Increased nerve stimulation induced release of noradrenaline from the rabbit heart after inhibition of prostaglandin synthesis

Properties of the peripheral opiate receptors in the cat nictitating membrane

The noradrenaline overflow and the contractile response elicited by nerve stimulation of the muscle were inhibited by 1 micron morphine in the cat nictitating membrane. This concentration of morphine did not modify the response of the muscle to exogenous noradrenaline. The inhibitory effect of morphine was increased by low Na+ (50 mM), whereas the capacity of naloxone as antagonist to morphine was higher with 150 mM than with 50 mM Na+. These results suggest that the peripheral opiate receptors which interact with noradrenergic neurotransmission could show a sodium allosteric transformation similar to that described for the brain opiate receptor. The effect of morphine was enhanced by manganese ion in the presence of normal Na+. The responses of the cat nictitating membrane to nerve stimulation were not altered in the presence of the protein modifying reagent DTNB [5,5'-dithiobis-(2-nitrobenzoic acid)] but the effect of morphine on the adrenergic neurotransmission was diminished by DTNB with 150 mM Na+. It is postulated that the affinity of the ligands for presynaptic receptors which regulate adrenergic neurotransmission might be modified during the physiological changes in ion concentration which accompany nerve depolarization.

Frequency-dependence of 3H-noradrenaline secretion from human vasoconstrictor nerves: modification by factors interfering with alpha-or beta-adrenoceptor or prostaglandin E2 mediated control

Isolated superfused field stimulated human omental arteries and veins, preincubated with 3H (-)-noradrenaline (NA) were used to study the frequency dependence of NA secretion and of the mechanisms for its local feedback control. 3H-NA secretion per shock was found to be basically a simple hyperbolic function of the stimulation frequency from 1 to 30 Hz, as long as secretion was restricted by prostaglandin E2 (PGE2). In the absence of restriction, or during facilitation, 3H-NA secretion per shock reached its maximum at 10 Hz and then declined at 30 Hz, indicating 'overload' in some link in the secretory mechanism. 3H-NA secretion was depressed by exogenous NA and by PGE2, and enchanced by isoprenaline, phentolamine and by blockade of PGE2 formation. Most of these effects were inversely related to the stimulation frequency. Attempts were made to study interactions between the different control mechanisms and to evaluate possible in vivo consequences of disturbance of adrenergic neuroeffector transmission by interference with the local control of the secretory mechanisms.

Influence of inhibitors of prostaglandin synthesis on renal vascular resistance and on renal vascular responses to vasopressor and vasodilator agents in the cat

We determined the effects of indomethacin and meclofenamate, two inhibitors of prostaglandin synthesis, on renal vascular resistance and on renal responses to nerve stimulation, pressor and depressor hormones in the in situ feline kidney under conditions of controlled blood flow. Both inhibitors produced a gradual rise in renal vascular resistance which became maximal 15-20 minutes after administration. The increase in renal resistance after indomethacin was not attenuated during intrarenal infusion of either phentolamine or SQ 20881. Pretreatment with propranolol, in a dose sufficient to inhibit renin secretion, also did not attenuate the increase in renal resistance produced by indomethacin. However, infusion of [Sar1-, Ala8]angiotensin II, an angiotensin II antagonist, did attenuate the indomethacin-induced increase in renal vascular resistance. After indomethacin, the vasoconstrictor response to norepinephrine was enhanced, whereas responses to nerve stimulation and angiotensin were unaffected. Although meclofenamate enhanced renal vascular resistance, its effects on vasoconstrictor responses were inconsistent. After indomethacin, the renal dilator response to bradykinin was enhanced; however, dilator responses to nitroglycerin were unaltered. The present data indicate that the increase in renal vascular resistance after indomethacin does not depend on the adrenergic system but may be dependent on the renin-angiotensin system. The inconsistent effect of the inhibitors of synthesis on renal constrictor responses to nerve stimulation suggests that endogenous prostaglandins do not serve to modulate the effects of the sympathetic nervous system on the feline renal vascular bed. These results also indicate that renal dilator responses to bradykninin are not mediated by prostaglandins in the cat.

Prostaglandins in adrenergic transmission of isolated perfused rat pancreas

In the isolated, perfused rat pancreas, prostaglandins (PGs) E1 and E2 1-5 ng/ml, reduced the vasoconstrictor responses to periarterial nerve stimulation and variably affected those to injected norepinephrine. Prostaglandin F2alpha had no consistent effect on the vasoconstrictor responses to both adrenergic stimuli. Stimulation of adrenergic nerves or administration of norepinephrine released a PGE-like substance from the perfused pancreas which was abolished by inhibitors of PG synthesis, acetylsalicylic acid, indomethacin, meclofenamate, and eicosa-5,8,11,14-tetraynoic acid. The latter three agents did not potentiate, but rather reduced the vasoconstrictor responses to both adrenergic stimuli. Arachidonic acid that was converted by the pancreas into PGE2 and PGF2alpha inhibited the vasoconstrictor responses to adrenergic stimuli. The latter effect of arachidonic acid was not altered by the simultaneous infusion of PG synthetase inhibitors. Although these results, which could be attributed to a direct effect of inhibitors of PG synthesis and arachidonic acid on adrenergic neuroeffector junction, fail to establish the role of endogenous PGs in modulating adrenergic responses in rat pancreatic vessels, they emphasize the differences in the effect of PGE1 and PGE2 on adrenergic responses in various vascular beds of the rat.

Nicotine stimulates prostaglandin formation in the rabbit heart

Rabbit isolated hearts were perfused according to Langendorff with Tyrode solution. Prostaglandin-like substances (PLS) in the effluent were purified and assayed on the rat stomach strip Noradrenaline (NA) in the effluent was assayed fluorimetrically. 2 Infusion of nicotine (1 muM-50 muM) caused a dose-dependent, brief increase, from 1.2+/-0.4 to maximally 8.3+/-2.1 ng/min, in the outflow of PLS from the heart. The increase was abolished by pretreatment of the heart with indomethacin. 3 Activation of nicotinic receptors in the heart with acetylcholine (ACh, 200 muM) in the presence of atropine (1 muM) also elicited an increase in the release of PLS. This release was smaller than that caused by nicotine. 4 Nicotine (50 muM) and ACh (200 muM) in the presence of atropine (1 muM) each caused a pronounced but brief release of NA into the effluent. There was no evident correlation between the ability of the drugs to cause release of PLS on the one hand, and NA on the other. 5 It is concluded that nicotine acts as a direct stimulus for the synthesis of prostaglandins in the rabbit heart.

Absence of a prostaglandinic mediator in the lateral hypothalamic cardio-vascular inhibition

The cardio-vascular inhibition elicited by electrical stimulation of the paraventricular nucleus in the lateral hypothalamus of anaesthesized and desafferentiated dogs is not linked with a mediator release like PGE. The effect of this biological agent is not registered in the isolated femoral artery of a receiver, after deviation by a cruised circulation of the arterial blood of a hypothalamic stimulated donor dog. Antipyretics, which are also prostaglandin-synthetase inhibitors, enhance the lateral hypothalamic reactions. As the thermoregulation centre is localized in the same region as the cardio-vascular inhibition centre and because lowering of temperature depends upon vasodilation and decrease in the general cellular metabolism, both functions of the paraventricular nucleus activity, a hypothesis is proposed that thermoregulation and cardio-vascular inhibition centres are a functional and anatomical unity.

Modification by prostaglandins E1 and E2, indomethacin, and arachidonic acid of the vasoconstrictor responses of the isolated perfused rabbit and rat mesenteric arteries to adrenergic stimuli

In isolated perfused rabbit mesenteric arteries, prostaglandin (PG) E1 and E2, 1-5NG/ML, did not alter the basal perfusion pressure, but reduced the vasoconstrictor responses to sympathetic nerve stimulation; the responses to injected norepinephrine were reduced by PGE1 and variably affected by PGE2. In contrast, in rat mesenteric arteries PGE1 and PGE2, 1-5 ng/ml, potentiated the vasoconstrictor responses to nerve stimulation and to injected norepinephrine. In rabbit mesenteric arteries, the inhibitor of PG synthesis, indomethacin, augmented the responses to sympathetic nerve stimulation and to injected norepinephrine, whereas in rat mesenteric arteries indomethacin inhibited the responses to both adrenergic stimuli. Arachidonic acid, a PG precursor, reduced the vasoconstrictor responses to sympathetic nerve stimulation and to injected norepinephrine in rabbit, whereas in rat, potentiation of the responses to adrenergic stimuli occurred. Since these effects of arachidonic acid were abolished by the simultaneous infusion of indomethacin, they appear to be mediated through conversion of arachidonic acid to PG. We conclude that prostaglandins modulate adrenergic transmission in mesenteric arteries and this effect is species dependent.

Prostaglandin-mediated inhibition of noradrenaline release: its resistance to variations in rate of prostaglandine synthesis

Isolated rabbit hearts were perfused according to Langendorff. The bilateral sympathetic nerve supply to the organ was stimulated at intervals, and the overflow of noradrenaline and of prostaglandins of the E series in the effluent was assayed, using fluorimetric and bioassay methods, respectively. The synthesis of prostaglandins in the organ was stimulated, either by perfusing the heart at a low pO2, or by infusing nicotinic acid. Hypoxia increased the coronary flow, provided the prostaglandin synthesis was not inhibited, probably as a consequence of hypoxia stimulation was however, unaffected by hypoxia. Nicotinic acid also stimulated prostaglandin formation, doubling the overflow of the lipid in response to nerve stimulation. In this series, too, the release of NA induced by nerve stimulation was unaffected by stimulation of prostaglandin synthesis. It is concluded that local variations in the rate of prostaglandin synthesis are unable to change the degree to which the release of sympathetic neurotransmitter is inhibited. Furthermore, it is suggested that the prostaglandin synthesis in rabbit heart takes place in compartments, separated functionally or morphologically.

On the relation between release of prostaglandins and contractility of rabbit splenic capsular strips

Rabbit splenic capsular strips release prostaglandins E and F when contracted by noradrenaline or methoxamine. Contractions and prostaglandin release are dose-dependent. Cocaine increases significantly the effect of noradrenaline, but not that of methoxamine, on contraction of the strips and release of prostaglandin E. Release of prostaglandin F was increased by the addition of cocaine not only when noradrenaline was used as an agonist but also at two of three dose levels of methoxamine. When indometacin is added to the bath fluid, it inhibits prostaglandin release and at the same time potentiates the contractile effects of noradrenaline and methoxamine on the rabbit splenic capsular strips. The prostaglandin-synthetase blocker 5,8,11,14-eicosatetraynoic acid also potentiates the contractions induced by noradrenaline and methoxamine. Both the effects on prostaglandin synthesis and on contraction exerted by indometacin can be reversed, when indometacin is washed out. Exogenous prostaglandins E1, E2 and F2alpha in concentrations up to 150 ng/ml do not influence contractions of the strips induced by either noradrenaline or methoxamine. At higher concentrations prostaglandin E1 decreases, but prostaglandins E2 and F2alpha increase the contractions induced by both agonists. The potentiation of the effects of noradrenaline and methoxamine on rabbit splenic strips by indometacin and 5,8,11,14-eicosatetraynoic acid cannot be explained by inhibition of uptake1 or uptake2, release of endogenous noradrenaline or inhibition of metabolism of the agonists. It is suggested that the potentiation is caused by inhibition of synthesis of endogenous prostaglandins, although an undefined sensitizing effect of indometacin and 5,8,11,14-eicosatetraynoic acid cannot be completely exclused.

Hormone interactions in the isolated rabbit heart. Synthesis and coronary vasomotor effects of prostaglandins, angiotensin, and bradykinin

In the present study, the synthesis and degradation of several potent vasoactive substances influencing coronary resistance were characterized in the isolated perfused rabbit heart. Prostaglandin synthetase activity, angiotensin converting enzyme activity, and bradykininase activity (without angiotensinase) were observed. A prostaglandin E2-like substance appeared to be the ednogenous mediator of the coronary vasodilation produced by bradykinin and angiotensin II (AII). (1) The concentration of this prostaglandinlike substance in the coronary venous effluent was directly proportional to the concentration of the coronary vasocilator stimulus (bradykinin or AII). (2) The prostaglandinlike substance released and the coronary dilation produced by the agonists correlated temporally and quantitatively. (3). Abolition of cardiac biosynthesis of the prostaglandinlike substance by indomethacin also abolished the decrease in coronary resistance produced by the agonists. AII, the most potent naturally occurring vasoconstrictor substance, produced a paradoxical coronary vasodilation because it stimulated cardiac prostaglandin biosynthesis, but the direct coronary vasoconstrictor action of AII could be readily unmasked by indomethacin, which blocks prostaglandin synthesis. The nonapeptide SQ-20881 blocked cardiac biosynthesis of AII (from angiotensin I) and enhanced the coronary vascular effects of bradykinin by interfering with bradykininase activity. Similarly, the AII-receptor antagonist, 1-Sar-8-Ile-AII, blocked the coronary vascular effect of AII.

Antagonism of the effects of furosemide by indomethacin in normal and hypertensive man

Furosemide and the prostaglandin synthetase inhibitor, indomethacin, were administered singly and in combination to four normal subjects and six patients with essential hypertension in order to determine whether the antihypertensive, natriuretic and other effects of furosemide could be altered by inhibition of prostaglandin synthesis. In all subjects indomethacin treatment alone resulted in a significant elevation of blood pressure and a fall in plasma renin without any change in sodium excretion. Furosemide alone resulted in a significant blood pressure fall with a rise in plasma renin and urinary aldosterone with a marked increase in urinary sodium loss. These effects were either obviated or blunted by addition of indomethacin. The results are compatible with hypothesis that the antihypertensive and natriuretic effects of furosemide might be mediated at least in part by prostaglandin synthesis. In addition, the effects of indomethacin should be considered when using this drug in hypertensive patients and in subjects requiring diuretic therapy.

The effects of prostaglandins E1, E2 and F2alpha on vagal bradycardia in the anaesthetized mouse

1 In anesthetized mice prostaglandins E1 and E2 reduced the bradycardia caused by electrical stimulation of the sectioned peripheral vagus nerve; prostaglandin F2alpha produced only a slight inhibition of the vagal response. 2 None of the prostaglandins studied affected acetylcholine-induced bradycardia. 3 Prostaglandins modify parasympathetic nerve activity in vivo presumably by a pre-synaptic action.

Prostaglandin release from cat and dog spleen

1. The output of prostaglandins from the spleens of cats and dogs was studied. Comparison is made between the results found in the two species. 2. The release of prostaglandins was investigated in isolated saline-perfused spleens and in incubates of spleen slices. Release in response to nerve stimulation, and exposure to adrenaline or noradrenaline was compared with resting release. 3. A resting release of prostaglandins was found in the dog but not in the cat spleen. 4. Whereas stimulated dog spleens released microgram quantities of prostaglandins E2 and F2alpha, prostaglandin output from the cat spleen under similar conditions was undectectable or barely detectable. 5. The identity of the prostaglandins released from the dog spleen (prostaglandins E2 and F2alpha) was confirmed by mass spectrometry. 6. The species difference in prostaglandin output from the spleen is discussed in relation to the hypothesis that endogenous prostaglandins modify the responses of this organ to nervous stimuli.

Indomethacin-induced increase in noradrenaline turnover in some rat organs

1 The effect of a prostaglandin synthesis inhibitor, indomethacin, on noradrenaline turnover rate in various rat tissues was determined from the product of the endogenous noradrenaline concentration and of the rate constant of (-)-[3H]-noradrenaline decline after injection of the labelled amine in tracer doses. 2 Treatment of the rats with indomethacin (5 mg/kg p.o. five times during 2.5 days) increased noradrenaline turnover rate 32-36% in submandibular gland, spleen and heart, and 4-17% in epididymal and subcutaneous adipose tissue. 3 The extrapolated [3H]-noradrenaline content at time 0 was greater in heart and adipose tissue of indomethacin-treated animals than in controls, while it was not significantly changed in spleen and salivary gland. 4 There was no apparent relationship between tissue concentrations of intravenously injected [14C]-indomethacin and the effect of indomethacin on noradrenaline uptake and turnover rate in the different tissues. 5 Indomethacin treatment did not affect monoamine oxidase and catechol O-methyl-transferase activities in the different tissues. 6 The results are consistent with the hypothesis that indomethacin increases noradrenaline turnover in the rat by blockade of a locally operating feed back inhibition of transmitter release by prostaglandins. However, additional effects, such as an increased impulse traffic, cannot be ruled out.

Reduction of the tone of the isolated human umbilical artery by indomethacin, eicosa-5,8,11,14-tetraynoic acid and polyphloretin phosphate

The effects of 2 prostaglandin synthetase inhibitors, indomethacin and eicosa-5,8,11,14-tetraynoic acid (ETA) and of the prostaglandin antagonist, polyphloretin phosphate (PPP), on the tone of the isolated human umbilical artery and on the responses of this preparation to 5-hydroxytryptamine (5-HG) and prostaglandin F2alpha (PGF2alpha) were investigated. Indomethacin (8 mug/ml), ETA (5 mug/ml) or PPP (40 mug/ml) reduced the tone of human umbilical arteries but had no influence on the responses to 5-HT. In these concentrations ETA and PPP but not indomethacin antagonized the action of PGF2alpha. When the concentration of indomethacin or PPP was increased 5-fold both 5-HT- and PGF2alpha-induced contractions were antagonized indicating a non-specific inhibition at these concentration levels. A 10-fold increase in the concentration of ETA had no antagonizing action on 5-HT-induced contractions suggesting a more selective inhibition of the PGF2alpha action than displayed by the other compounds. The effects on the tone of the human umbilical artery of the compounds studied might reflect inhibition of prostaglandin biosynthesis and/or antagonism of the action of formed prostaglandins. The findings are compatible with the view that intramural synthesis of prostaglandins contributes to the maintenance of the tone of the isolated human umbilical artery.

Prostaglandin release and mechanical perfromance in the isolated rabbit heart during induced changes in the internal environment

The isolated rabbit heart was perfused according to the Langendorff technique. Prostaglandins in the effluent from the organ were identified by use of thin layer chromatography and assayed on the rat stomach strip. The effect of alterations of the physical and chemical conditions of the perfusion medium on the overflow of prostaglandins from the heart was studied. In addition, the capacity of noradrenaline and acetylcholine to release prostaglandins was tested. Acidosis, hyperthermia, hypothermia, hypotension, hyperosmoaarity and increased [K+] OR [Ca++] levels, while all inducing marked changes in the mechanical activity of the heart, did not induceporstaglandin release. Hypoxia, on the other hand, stimulated the liberation of prostaglandins. Noradrenaline was a potent agent for stimulation of prostaglandin release, in the absence of alpha- and betaadrenergic receptor blockade. Acetylcholine was also found to liberate prostaglandins, by activation of muscarinic receptors. The prostaglandin releasing capacity of acetylcholine was about 3 times weaker than that of noradrenaline. It is concluded that the release of prostaglandins from the rabbit heart is not dependent on the mechanical activity of the organ. Furthermore, it is suggested that prostaglandins released by hypoxia may play an important roli in the development of reactive hyperemia. Finally it is stated that the release of prostaglandins from the heart caused by acetylcholine may constitute the negative link in an endogenous prostaglandin mediated feed-back inhibition of the release of acetycholine from parasympathetic nerve endings.

Release of prostaglandin from the rabbit isolated heart following vagal nerve stimulation or acetylcholine infusion

1 Rabbit hearts were perfused by the Langendorff technique. The lipid fraction in the perfusate from the heart was isolated and analysed for prostaglandins by thin layer chromatography and quantitative assay on the rat isolated stomach strip.2 Infusion of acetylcholine at a rate of 8 mug/min significantly increased the outflow of prostaglandins from the heart, from 1.8 to 6.2 ng/minute.3 Addition of atropine (1 mug/ml) to the perfusing medium completely abolished not only the mechanical response but also the increase in outflow of prostaglandins caused by infusion of acetylcholine.4 Bilateral stimulation of the parasympathetic nerves to the heart at 5 Hz also significantly increased the outflow of prostaglandins from the organ from 5.2 to 8.3 ng/minute.5 Both prostaglandin E(1) and E(2) were isolated from the lipid fraction of the perfusate.6 The role of prostaglandins in relation to autonomic neurotransmission in the heart is discussed.

The effects of indomethacin and eicosa-5,8,11,14-tetraynoic acid on the response of the rabbit portal vein to electrical stimulation

1 The effects of indomethacin and eicosa-5,8,11,14-tetraynoic acid (ETYA) on the contractile response of the transmurally stimulated rabbit portal vein were studied in vitro.2 When the veins were stimulated for 240 pulses at 1 and 2 Hz, the responses were potentiated by indomethacin and ETYA. However, responses to 4 and 8 Hz were not potentiated. The responses to continuous electrical stimulation at 2 Hz were also potentiated by indomethacin and ETYA. This potentiating effect was attenuated when the veins were pretreated with alpha-methyl-p-tyrosine. The responses of the veins to noradrenaline were not altered by either indomethacin or ETYA.3 Prostaglandin E(2) inhibited the responses of the portal vein to electrical stimulation. The magnitude of this inhibition was inversely related to the frequency of stimulation. The responses of the vein to noradrenaline were not altered by prostaglandin E(2).4 It is concluded that potentiation by indomethacin and ETYA of the response of the isolated portal vein is due to an increased release of newly synthesized noradrenaline as a result of inhibition of prostaglandin synthesis.

A vasodilator innervation to the central artery of the rabbit ear

1 The possibility of a vasodilator innervation to the isolated and perfused central artery of the rabbit ear was examined.2 Stimulation of the periarterial nerves in the presence of noradrenaline or other agonist used to maintain a partial constriction of the ear artery, led to a decrease in intraluminal flow followed after the cessation of stimulation by an increase in flow beyond the pre-stimulation level.3 After blockade of adrenergic transmission with bretylium or guanethidine or of the alpha- and beta-adrenoceptors with phentolamine and propranolol, stimulation of the periarterial nerves in the presence of a background tone, led to a clearly detectable vasodilation. This dilatation was not blocked by treatment with atropine or mepyramine; nor was it enhanced by physostigmine.4 Pretreatment of rabbits with reserpine (2 mg/kg) to deplete catecholamine stores, eliminated both the vasoconstrictor and vasodilator responses to nerve stimulation. However, a lower dose of reserpine (0.2 to 0.5 mg/kg) selectively eliminated the vasoconstrictor component of periarterial nerve activation.5 The ear artery dilated in response to low concentrations of prostaglandin E(1), and E(2), in the presence of noradrenaline, but treatment with inhibitors of prostaglandin synthesis, indomethacin, aspirin or eicosa-5,8,11,14-tetraynoic acid did not reduce the vasodilator response. Attempts to extract a prostaglandin in the bathing medium were unsuccessful.6 The involvement of a purine nucleotide appeared unlikely since the ear artery dilated only in response to fairly high concentrations of adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP) and adenosine 5'-monophosphate (AMP). Furthermore, dipyridamole, an inhibitor of adenosine uptake, enhanced dilation due to exogenous ATP but not to periarterial nerve stimulation.7 It is concluded that the central artery of the rabbit ear has a vasodilator innervation but the identity of the transmitter remains to be established.

The effect of indomethacin on the release of prostaglandin E2 and acetylcholine from guinea-pig isolated ileum at rest and during field stimulation

1 Guinea-pig ileum suspended in Krebs solution showed a continuous increase of tone which was lost by changing the bath fluid. Prostaglandin E(2) was released from the ileum during incubation, and its concentration in the bath fluid appeared to correlate with the increase in tone.2 Supramaximal field stimulation (10 Hz) resulted in increased release of prostaglandin E(2) from the ileum. At lower rates of stimulation, the increase in the release of E(2) compared with the resting output was not significant.3 Indomethacin (1 and 10 mug/ml) produced a significant, dose-related reduction of the amount of prostaglandin E(2) measured in the bath fluid at rest and with field stimulation. Indomethacin inhibited the contraction of the ileum during incubation in Krebs solution.4 Indomethacin (10 and 20 mug/ml) had no significant effect on the release of acetylcholine during field stimulation, but reduced the resting output of acetylcholine from guinea-pig ileum in some experiments.5 The results are discussed in the context of the role ascribed to prostaglandins as physiological modulators in transmitter release. No evidence for a prostaglandin-mediated negative feedback mechanism on acetylcholine release was obtained.