Failure of antioxidants to protect against angiotensin II-induced aortic rupture in aged apolipoprotein(E)-deficient mice.

BACKGROUND AND PURPOSE: Oxidative stress may be involved in the development of abdominal aortic aneurysms (AAAs). Previous studies indicate that antioxidants protect against AAA formation during chronic angiotensin (Ang) II infusion in apolipoprotein E-deficient (ApoE(0)) mice. We here examine if these protective effects also occurred in aged ApoE(0) mice. EXPERIMENTAL APPROACH: Male ApoE(0) mice (50-60 weeks) were randomly divided into 4 groups: saline, Ang II (1000 ng kg(-1) min(-1) for 4 weeks), Ang II plus antioxidants (0.1% vitamin E in food plus 0.1% vitamin C in drinking water), and Ang II plus losartan (30 mg kg(-1) day(-1)). KEY RESULTS: Exogenous Ang II increased systolic blood pressure by 40 mmHg and resulted in the formation of pseudoaneurysms (rupture and extramural haematoma) in the abdominal aorta in 50% of animals. True aneurysmal dilatation was rarely observed. Antioxidants decreased systemic oxidative stress (plasma malondialdehyde), but had only minor effects on aortic rupture, relative to the complete prevention by losartan. Immunohistochemistry revealed strong matrix metalloproteinase-9 (MMP-9) expression in atherosclerotic plaques and at the sites of rupture. Antioxidants did not affect tumour necrosis factor-alpha-stimulated MMP-9 release from U937 cells. In addition, antioxidants had little effects on Ang II-induced renal dysfunction. CONCLUSIONS AND IMPLICATIONS: In contrast to previous findings in younger mice, antioxidants had only minor effects on Ang II-induced aortic rupture in aged mice. Our results demonstrate that the pathological features of the aneurysmal remodelling induced by Ang II in old ApoE(0) mice are distinct from those of human AAA.

Prostacyclin and vascular function: implications for hypertension and atherosclerosis.

Prostacyclin and endothelium-derived relaxing factor (or nitric oxide) are unstable mediators produced by the vascular endothelium, that are important for local regulation of platelet behavior and blood flow. This review focuses on the basic biochemistry and pharmacology of prostacyclin, its interactions with nitric oxide and nitrovasodilator drugs, and the implications of disturbances in this system for vascular disease, particularly hypertension and atherosclerosis. Prostacyclin and its stable analogs are also finding limited therapeutic applications in preservation of platelet function, pulmonary hypertension, and investigation into the cytoprotective and antiatherosclerotic properties is continuing.

Cholinergic neurogenic vasodilatation is mediated by nitric oxide in the dog hindlimb.

OBJECTIVE: The aim was to investigate the role of nitric oxide (NO) in cholinergic neurogenic vasodilatation in the dog hindlimb using the NO synthase inhibitor, N-nitro-L-arginine (NOLA), and the NO precursor, L-arginine. METHODS: 20 dogs were anaesthetised with thiopentone and alpha chloralose and experiments were performed in the presence of noradrenergic neurone blockade with guanethidine (15 mg.kg-1 subcutaneously). Using stereotaxic procedures, specific sites in the hypothalamus were electrically stimulated (HS) to produce depressor and hindlimb vasodilator responses. In each experiment, responses to intra-arterial (ia) injections of acetylcholine and glyceryl trinitrate produced increases in femoral blood flow similar to those caused by HS. RESULTS: Vasodilator responses to HS and acetylcholine but not glyceryl trinitrate were reduced by the muscarinic receptor antagonists tropicamide (3-12 mg ia) or atropine (0.5 mg.kg-1 intravenously, i.v.). Administration of NOLA (5-15 mg.kg-1 ia) significantly attenuated the HS induced decrease in arterial pressure [delta AP: control = -21 (SEM 3) mm Hg v NOLA treated = -9(3) mm Hg, p < 0.005] and the increase in femoral blood flow [delta FBF: control = 43(7) ml.min-1 v NOLA treated = 17(4) ml.min-1, p < 0.005]. NOLA also significantly inhibited femoral vasodilator responses to acetylcholine [delta FBF: control = 47(6) ml.min-1 v NOLA treated = 35(6) ml.min-1, p < 0.05] whereas responses to glyceryl trinitrate were enhanced [delta FBF: control = 54(9) ml.min-1 v NOLA treated = 69(9) ml.min-1, p < 0.005]. In addition L-arginine (150-300 mg.kg-1 i.v.), but not D-arginine (150 mg.kg-1 i.v.), reversed the inhibitory effect of NOLA on HS induced dilator responses [delta FBF: NOLA treated = 14(4) ml.min-1 v L-arginine treated = 35(8) ml.min-1, n = 8; greater than NOLA treated, p < 0.05]. CONCLUSIONS: Vasodilatation in the dog hindlimb evoked by activation of cholinergic nerves involves the synthesis of NO; however the source of this NO remains to be determined.

Prostacyclin (PGI2) induces coronary vasodilatation in anaesthetised dogs.

Prostacyclin (PGI2), the predominant metabolite of arachidonic acid in isolated hearts, relaxes strips of bovine coronary artery and is a potent vasodilator in isolated perfused hearts. We have examined the actions of prostacyclin on coronary blood flow in open chest dogs anaesthetised with chloralose. An electromagnetic flow probe was fitted to the left circumflex artery and phasic coronary flow, mean coronary flow (a measure of coronary volume flow over 4 s intervals), and coronary vascular resistance were recorded together with aortic pressure and heart rate. Intravenous infusion of prostacyclin (0.05 to 1.0 microgram.kg.1.min.1), reduced coronary vascular resistance and aortic pressure according to dose, but had only small effects on phasic coronary flow or mean coronary flow. Both tachycardia and bradycardia occurred during infusion of prostacyclin, but 6-oxo-prostaglandin F1alpha (infused at 10 micrograms.kg-1.min-1), the stable degradation produce of prostacyclin, had no cardiovascular effects. The coronary vasodilator effects of prostacyclin were clear when it was injected into the left circumflex artery via a fine catheter distal to the flow probe. Prostacyclin (0.05 to 0.5 microgram) increased phasic coronary flow and mean coronary flow up to 3 fold and reduced coronary vascular resistance without affecting aortic pressure or heart rate, although higher doses had systemic effects. Prostaglandin E1 (0.1 to 0.5 microgram), which also dilated the coronary vessels, had a longer lasting effect and was 1 to 4 times more potent than prostacyclin. Prostaglandin E2, (0.5 to 4 microgram) was less potent than prostacyclin. In four dogs prostacyclin (20 to 500 micrograms) applied epicardially to the left ventricle caused marked and prolonged coronary vasodilatation. Epicardial application of prostacyclin (10 to 25 micrograms) to the right ventricle increased coronary sinus oxygen content with minimal changes in blood pressure. The endoperoxide prostaglandin H2 was a coronary vasodilator of similar potency to prostacyclin, but its analogue U46619 is a vasoconstrictor. Inhibition of cyclo-oxygenase with indomethacin (5 mg.kg-1 i.v.) or sodium meclofenamate (2 mg.kg-1 i.v.) potentiated the coronary dilator effects of prostacyclin given intravenously or into the coronary artery. Cyclo-oxygenase inhibition did not alter the hypotensive effects and increased the coronary vasodilator potency of prostacyclin relative to prostaglandin E2. Thus the sensitivity of the coronary vascular bed to prostacyclin is enhanced when endogenous biosynthesis of prostaglandin-like substances is inhibited. Although the importance of arachidonic acid metabolites in the coronary circulation still requires validation in vivo, it is clear that prostacyclin, and not prostaglandin E2, is the prostaglandin most likely to be involved.

Dexamethasone inhibits endotoxin-induced changes in calcium and contractility in rat isolated papillary muscle.

This study investigates whether endotoxin-induced contractile dysfunction is associated with a defect in the modulation of calcium homeostasis and the potential mechanisms involved. Treatment of rats in vivo with endotoxin significantly decreased the magnitude of contractile transients in electrically stimulated left ventricular papillary muscle isolated after an equilibration period of 6 hours. Although no significant difference was found in the peak intracellular calcium concentration ([Ca2+]i) between the endotoxin-treated and control groups, resting [Ca2+]i) was significantly elevated in the endotoxin-treated group, producing a smaller Ca2+ transient (basal-peak difference) in this group. Pretreatment of rats with dexamethasone prevented the endotoxin-induced decrease in peak tension and inhibited the elevation in resting [Ca2+]i, with a resultant maintenance of Ca2+ transient magnitude. Similar observations were made during stimulation of the muscles by the beta-adrenoceptor agonist, isoprenaline. These results show that endotoxin-induced reduction of cardiac contractile performance is mediated, at least in part, by elevating resting [Ca2+]i, and a glucocorticoid protected from these negative effects. While endotoxin reduces the magnitude of the Ca2+ transient it does not alter peak [Ca2+]i availability. Further investigation is required to determine whether endotoxin decreases contractile performance by reducing the sensitivity of cardiac myofilaments to calcium.

Increased vascular reactivity induced by essential fatty acid deficiency in rat autoperfused hindquarters.

The effects of essential fatty acid deficiency (EFAD) on vascular reactivity to vasoconstrictor stimuli were studied in rat autoperfused hindquarters. Weanling male Sprague-Dawley rats (aged 21 days) were fed diets containing 8% (weight/weight) of stearax plus 2% safflower oil (control diet) or 10% stearax (EFAD diet) for 8 weeks. There was no difference in systemic blood pressure or body weight between the two groups. Basal production of immunoreactive 6-keto-PGF1 alpha by aortic segments was much less in EFAD aortae than in control aortae. In contrast, immunoreactive 6-keto-PGF1 alpha produced by incubating aortic segments with exogenous arachidonic acid (12 mumol/l) was much greater in EFAD aortae than in control aortae. Moreover, conversion of [14C]-arachidonate to [14C]-6-keto-PGF1 alpha was more pronounced in EFAD aortae than in control aortae. Vasoconstrictor responses to noradrenaline (0.01-1.0 mumol/l) and angiotensin II (0.001-1.0 mumol/l) infused into the blood perfused hindquarters were then examined. The rats on the EFAD diet were more sensitive to both noradrenaline and angiotensin II than rats on the control diet (P less than 0.05, two-way ANOVA). Thus, a deficiency of essential fatty acids can lead to increased vascular sensitivity to vasoconstrictor stimuli. Deficiency of arachidonic acid in phospholipid stores is also accompanied by augmented cyclo-oxygenase activity in the vessel wall, similar to that observed previously in spontaneously hypertensive rats (SHR) and rats with one kidney renovascular hypertension.

Vascular prostacyclin and Goldblatt hypertensive rats.

Vascular prostacyclin production in Goldblatt hypertension was examined in one-kidney, one clip (1K, 1C) and two-kidney, one clip (2K, 1C) rat models. Vasodepressor responses to prostacyclin and nitroprusside correlated well with resting blood pressure in both groups of rats, but when measured as a percentage of resting blood pressure the responses did not differ significantly between hypertensive rats and the normotensive controls within each group. In contrast, the vasodepressor effects of arachidonic acid (1-3 mg/kg, i.v.) were much greater in the 1K, 1C rats than in their normotensive controls, but did not differ significantly between hypertensive 2K, 1C rats and sham-operated controls. The effects of arachidonic acid were virtually abolished by indomethacin (10 mg/kg, i.v.). The metabolism of [14C]-arachidonic acid was also studied in isolated aortae of both one- and two-kidney rats by high pressure liquid chromatography of extracts of the incubation mixture. [14C]-6-oxo-PGF1 alpha was the only prostanoid conversion product recovered from the incubations and significantly more of this metabolite was produced by aortic tissue from 1K, 1C rats than from normotensive controls. There was no difference in [14C]-6-oxo-PGF1 alpha production between 2K, 1C rats and controls. These results demonstrate an enhanced ability of vascular tissue from 1K, 1C hypertensive rats to convert exogenous arachidonate to vasodilator prostacyclin, but this is not evident in the two-kidney model. Although enhanced biosynthetic capacity for prostacyclin in the one-kidney model and spontaneously hypertensive rats does not lessen peripheral vascular resistance, it might reflect a fundamental disturbance in phospholipid metabolism which contributes to increased vascular resistance.

Angiotensin-converting enzyme inhibitors reduce neointimal thickening and maintain endothelial nitric oxide function in rabbit carotid arteries.

Application of periarterial collars induced atheroma-like lesions in the carotid arteries of normocholesterolemic rabbits. Vessel segments taken from the mid-region of the collar (cuffed region) and control regions of the same artery were studied at 7 days after surgery. A group of placebo rabbits was provided ad libitum with regular tap water, and treated animals were supplied for 14 days (beginning 7 days before collar application) with water containing perindopril (daily intake of approximately 0.3 mg/kg). Perindopril treatment reduced plasma angiotensin-converting enzyme (ACE) activity by 88% but did not significantly alter arterial blood pressure or heart rate. The sensitivity of arterial rings to angiotensins I and II did not differ between control and cuffed arteries in either placebo or perindopril-treated rabbits, but in rings from both groups of rabbits the sensitivity to the vasoconstrictor action of serotonin was higher in the cuffed segments, as in previous studies. In addition, in placebo rabbits the endothelium-dependent vasorelaxant response to acetylcholine (which results from the release of nitric oxide) was weaker in cuffed arteries than in controls, whereas in the perindopril-treated animals, this impairment of relaxation was restored to the extent that, in cuffed vessels, it was no longer significantly different from the controls. Similar results were obtained in rabbits treated with another ACE inhibitor (ramipril). In contrast, acute treatment with the metabolite, perindoprilat, in vitro (1.0 microM) did not alter the response to acetylcholine in control or cuffed rings from placebo rabbits. Morphologically, vessel segments taken from the center of the cuffed artery of placebo rabbits showed a thickened intima with marked smooth muscle cell proliferation.(ABSTRACT TRUNCATED AT 250 WORDS)

Prostacyclin produced by the pericardium and its influence on coronary vascular tone.

To determine the influence of pericardial fluid prostacyclin on coronary blood flow, the latter was measured in the circumflex artery of anesthetized dogs. Intraaortic infusions of angiotensin II (25 ng . kg-1 . min-1) reduced blood flow and released prostacyclin into pericardial fluid. Epicardial and pericardial superfusion with indomethacin (1 micrograms/ml) abolished prostacyclin release and significantly increased the coronary vasoconstrictor effect of angiotensin II; this treatment did not appear to affect vascular synthesis of prostacyclin. Pericardial prostacyclin may modulate the coronary vasoconstrictor effect of angiotensin, but its general role as a regulator of coronary vascular resistance is probably limited. A more important effect of pericardial prostacyclin may be exerted on the large coronary vessels in the epicardial surface. Release of prostacyclin into pericardial fluid represents a potential mechanism for opposing coronary vasospasm, especially if platelet activation is found to be a contributory factor in vasotonic angina pectoris.

Prostanoids in platelet-vascular interactions.

Prostacyclin, the labile prostanoid product of arachidonic acid metabolism in vascular endothelium, is the most potent known inhibitor of platelet aggregation and is highly effective in relaxing vascular smooth muscle. Its production is probably critically important in the maintenance of an intact vasculature. Although there is some evidence that prostacyclin circulates as a hormone, it is probably most important as a locally active agent in preventing thrombosis and maintaining patent vessels. Several factors can influence prostacyclin production, the most important of which probably act locally at sites of vessel wall injury. The most promising therapeutic approaches toward using prostacyclin's beneficial effects in vascular disease may lie in the use of drugs aimed at increasing prostacyclin production. Among these are thromboxane synthesis inhibitors, which act by diverting prostaglandin endoperoxides through the prostacyclin synthetase pathway, and lipoxygenase inhibitors, which might act chiefly by preventing formation of metabolites capable of inhibiting prostacyclin synthetase.

Characterization of receptors for platelet-activating factor on platelets, polymorphonuclear leukocytes and macrophages.

1. We have compared the potency of the putative platelet-activating factor (Paf) receptor antagonists (WEB 2086, L-652,731 and BN 52021) against Paf-induced aggregation of rabbit and guinea-pig platelets, aggregation of rabbit polymorphonuclear leukocytes (PMNLs) and prostacyclin generation by guinea-pig resident peritoneal macrophages. 2. On rabbit washed platelets and PMNLs WEB 2086, L-652,731 and BN 52021 each antagonized competitively Paf-induced aggregation. The rank order of potency was WEB 2086 congruent to L-652,731 greater than BN 52021 and was the same for the two cell types. 3. The pA2 values for each of the three antagonists were similar on rabbit washed platelets and PMNLs. Moreover, the pA2 for WEB 2086 on rabbit platelets (7.58) did not differ significantly from that on guinea-pig platelets (7.69). 4. On guinea-pig resident peritoneal macrophages WEB 2086 was 10 fold less potent for receptors mediating increased generation of 6-oxo-prostaglandin F1 alpha (6-oxo-PGF1 alpha) than for those mediating platelet aggregation. 5. The potencies of L-652,731 and BN 52021 were also markedly less (2 log units) for the macrophage receptors than for platelet or PMNL receptors and BN 52021 was more potent than L-652,731 in the macrophages. 6. WEB 2086 and L-652,731 significantly reduced basal 6-oxo-PGF1 alpha produced by macrophages, but none of the antagonists affected 6-oxo-PGF1 alpha production during stimulation by A23187. 7. These data raise the possibility that there may be a Paf receptor-subtype mediating prostacyclin generation in macrophages that is different from that on the platelet and PMNL. Hence, the potency of Paf antagonists against platelet aggregation would not be a good predictor of antagonist potency in disorders involving macrophages.

Catecholamine release and potentiation of thromboxane A2 production by nicotine in the greyhound.

Thromboxane A2 was generated by infusing arachidonic acid (2.5 micrograms ml-1) into an extra-corporeal circuit of blood withdrawn from anaesthetized dogs, and assayed on a blood-bathed bioassay cascade of porcine and bovine coronary artery strips, chick rectum and rat stomach strip. All tissues except chick rectum were treated with phentolamine and propranolol to abolish direct effects of catecholamines. The arachidonate-induced contractions of artery strips were abolished by a thromboxane synthetase inhibitor UK-38485 (3 mg kg-1, i.v.), but were not altered by the 5-hydroxytryptamine antagonist ketanserin (10 microM) administered over the tissues. Intravenous infusion of adrenaline (0.2 and 0.4 micrograms kg-1 min-1) reversibly potentiated the coronary contractions produced by arachidonate, but did not alter contractions when applied directly over the bioassay tissues. Intra-aortic infusion of nicotine (5 or 10 micrograms kg-1 min-1) also increased the arachidonate-induced contractions of the bioassay tissues but only on those experiments where nicotine caused appreciable adrenaline release, as indicated by relaxation of chick rectum. Phenoxybenzamine (2 mg kg-1, i.v.) blocked the potentiation effect of adrenaline and nicotine on coronary contractions. The specific alpha 2-adrenoceptor antagonist, idazoxan (1 mg kg-1, i.v.), also blocked nicotine-induced potentiation of the contractions. These findings suggest that the ability of nicotine to potentiate thromboxane release from circulating platelets and blood cells is dependent upon the release of adrenaline, and probably involves an action on alpha-adrenoceptors of the circulating blood elements.

Endothelium-dependent vasorelaxation independent of nitric oxide and K(+) release in isolated renal arteries of rats.

1. We investigated whether K(+) can act as an endothelium-derived hyperpolarizing factor (EDHF) in isolated small renal arteries of Wistar-Kyoto rats. 2. Acetylcholine (0.001 - 3 microM) caused relaxations that were abolished by removal of the endothelium. However, acetylcholine-induced relaxations were not affected by the nitric oxide (NO) synthase inhibitor N:(omega)-nitro-L-arginine methyl ester (L-NAME, 100 microM), by L-NAME plus the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ, 1 microM) or by L-NAME plus the cyclo-oxygenase inhibitor indomethacin (10 microM). In rings precontracted with high-K(+)(60 mM) physiological salt solution in the presence of L-NAME, acetylcholine-induced relaxations were abolished. 3. L-NAME-resistant relaxations were abolished by the large-conductance Ca(2+)-activated K(+) channel inhibitor charybdotoxin plus the small-conductance Ca(2+)-activated K(+) channel inhibitor apamin, while the inward rectifier K(+) channel inhibitor Ba(2+) or the gap junction inhibitor 18alpha-glycyrrhetinic acid had no effect. Acetylcholine-induced relaxation was unchanged by ouabain (10 microM) but was partially inhibited by a higher concentration (100 microM). 4. In half of the tissues tested, K(+)(10 mM) itself produced L-NAME-resistant relaxations that were blocked by ouabain (10 microM) and partially reduced by charybdotoxin plus apamin, but not affected by 18alpha-glycyrrhetinic acid or Ba(2+). However, K(+) did not induce relaxations in endothelium-denuded tissues. 5. In conclusion, acetylcholine-induced relaxations in this tissue are largely dependent upon hyperpolarization mechanisms that are initiated in the endothelium but do not depend upon NO release. K(+) release cannot account for endothelium-dependent relaxation and cannot be an EDHF in this artery. However, K(+) itself can initiate endothelium-dependent relaxations via a different pathway from acetylcholine, but the mechanisms of K(+)-induced relaxations remain to be clarified.

N-nitro L-arginine causes coronary vasoconstriction and inhibits endothelium-dependent vasodilatation in anaesthetized greyhounds.

1. The effect of N-nitro-L-arginine (L-NNA), an inhibitor of nitric oxide biosynthesis, on large coronary artery diameter and coronary blood flow was examined in anaesthetized greyhounds. The effects of L-NNA on the coronary vascular responses to acetylcholine (ACh), glyceryl trinitrate (GTN) and 5-hydroxytryptamine (5-HT) were also assessed. 2. L-NNA (5 mg kg-1), infused into the left circumflex coronary artery, increased systemic mean arterial pressure and decreased the external diameter of the artery. Infusion of L-NNA decreased coronary blood flow in 5 of the 7 dogs tested and increased mean coronary resistance but neither of these effects was statistically significant. There was no change in heart rate. 3. Intra-arterial injection of both ACh (0.01-0.05 micrograms kg-1) and GTN (0.1-0.5 micrograms kg-1) increased large coronary artery diameter and coronary blood flow. Coronary vascular responses to the endothelium-dependent vasodilator ACh were significantly reduced by L-NNA, whereas the responses to the endothelium-independent vasodilator GTN were not significantly affected. 4. 5-HT (0.1 microgram kg-1, injected into the left circumflex coronary artery) decreased coronary artery diameter but increased coronary blood flow. After the administration of L-NNA the 5-HT-induced dilatation of the coronary resistance vessels was significantly attenuated whereas the constriction of the circumflex coronary artery was increased in 3 out of 3 dogs in which diameter could be measured, although the latter effect was not statistically significant. 5. These data indicate that L-NNA causes coronary and systemic vasoconstriction and selectively inhibits endothelium-dependent vasodilatation in the coronary circulation of the anaesthetized greyhound. Therefore endothelium-derived NO has an important role in the regulation of coronary vascular tone in the large arteries and the resistance vessels.

Depressor responses to spinal stimulation in the pithed rat.

1. Electrical stimulation of the spinal nerves in the pithed rat preparation produces a pressor response due to sympathetic vasoconstriction.2. When the vasoconstrictor effect of sympathetic stimulation is abolished by guanethidine or hexamethonium and the blood pressure is raised by noradrenaline infusion, spinal stimulation produces depressor responses or complex responses containing depressor components.3. Contractions of skeletal muscle caused by stimulation of motor nerves result in complex changes in blood pressure consisting of a pressor component due to clamping of muscle blood vessels and a secondary depressor phase due to functional hyperaemia.4. The depressor response is partly due to stimulation of cholinergic postganglionic fibres. The acetylcholine released, which causes vasodilatation, may be the overflow from neuromuscular junctions or ganglionic synapses.5. Stimulation of the nerves to the adrenal medulla causes release of adrenaline which has a vasodilator effect during noradrenaline infusion.

Stimulation of prostacyclin release from the epicardium of anaesthetized dogs.

1 The generation of prostanoids in the hearts of anaesthetized dogs was studied by irrigating in situ the epicardial surface with Krebs solution. Prostanoids were measured by direct bioassay on smooth muscles and by radioimmunoassay of 6-oxo-prostaglandin F1 alpha (6-oxo-PGF1 alpha) and prostaglandin E2 (PGE2) in the epicardial irrigation fluid. 2 The epicardial irrigation fluid contained a prostacyclin-like substance, as indicated by the bioassay tissues, and immunoreactive 6-oxo-PGF1 alpha; PGE2-like materials were also detected. By both methods the output of the prostacyclin-like substance, which decreased with time of epicardial irrigation, was increased by manipulating the heart and by adding arachidonic acid (3 microgram/ml), and decreased by adding indomethacin (1 microgram/ml) to the irrigation fluid. 3 Bioassayed prostacyclin and immunoreactive 6-oxo-PGF1 alpha in the epicardial irrigation fluid increased by about 3-5 ng/ml during and after infusion of isoprenaline (0.1 microgram kg-1 min-1). The substance was not released by isoprenaline when indomethacin was added to the irrigation fluid, or when propranolol (0.5 mg/kg) was given intravenously. 4 Aortic constriction, bilateral carotid artery occlusion and intravenous angiotensin infusion all increased output of the prostacyclin-like substance into the epicardial irrigation fluid. The output was abolished by treating the heart with indomethacin (10 mg/kg intravenously or 1 microgram/ml epicardially). 5 The prostacyclin-like substance was also released by all of the above stimuli after the parietal pericardium had been removed and replaced by a plastic sheet. 6 It is concluded that prostacyclin is continually released from tissues close to the epicardial surface and from the pericardium, and that prostacyclin generation increases when cardiac workload increases. Prostacyclin of epicardial or pericardial origin might therefore contribute to metabolic regulation of coronary blood flow.

CGRP and nitric oxide of neuronal origin and their involvement in neurogenic vasodilatation in rat skin microvasculature.

1. Sensory nerves are important for the initiation of neurogenic inflammation and tissue repair. Both calcitonin gene-related peptide (CGRP) and nitric oxide (NO) have been implicated in neurogenic vasodilatation and inflammatory responses. 2. A blister model in the rat hind footpad was used as a site to induce neurogenic vasodilatation in response to antidromic electrical stimulation of the sciatic nerve. Blood flux was monitored with a laser Doppler flow monitor. 3. The quantitative contributions of CGRP and NO to vasodilatation were examined by use of the CGRP receptor antagonist CGRP8-37 and NO synthase inhibitors 7-nitroindazole (7-NI), 3-bromo 7-NI and N(G)-nitro L-arginine methyl ester (L-NAME). The potential modulatory role of endothelin was examined by use of the ET(A) receptor antagonist BQ-123. 4. CGRP8-37 (10 microM) was perfused over the blister base before nerve stimulation and continuously throughout the post-stimulation period, resulting in a significant reduction (41%) in the blood flux vascular response. 5. Pretreatment with the specific neuronal NO synthase inhibitors, 7-NI and 3-bromo 7-NI (10 mg kg(-1), i.v.), and of the non-specific L-NAME (100 microM), resulted in significant inhibition of the blood flux response (36%, 72% and 57% decrease, respectively). In contrast, 7-NI treatment in young rats pretreated with capsaicin had no further effect on the vascular response, suggesting that the source of NO is the sensory nerves. 6. BQ-123 (10 microM) significantly enhanced the stimulation-induced blood flux response (61% increase). When 7-NI was co-administered with either CGRP8-37 or BQ-123, the drug actions were additive, suggesting that there was no interaction between NO and CGRP or endothelin. 7. These data suggest that both NO and CGRP participate in neurogenic vasodilatation in rat skin microvasculature and that this response is modulated by endogenous endothelin.

Recirculation of prostacyclin (PGI2) in the dog.

1 The inactivation of prostacyclin (PGI2) in the circulation of anaesthetized dogs has been studied by the blood-bathed organ bioassay technique. 2 Spiral strips of bovine coronary and rabbit coeliac or mesenteric artery detected concentrations of PGI2 of 2 to 5 ng/ml. These tissues were insensitive to concentrations at least 200 fold higher of 15-oxo-PGI2 and 6-oxo-PGF1alpha. 3 PGI2 assayed on bovine coronary artery, rabbit coeliac artery or rat stomach strip, had a half life in blood of 3.0 +/- 0.3 min, indicating non-enzymatic degradation. 4 No disappearance could be detected by bovine coronary artery when PGI2 was infused across the lungs (0.1 to 0.5 microgram kg-1 min-1). However, PGI2 was partially inactivated in passage through vascular beds of hindquarters and liver. 5 Of PGI2 infused into the aorta 35 to 65% escaped inactivation in one complete circulation. Therefore, endogenous PGI2 released from the lungs may function as a circulating hormone.

Some direct and reflex cardiovascular actions of prostacyclin (PGI2) and prostaglandin E2 in anaesthetized dogs.

1 The aim of the study was to determine the mechanism of the hypotension and bradycardia produced by prostacyclin (PGI2). 2 Haemodynamic studies were carried out in nineteen open-chest beagle dogs anaesthetized with chloralose. PGI2 was infused intravenously or into the left atrium. 3 Infusions of PGI2 either intravenously or into the left atrium equally reduced arterial pressure and total peripheral resistance but bradycardia was greater after infusion into the left atrium. 4 Comparison of effects of PGI2 with those of prostaglandin E2 (PGE2) showed that although left atrial infusions both reduced aortic pressure and total peripheral resistance, PGE2 always increased heart rate, cardiac output and maximum acceleration. 5 Similar effects were observed with sodium nitroprusside except that it always caused tachycardia and reduced stroke volume. 6 Atropine (0.05 or 1 mg/kg i.v.) reduced or reversed the bradycardia induced by PGI2 but its hypotensive effects were reduced only after 1 mg/kg atropine. After vagotomy changes in cardiac output, stroke volume and maximum acceleration were increased, the hypotensive effects of PGI2 were reduced and the bradycardia was reversed; effects induced by PGE2 were not significantly altered. 7 The hypotension induced by prostacyclin is due to two components, a direct relaxation of vascular smooth muscle and a reflex, non-cholinergic vasodilatation. The bradycardia is reflex in nature and is partially mediated by the vagus pathway.

Acetylcholine induces vasodilatation in the rabbit isolated heart through the release of nitric oxide, the endogenous nitrovasodilator.

1. Acetylcholine (ACh, 0.03-3.0 microM) induced a dose-dependent vasodilatation in the isolated Langendorff-perfused heart of the rabbit. The vasodilatation was mimicked by exogenous nitric oxide (NO, 0.045-4.5 nmol). 2. There was no detectable vascular relaxing activity in the cardiac effluent when these concentrations of ACh or NO were injected through the heart, even in the presence of an infusion of superoxide dismutase (SOD). 3. Acetylcholine (0.03-3.0 microM), however, induced the release into the cardiac effluent of a material which produced a chemiluminescent signal when reacted with ozone, a response which could be mimicked with exogenous NO (0.045-4.5 nmol) injected through the heart. 4. The effects of ACh, but not those of NO, were antagonized by atropine (2 microM). Prostacyclin (1 microM) injected through the heart induced vasodilatation without the release of a biologically active or chemiluminescent material. 5. During passage through the heart, greater than 99% of the biological activity of exogenous NO disappeared, whereas there was approximately 50% reduction of its chemiluminescent response. This indicates complete transformation into a mixture containing approximately 50% NO2- and 50% of other non-chemiluminescent material(s), presumably NO3-. 6. This study suggests that ACh induces endothelium-dependent vasodilatation in the coronary circulation through the release of the endogenous nitrovasodilator, NO, which is rapidly converted to NO2- and NO3-.