Prostaglandin-mediated inhibition of noradrenaline release: V. A comparison of the neuroinhibitory effect of three prostaglandins: E2, I2, and 6-keto-PGF1alpha

Pheochromocytoma crisis caused by contemporary ergotamine, caffeine, and nimesulide administration

Pheochromocytoma is a rare tumor that secretes excess catecholamines. Pheochromocytoma crises may be precipitated by the use of several drugs. This article describes the case of a patient affected by pheochromocytoma in whom multiple organ failure developed after contemporary administration of ergotamine, caffeine, and nimesulide. The patient recovered completely long after surgical intervention.

The effect of indomethacin on the circulatory and plasma noradrenaline responses to cold pressor testing in normal subjects

Prostaglandins may alter neuronal noradrenaline release or vascular responsiveness to sympathoexcitation. The purpose of this study was to determine if indomethacin, a prostaglandin synthesis inhibitor, influences the sympathetic and circulatory adjustments to a commonly utilized laboratory stressor in the clinical assessment of autonomic function, the cold pressor test. Venous plasma noradrenaline levels (n = 8), mean arterial pressure and heart rate (n = 10) were measured in healthy male subjects during immersion of the non-dominant hand in cold water (1 degree C) for 90 s. The subjects were given either placebo or indomethacin (100 mg) in a double-blind manner. The order of administration was counterbalanced and a 1 week period was given for systemic clearance of the drug. The absolute level of mean arterial pressure was elevated during the resting control period after indomethacin treatment (88 in placebo vs. 92 mmHg in indomethacin). Both heart rate and venous plasma noradrenaline levels were similar between trials during the resting control period. Mean arterial pressure and heart rate increased similarly during cold pressor testing in both indomethacin and placebo. Venous plasma noradrenaline levels increased during cold pressor testing 162 +/- 39 vs. 200 +/- 69 pg/ml in indomethacin vs. placebo (p > 0.05), respectively. In addition, perceived pain (peak level = 7 +/- 1 vs. 6 +/- 1 units; indomethacin vs. placebo, respectively) was not different between the trials. These results suggest that administration of indomethacin in a maximal single therapeutic dose, does not affect the sympathetic nervous system or circulatory responsiveness to cold pressor testing. It may not be necessary to discontinue indomethacin prior to autonomic function testing.

Effects of prostacyclin analogue, OP-2507, on function and metabolism in the ischemic working rat heart

We examined the effects of a new stable prostacyclin analogue, OP-2507, on myocardial function and metabolism in the ischemic working rat heart preparation. The hearts were perfused with Krebs-Henseleit bicarbonate (KHB) buffer, and whole heart ischemia was induced by one-way aortic valve for 15 min follows by reperfusion for 30 min. In the treated hearts, OP-2507, 20 ng.ml(-1), was administered to KHB buffer from the beginning to the end of experiment. During ischemia, coronary flow in the OP-2507 group increased significantly more than that in the control group. The mechanical performance of both groups was impaired after ischemia. However, the recovery of coronary flow, cardiac output, peak systolic pressure and LV dP/dT(max) was significantly higher in the treated group than in the control group. The incidence of ventricular fibrillation during reperfusion was 100% and 25% in the control and the OP-2507 groups, respectively. Myocardial ATP content was significantly higher in the treated hearts than that in the control hearts. These results indicate that this stable prostacyclin analogue is beneficial in myocardial ischemia, even without its well known action of preventing platelet aggregation.

Repeated sympathetic stimuli elicit the decline and disappearance of prostaglandin modulation and an increase of vascular resistance in humans

To investigate the role of prostaglandins I2 and E2 in modulating the vasoconstrictor response to sympathetic stimulation, repeated and proximate cold pressor tests were performed in 23 healthy young volunteers. Limb vascular resistance (blood flow measured by venous occlusion plethysmography), prostaglandin I2 (as 6-ketoprostaglandin F1 alpha) and prostaglandin E2 plasma levels (detected by radioimmunoassay), and plasma catecholamines (detected by high-performance liquid chromatography and electrochemical detection) were measured. A progressive increase in the duration of the vasoconstrictor response was observed with repetition of cold applications (p less than 0.001, by analysis of variance for trends). This increase was associated with a progressive decrease in cold-induced elevation of 6-ketoprostaglandin F1 alpha and prostaglandin E2 plasma levels until, after five stimulations, neither prostaglandin was detectable. The maximum detected concentration of norepinephrine did not significantly change, but its area under the curve in time showed a trend toward an increase. Epinephrine levels did not significantly change. The increase of vascular resistance was significantly correlated with the decrease of both prostaglandins (r = 0.93, p less than 0.05 for prostaglandin E2 and r = 0.89, p less than 0.05 for 6-ketoprostaglandin F1 alpha), whereas no significant correlations were found between variations of vascular resistance and catecholamines. Prostaglandin blockade induced by diclofenac sodium administration caused, from the first cold application, a pattern of the vasoconstrictor response and plasma prostaglandin and norepinephrine changes similar to that observed at the fifth cold application in untreated subjects, when prostaglandins are no longer detectable in plasma. We conclude that an increased vasoconstrictor response to sympathetic stimulation in humans may result from a diminished inhibitory influence of prostaglandins on adrenergic transmission.

The effect of acetylsalicylic acid and indomethacin on the catecholamine- and oxytocin-induced contractility and prostaglandin (6-keto-PGF1 alpha, PGF2 alpha)-production of human pregnant myometrial strips

The effects of acetylsalicylic acid (ASA) and indomethacin (IND) on the epinephrine and oxytocin stimulated contractility and prostaglandin (6-keto-PGF1 alpha, PGF2 alpha) production of superfused myometrial strips from the pregnant human uterus at term are reported. Without preincubation in ASA or IND epinephrine dose-dependently (10 ng/ml to 1 microgram/ml) stimulated the contractility and significantly increased the PG-release of the myometrial strips. The epinephrine induced increase in contractility was correlated to a higher increase in PGF2a production and a decreased 6-keto-PGF1 alpha/PGF2 alpha ratio (5.4 to 1.8). Superfusion of oxytocin increased myometrial contractions and PGF2 alpha release according to dose (3-12 microU/ml). However, 6-keto-PGF1 alpha production was not affected by oxytocin. Myometrial strips preincubated with ASA (100 micrograms/ml) or IND (10 micrograms/ml) demonstrated little spontaneous activity and the PG production was below the detection limit of the RIA. The stimulating effect of epinephrine and oxytocin on the contractility and PGF2 alpha release of the myometrial strips was inhibited significantly. During continuous superfusion of the ASA and IND preincubated myometrial strips with Tyrode's solution the inhibitory effect on spontaneous, epinephrine-, and oxytocin-stimulated contractility and PGF2 alpha release gradually declined over a period of 2 hours. This decrease of the inhibitory effect was more significant in ASA preincubated specimens. Our results demonstrate that spontaneous, epinephrine-, and oxytocin-stimulated contractility and PG release of human myometrial strips can be inhibited by ASA and IND and that this inhibitory effect is reversible. Furthermore our results suggest that in pregnant human myometrium the inhibition of PGF2 alpha production by ASA and IND is more pronounced than that of 6-keto-PGF1 alpha (PGI2).

Diminished prostaglandin-mediated inhibition of norepinephrine release from the sympathetic nerve endings in spontaneously hypertensive rats

This study was performed to investigate prostaglandin (PG)-mediated regulation of norepinephrine release from the sympathetic nerve endings in spontaneously hypertensive rats (SHR). The effects of PGE2 on the pressor responses and norepinephrine overflow during the periarterial nerve stimulation were examined in the perfused mesenteric vascular beds from SHR and age-matched Wistar Kyoto rats (WKY). In preliminary studies using normotensive Wistar rats, PGE2 inhibited the overflow of norepinephrine in response to the electrical nerve stimulation. On the contrary, when indomethacin was infused into the preparations, the norepinephrine overflow induced by the nerve stimulation was increased. In SHR, the neurogenic vasoconstriction and norepinephrine overflow during the nerve stimulation were significantly greater than in WKY. PGE2 in the medium inhibited the pressor responses and norepinephrine overflow during the nerve stimulation dose-dependently in WKY, while the inhibitory effects of PGE2 in SHR were blunted. These results suggest that PGE2 is an important hormone in regulating sympathetic vascular tone and that presynaptic inhibition of vascular adrenergic transmission by PGE2 is attenuated in SHR.

Effects of thromboxane synthase inhibition on vascular responsiveness in the in vivo rat mesentery

The purpose of this investigation was to determine the effects of thromboxane synthase inhibition on vascular responsiveness. To achieve this goal, the effects of thromboxane synthase inhibitors on mesenteric vascular responses to sympathetic nerve stimulation, norepinephrine, and angiotensin II were determined in vivo. In normotensive rats, chronic treatment with the thromboxane synthase inhibitor, UK38,485 (100 mg/kg X d X 7 d), attenuated vascular responses to nerve stimulation and angiotensin II, but not to norepinephrine. Indomethacin treatment (5 mg/kg X three doses) did not attenuate vascular responses, but did prevent chronic UK38,485 administration from attenuating vascular reactivity. A single dose of UK38,485 (100 mg/kg) did not modify vascular responses to nerve stimulation or angiotensin II, even though platelet thromboxane synthase was inhibited completely. In spontaneously hypertensive rats, chronic administration (100 mg/kg X d X 7 d) of either UK38,485, OKY1581, or U-63557A (three structurally distinct thromboxane synthase inhibitors) attenuated vascular responses to nerve stimulation and angiotensin II. Only U-63557A suppressed responses to norepinephrine. Chronic treatment with UK38,485 or U-63557A did not influence vascular reactivity in hypertensive rats treated with indomethacin. Also, chronic administration of lower doses of UK38,485 or U-63557A (30 mg/kg X d X 7 d) did not affect vascular responsiveness in hypertensive rats, despite complete blockade of platelet thromboxane synthase. These data indicate that chronic administration of high doses of thromboxane synthase inhibitors attenuates vascular responses to sympathetic nerve stimulation and angiotensin II, but not usually to norepinephrine. This action may be mediated by endoperoxide shunting within the blood vessel wall.

The effects of alpha- and beta-adrenergic stimulation on contractility and prostaglandin (prostaglandins E2 and F2 alpha and 6-keto-prostaglandin F1 alpha) production of pregnant human myometrial strips

The effects of catecholamines and alpha- and beta-adrenergic agonists and antagonists on the spontaneous contractility of superfused pregnant human myometrial strips are reported. Prostaglandins (prostaglandins E2 and F2 alpha and 6-keto-prostaglandin F1 alpha) were analyzed in the effluent of the superfusion medium by specific radioimmunoassays. Both epinephrine and norepinephrine dose-dependently (10 ng/ml to 1 microgram/ml) stimulated the contractility of the myometrial strips and significantly increased the synthesis of all prostaglandins assayed. alpha-Adrenergic blockers inhibited the catecholamine-induced increase in contractility. This was associated with decreased prostaglandins F2 alpha and E2 concentrations and a further increase in 6-keto-prostaglandin F1 alpha levels. Exclusive beta-adrenergic stimulation with beta-mimetic drugs had the same effect. Conversely, epinephrine stimulation together with beta-blockers resulted in a further increase in the prostaglandins F2 alpha and E2 release of the myometrial strips. This effect was even more pronounced with specific alpha-adrenergic stimulant drugs. Our results demonstrate the interrelationship of alpha- and beta-adrenergic stimulation and the prostaglandin system. alpha-Adrenergic stimulation increases myometrial contractility and the synthesis of prostaglandins F2 alpha and E2. beta-Adrenergic stimulation reduces contractility by further enhancing 6-keto-prostaglandin F1 alpha production.

Attenuation of noradrenergic neurotransmission by the thromboxane synthetase inhibitor, UK 38,485

The purpose of this study was to determine the effects of chronic administration of the thromboxane synthetase inhibitor, UK 38,485, on noradrenergic neurotransmission. Male Sprague Dawley rats (n = 14) were treated once daily with either UK 38,485 (100 mg/kg; n = 7) or the vehicle of UK 38,485 (olive oil; n = 7) by gavage. The dose of UK 38,485 chosen was sufficient to inhibit ex vivo platelet TXB2 production by greater than 90% for 24 hours. One week into the treatment animals were prepared for in situ perfusion of their mesenteric vascular beds. Vasoconstrictor responses to both exogenous norepinephrine and periarterial nerve stimulation were determined both before and during an infusion of angiotensin II (9 ng/min) into the superior mesenteric artery. UK 38,485 significantly (P less than 0.02) attenuated the vascular response to periarterial nerve stimulation without altering the vascular response to either norepinephrine or angiotensin II. UK 38,485 did not influence the baseline perfusion pressure, the mean arterial blood pressure or the potentiation of neurotransmission by angiotensin II. These data indicate that in the in situ rat mesentery UK 38,485 attenuates the release of neurotransmitter from sympathetic nerve terminals.

Minireview. Presynaptic receptors and alterations in norepinephrine release in spontaneously hypertensive rats

The ability of blood vessels to constrict to a given stimulus is significantly increased in spontaneously hypertensive rats (SHR). Such an increase in the vasoconstrictor responsiveness contributes to the elevated peripheral vascular resistance noted in SHR. The present review discusses evidence in support of the concept that an increased release of norepinephrine during sympathetic nerve stimulation may contribute to the increase in vasoconstrictor responsiveness and, subsequently, to an increase in vascular resistance in the SHR. Several studies suggest that the exocytotic release of norepinephrine from sympathetic nerves may be altered by endogenously occurring neurohumoral substances which produce their effects by interacting with presynaptic receptors located on postganglionic sympathetic nerves. Therefore, it is postulated that alterations in presynaptic regulation of norepinephrine release, resulting from changes in the functioning of one or more of these presynaptic receptors, may lead to a greater release of norepinephrine in the SHR. This review summarizes the results of studies evaluating presynaptic receptor mechanisms and norepinephrine release in the SHR. These studies suggest that norepinephrine release during sympathetic nerve stimulation is greater in the SHR and that alterations in some of the presynaptic receptor mechanisms may be responsible for this phenomenon.

Interaction of prostaglandins and clonidine in the rat vas deferens

Stimulation of presynaptic alpha 2-adrenoceptors by clonidine may lead to local synthesis of prostaglandins which contribute to the inhibition of noradrenaline release observed with this drug. The present investigation was undertaken to determine the role of prostaglandins in the effect of clonidine and xylazine on the rat vas deferens. Both drugs inhibited the twitch response to field stimulation in this preparation. Inhibition was reversed by yohimbine. This effect of clonidine (but not xylazine) was reduced by preincubating vasa deferentia in Krebs containing indomethacin for 1h. Clonidine (but not xylazine) stimulated the synthesis of prostaglandin-like activity in pieces of intact vas deferens incubated in Krebs containing arachidonic acid. Such stimulation was prevented by inclusion of yohimbine (but not prazosin) in the incubation medium. Clonidine did not stimulate prostaglandin synthesis in a cell-free preparation of sheep seminal vesicle microsomes incubated with arachidonic acid or inhibit PGE2 catabolism by purified swine lung 15-PGDH. We conclude that clonidine (but not xylazine) stimulates prostaglandin synthesis possibly by activating phospholipase activity and releasing arachidonic acid from membrane phospholipids. This effect on prostaglandin production is secondary to activation of alpha 2-adrenoceptors.

Eicosanoids: prostaglandins, thromboxanes, leukotrienes, and other derivatives of carbon-20 unsaturated fatty acids

In recent years, knowledge of the biochemistry of oxygenated metabolites of arachidonic acid has greatly increased. Their biological functions in acceleration and prevention of platelet aggregation and in inflammatory and immune reactions are becoming much clearer. The therapeutic value, particularly of PGI2 as well as selective inhibitors of synthesis, is also rapidly advancing. Despite much effort, the functional importance of prostaglandins and thromboxanes in the cNS in normal ongoing physiological processes is still quite uncertain. However, when parenchymal or vascular elements are damaged or invaded by extraneural cells, the synthesis of one or the other member of the eicosanoids is greatly increased and contributes significantly to pathophysiological reactions. Thus, prevention of synthesis is likely to have increasing importance in clinical neurology, particularly in cerebrovascular diseases.

Is PGI2 arrhythmogenic? Investigations on ouabain induced arrhythmias in cats

The present study investigated the effect of PGI2 on ouabain induced arrhythmias in cats. PGI2 was infused at a dose of 1.0, 2.0 and 5.0 microgram/kg . min. PGI2 in a dose of 1.0 microgram/kg . min was without any antiarrhythmic effect. Infusions of 2.0 and 5.0 microgram/kg . min PGI2 in 3 of 10 and 7 of 10 animals, respectively, converted the ouabain induced arrhythmias into a sinus rhythm. The duration of the antiarrhythmic effect after infusion of 2.0 microgram/kg . min lasted about 21 min, whereas the higher dose (5.0 microgram/kg . min) had a shorter effect (about 2 min). Under these conditions and doses the systolic blood pressure decreased between 35% and 39% and the diastolic blood pressure was reduced by about 50%. Thus PGI2 showed when used in the given dose range, an antiarrhythmic effect on ouabain induced arrhythmias in cats. The importance of dosage, methods and species is here discussed.

A possible antihypertensive mechanism of propranolol: antagonism of angiotensin II enhancement of sympathetic nerve transmission through prostaglandins

The effects of propranolol on angiotensin II (AII) enhancement of sympathetic nerve transmission were investigated in the in situ blood-perfused mesenteric vascular bed of the rat. Angiotensin II in subpressor concentrations (3 ng/ml) potentiated the vasoconstrictor responses to both sympathetic nerve stimulation (NS) and exogenous norepinephrine (NE). The dl-propranolol had no effect on the basal vasoconstrictor responses to NS and NE, yet inhibited the AII-enhanced vasoconstrictor responses to NS by 47% (p less than 0.05) and 81% (p less than 0.001) at 100 and 300 ng/ml respectively. In contrast, the potentiation of NE responses by AII was unaffected by propranolol. A similar blockade of AII enhancement of NS was observed with the d-isomer of propranolol. Dibucaine (300 ng/ml), a local anesthetic, failed to alter the basal or AII-enhanced responses to either NS or NE. Indomethacin, a prostaglandin synthetase inhibitor (5 mg/kg, s.c.), abolished the inhibitory effect of dl-propranolol on AII enhancement of NS. Prostaglandin E2 (PGE2), but not prostaglandin I2, (3 ng/ml) inhibited AII enhancement of NS without altering the basal response to NS or NE in indomethacin-pretreated animals. Intraarterial infusions of dl-propranolol, d-propranolol, AII, and dl-propranolol-plus-AII into the superior mesenteric artery increased mesenteric venous PGE2 concentrations from 216 +/- 33 to 355 +/- 33 (p less than 0.01), 328 +/- 44 (p less than 0.05), 325 +/- 27 (p less than 0.02), and 407 +/- 44 pg/ml (p less than 0.01) respectively. We conclude that propranolol antagonizes AII enhancement of NS by increasing prostaglandin levels in vascular tissue. Furthermore, these findings suggest that propranolol may exert its antihypertensive effect through the release of prostaglandins when used in therapeutic doses in excess of those required for beta-adrenergic blockade.

Analysis of hemodynamic and respiratory effects of PGI2 in the pig

Cardiovascular and pulmonary effects of prostacyclin (PGI2) were investigated in the anaesthetized pig before and after vagosympathectomy at 1',2',3',4',5',10',15' and 20' after the beginning of the infusion. The animals were perfused with PGI2,2ug/kg/min for 4 min. We measured the following parameters : mean arterial pressure, heart rate, cardiac output, stroke volume, mean pulmonary arterial pressure, mean wedge pressure, left stroke work, total systemic resistance, total pulmonary resistance, tidal volume, respiratory frequency, minute volume, lung resistance and compliance, end-tidal CO2, transpulmonary pressure. Our findings suggest that the hypotension observed is due to a reduction in total peripheral resistance by a direct effect of PGI2 on the vascular bed without inhibition of the sympathetic nervous system. Prostacyclin does not directly modify cardiac function and produces a decrease in preload by venous dilation. The changes in pattern of breathing are related to the hypotensive effect of PGI2.

Regulation of noradrenaline overflow in rat cerebral cortex by prostaglandin E2

1 The effects of prostaglandin E2 (PGE2), PGF2 alpha and PGI2 and of inhibitors of prostaglandin synthesis and action, on the K+-evoked [3H]-noradrenaline ([3H]-NA) overflow from rat cerebral cortex slices have been investigated. 2 PGE2 reduced, while indomethacin (a prostaglandin synthesis inhibitor) or SC 19220 (a prostaglandin receptor antagonist) increased, the evoked overflow compared with controls. 3 The inhibition of [3H]-NA overflow by PGE2 was dose-dependently antagonized by SC 19220. 4 The results indicate that PGE2 modulates NA release in rat cerebral cortex in vitro.

The influence of prostacyclin on coronary occlusion induced arrhythmia in cats

The present study examined the effect of prostacyclin (PGI2) on ventricular arrhythmias following coronary artery occlusion in the cat. The left anterior descending coronary artery (LAD) was occluded abruptly in 50 cats anesthetized with alpha-chloralose. The ECG (Lead II) along with arterial blood pressure were monitored before and for at least one hour after occlusion. Either vehicle or PGI2 was infused at a rate of 0.15 ml . min-1 into the left atrium 15 min before and 1 hour after LAD occlusion. PGI2 was infused at 4 doses: 2.7,27,270 and 2700 pmole . kg-1 . min-1. Infusion of PGI2 before occlusion produced a dose dependent decrease in mean arterial blood pressure with no significant change in heart rate. Abrupt occlusion of the LAD produced ventricular arrhythmia in all cats ranging from ventricular premature beats to ventricular fibrillation (VF). In cats infused with PGI2 the incidence of VF ranged from 30-40% at the low and middle doses to a maximum of 60% at the highest dose (2700 pmole . kg-1 . min-10. The incidence of VF in the latter group was two times greater than that observed in the control group (30%). In addition, the mean number of ventricular ectopic beats was greater at the 27 and 2700 pmole . kg-1 . min-1 doses of PGI2 than in the control group. An increase in ventricular ectopic beats was not observed at the 2.7 or 270 pmole . kg-1 . min-1 doses of PGI2. These data indicate the PGI2 can exert an arrhythmogenic effect following coronary artery occlusion and that this effect occurs in a biphasic manner i.e., the increase in arrhythmia observed at 27 did not occur at 270 but occurred again at the 10 fold higher dose (2700 pmole . kg-1 . min-1). At the lowest dose of PGI2 infarct size was approximately 14% less than in the control group while, at the highest dose, infarct size was 20% greater than control and approximatey 40% greater than the lowest dose of PGI2.