induced by indomethacin. When given subsequent to indomethacin PGI₂ and PGE₂ restored feedback responsiveness almost to normal. In contrast, PGF_2α did not influence feedback inhibition caused by indomethacin. Infusion of PGI₂ induced partial restoration of feedback responses in DOCA-salt treated animals in which the feedback system is virtually completely inactive. Our results indicate that availability of PGI₂ or PGE₂ is necessary for the normal operation of the tubuloglomerular feedback mechanism for control of nephron filtration rate.     

Separation of prostaglandin metabolites on sephadex LH 20 columns

Sephadex LH 20 columns have been investigated for the separation of initial prostaglandin metabolites. Solvent systems are described for the separation of the free acids of 15-keto-dihydro-PGE₁, 15-keto-PGE₁, PGE₁, and PGF_1α. Further, one of the solvent systems is described for the separation of pulmonary metabolites of PGE₁ and PGF_1α, and anotehr one for separation of dihydro-PGE₁ and PGE₁.     

Effect of angiotensin II and norepinephrine on release of prostaglandins E2 and I2 by the perfused rat mesenteric artery

Prostaglandin E₂ (PGE₂) and 6 keto-PGF_1α, the stable metabolite of prostacyclin (PGI₂), have been measured in the effluent of perfused rat mesenteric arteries by the use of a sensitive and specific radioimmunoadday (RIA) method. The PGE₂ and 6-keto-PGF_1α were continuousyl released by the unstimulated mesenteric artery over a period of 145 min. After 100 min of perfusion the release of PGE₂ and 6-keto-PGF_1α was 4.5 ± 8.4 pg/min and 254 ± 75 pg.min respectively, which is in accord with the general belief that PGI₂ is the major PG synthesized by arterial tissue. Angiotensin II (AII) 5 ng/ml) induced an increased of PGE₂ and 6-keto-PGF_1α release without changing the perfusion pressure. The effect of norepinephrine (NE) injections on release of PGs depended on the duration of the stabilization period. The changes of perfusion pressure induced by NE were not related to changes in release of PGs. Thus, it seems that the increase of PG release induced by AII and NE was due to a direct effect of the drugs on the vascular wall. This may represent an important modulating mechanism in the regulation of vascular tone.     

Determination of cyclooxygenase products and prostaglandin metabolites using high-pressure liquid chromatography and radioimmunoassay.

A method is described for measurement of the cyclooxygenase products, thromboxane,prostacyclin, and prostaglandins (PG), and several prostaglandin metabolites. The procedure involves separation of the compounds by high-pressure liquid chromatography combined with identification and estimation by serologic analysis. These combined procedures have been used to identify and estimate five such products, PGE₂, PGE₁ PGF2α, PGF_1α, and 6-keto-PGF_1α, in the culture fluids of dog kidney cells stimulated by a tumor-promoting phorbol diester. The prostaglandin metabolites, 13,14-dihydro-15-keto-PGE₂, 13,14-dihydro-15-keto-PF2_2α, 13,14-dihydro-PGE₂, and 13,14-dihydro-PGF_2α, were not found in these culture fluids.     

Prostacyclin induces a surprising long-lasting motility in quiescent uterine strips (indomethacin-treated) isolated from ovariectomized rats

Dose-response curves for several prostaglandins (PGI₂; PGD₂; PGF₂ and PGE₂); BaCl₂ or prostaglandin metabolites (15-keto-PGF_2α; 13, 14-diOH-15-keto-PGF_2α; 6-keto-PGF_1α and 6-keto-PGE₁ in quiescent (indomethacin-treated) uterine strips from ovariectomized rats, were constructed. All PGs tested as well as BaCl₂, triggered at different concentrations, evident phasic contractions. Within the range of concentrations tested the portion of the curves for the metabolites of PGF_2α was shifted to the right of that for PGF_2α itself; the curve for 6-keto-PGF_1α was displaced to the right of the curve for PGI₂ and that for 6-keto-PGE₁ to the left.It was also demonstrated that the uterine motility elicited by 10⁻⁵ M PGF_2α and its metabolites was long lasting (more than 3 hours) and so it was the activity evoked by PGI₂; 6-keto-PGF_1α and BaCl₂, but not the contractions following 6-keto-PGE₁, which disappeared much earlier. The contractile tension after PGF_2α; 15-keto-PGF_2α; 13, 14-diOH-15-keto-PGF_2α and PGI₂, increased as time progressed whilst that evoked by 6-keto-PGF_2α or BaCl₂ fluctuated during the same period around more constant levels.The surprising sustained and gradually increasing contractile activity after a single dose of an unstable prostaglandin such as PGI₂, on the isolated rat uterus rendered quiescent by indomethacin, is discussed in terms of an effect associated to its transformation into more stable metabolites (6-keto-PGF_1α, or another not tested) or as a consequence of a factor which might protects prostacyclin from inactivation.     

Effects of PGE1 or PGE2 on luteal function in pseudopregnant rats

Effects of PGE₁ or PGE₂ on luteal function were studied in 163 pseudopregnant rats. PGE₁ (10, 100, or 300μg) given intrauterine every 6 hr did not shorten pseudopregnancy (P < 0.05), however, the same doses of PGE₂ given intrauterine every 6 hr advanced luteolysis (P < 0.05). PGE₁ (100 or 300μg) given every 4 hr intramuscular maintained levels of progesterone in peripheral blood above controls (P < 0.05) while 100 or 300μg of PGE₂ hastened the decline in progesterone (P < 0.05). The antiluteolytic effect of PGE₁ was not via an inhibition of PGF secretion (P < 0.05) by the uterus or by induction of ovulation in treated animals. Moreover, PGE₁ (100, 200, or 500μg) given intramuscular every 4 hr from day 4 of pseudopregnancy until the next proestrus delayed luteal regression around 3 days (P < 0.05). PGE₂ at doses of 100, 200, or 500μg every 4 hr given intramuscular consistently shortened pseudopregnancy (P < 0.05). Lower doses were without effect (P < 0.05). Based on the above data it is concluded that PGE₂ is consistently luteolytic whereas PGE₁ is not luteolytic in pseudopregnant rats and that PGE₁ may be an antiluteolysin.     

Hepatic metabolism of prostacyclin (PGI2) in the rabbit: Formation of a potent novel inhibitor of platelet aggregation

Metabolism of [9-³H]-PGI₂ was studied in the isolated Tyrode's perfused rabbit liver. Five products, four radioactive and one non-radioactive, were identified in the perfusate: 19-hydroxy-6-keto-PGF_1α, 6-keto-PGF_1α, dinor-6-keto-PGF_1α, pentanor PGF_1α and a 6-keto-PGE₁-like substance. The first two, 19-hydroxy-6-keto-PGF_1α and 6-keto-PGF_1α, represented 5% and 45% respectively, of the total radioactivity; the last two accounted for 39%. The presence of dinor and pentanor derivatives of 6-keto-PGF_1α indicated that β -oxidation and oxidative-decarboxylation occurs in the liver as the major metabolic pathway of PGI₂. One non-radioactive metabolite which co-migrated with authentic 6-keto-PGE₁ was found to inhibit platelet aggregation, having a potency similar to authentic 6-keto-PGE₁, and its effect can be eliminated by boiling and by alkali treatment. This metabolite, having similar Rf value on TLC and biological behavior as 6-keto-PGE₁, may arise from oxidation of 6-keto-PGF_1α via the 9-hydroxyprostaglandin dehydrogenase pathway, as suggested by recovery of tritiated water in the aqueous phase of the perfusate. This material, a potent inhibitor of platelet aggregation, may arise from PGI₂ or its hydrolysis product, 6-keto-PGF_1α.     

6-Keto-prostaglandin E1 is not equipotent to prostacyclin (PGI2) as an antiaggregatory agent

A direct comparison of the relative potencies of the two anti-aggregatory prostaglandins PGI₂ and 6-keto-PGE₁ showed PGI₂ was at least 20 times more potent than 6-keto-PGE₁ when tested against ADP-induced human platelet aggregation. This marked difference in potency was even more evident when the ability of PGI₂ and 6-keto-PGE₁ to stimulate platelet cyclic AMP levels was determined. When cyclic AMP levels were measured direct comparisons were difficult because the respective dose response curves were not parallel, but 10 ng of PGI₂ was equivalent to 300 ng of 6-keto-PGE₁.PGI₂ was also more potent (10–20 times) than 6-keto-PGE₁ as a disaggregatory agent, and the disaggregatory activity of both prostaglandins was enhanced by the phosphodiesterase inhibitor 1-methyl-3-isobutylmethylxanthine.PGI₂ was also more active than 6-keto-PGE₁ as an inhibitor of thrombus formation in dog coronary arteries in vivo. In vivo, 6-keto-PGE₁ was at least 10 times less potent than PGI₂, the exact difference could not be determined because 6-keto-PGE₁ caused significant falls in blood pressure before anti-platelet activity could be detected.PGI₂ is an intrinsically more potent anti-aggregatory molecule than 6-keto-PGE₁, but these data do not rule out the possibility that some of the activities attributed to PGI₂ could be the result of the conversion of PGI₂ and/or 6-keto-PGF_1α to 6-keto-PGE₁.