Acetylcholine induces vasodilation and prostacyclin synthesis in rat lungs

Inhibition of nitric oxide synthase unmasks vigorous vasoconstriction in established pulmonary arterial hypertension

It is widely accepted that impaired bioavailability of endothelial nitric oxide (NO) plays a critical role in the pathophysiology of pulmonary arterial hypertension (PAH). However, there are published data that show that relatively many PAH patients respond favorably to acetylcholine‐induced pulmonary vasodilation during their follow‐up period, when diverse stages of the disorder are included. We hypothesized that NO bioavailability varies depending on the progression of PAH. Adult rats were exposed to the VEGF receptor blocker Sugen5416 and 3 weeks of hypoxia followed by return to normoxia for various additional weeks. All rats developed increased right ventricular systolic pressure (RVSP) and occlusive lesion formation at 1, 3, 5, and 8 weeks after the Sugen5416 injection. Acute NO synthase blockade did not change the elevated RVSP at the 1‐week time point, while it further increased RVSP markedly at the 3‐, 5‐, and 8‐week time points, leading to death in all rats tested at 8 weeks. Acetylcholine caused significant reduction in RVSP at the 8‐week but not the 1‐week time point, whereas sodium nitroprusside decreased the pressure similarly at both time points. Increased NO‐mediated cGMP production was found in lungs from the 8‐week but not the 1‐week time point. In conclusion, despite its initial impairment, NO bioavailability is restored and endogenous NO plays a critical protective role by counteracting severe pulmonary vasoconstriction in established stages of PAH in the Sugen5416/hypoxia/normoxia‐exposed rats. Our results provide solid pharmacological evidence for a major contribution of a NO‐suppressed vasoconstrictor component in the pathophysiology of established PAH.

What leads to different mediators of alkalosis-induced vasodilation in isolated and in situ pulmonary vessels?

We previously found that nitric oxide synthase (NOS) inhibition fully blocked alkalosis-induced relaxation of piglet pulmonary artery and vein rings. In contrast, NOS inhibition alone had no effect on alkalosis-induced pulmonary vasodilation in isolated piglet lungs. This study sought to identify factors contributing to the discordance between isolated and in situ pulmonary vessels. The roles of pressor stimulus (hypoxia vs. the thromboxane mimetic U-46619), perfusate composition (blood vs. physiological salt solution), and flow were assessed. Effects of NOS inhibition on alkalosis-induced dilation were also directly compared in 150-350-microm-diameter cannulated arteries and 150-900-microm-diameter, angiographically visualized, in situ arteries. Finally, effects of NOS inhibition on alkalosis-induced vasodilation were measured in intact piglets. NOS inhibition with N(omega)-nitro-L-arginine fully abolished alkalosis-induced vasodilation in all cannulated arteries but failed to alter alkalosis-induced vasodilation in intact lungs. The results indicate that investigation of other factors, such as perivascular tissue (e.g., adventitia and parenchyma) and remote signaling pathways, will need to be carried out to reconcile this discordance between isolated and in situ arteries.

Acetylcholine but not sodium nitroprusside exerts vasodilation in pulmonary hypertension secondary to chronic congestive heart failure

BACKGROUND

Reduced endothelium-dependent vasodilation contributes to the development of pulmonary hypertension in chronic congestive heart failure (CHF). We investigated pulmonary endothelium-dependent and independent vasodilation in patients with CHF.

METHODS

We studied 42 patients with CHF (age 55 +/- 10, NYHA Classes II-III, left ventricular ejection fraction 27 +/- 10%, mean PAP 29 +/- 12 mmHg). The endothelial vasodilator capacity of pulmonary resistance vessels was assessed by the infusion of acetylcholine into a pulmonary artery branch while measuring the blood flow velocity with a Doppler flow wire. For comparison endothelium-independent vasodilation was measured with the response to sodium nitroprusside. The conductance vessel diameter (4.4 +/- 0.2 mm) was determined by intravascular ultrasound. Acetylcholine was administered at concentrations of 10(-6) to 10(-4) mol/l, sodium nitroprusside was administered at concentrations of 0.125 and 0.25 microg/kg per min. The effects on conductance vessel diameter were investigated in 12 patients by the measurement of diameter and flow velocity following the administration of acetylcholine and sodium nitroprusside.

RESULTS

Acetylcholine markedly increased blood flow velocity (+39 +/- 7% at 10(-4) mol/l; p < .05). This correlated with the baseline PAP (r = 0.58; p < .05) and pulmonary vascular resistance (r = 0.58; p < .05). Sodium nitroprusside caused a small increase in the flow velocity (5 +/- 2% at 0.125, 12 +/- 4% at 0.25 microg/kg per minute; p < .05) that was accompanied by systemic vasodilation. The conductance vessel diameter was unchanged after acetylcholine was administered and was only marginally decreased after the administration of sodium nitroprusside.

CONCLUSIONS

In CHF acetylcholine reveals preserved receptor-mediated endothelial vasodilation, that is positively correlated to pulmonary hypertension, and cannot be reproduced by sodium nitroprusside.

Human placental cholinergic system

The occurrence of acetylcholine (ACh)-like activity in human placenta, a tissue without innervation, has been known for more than 60 years. However, the non-neuronal functions of ACh in human placenta are not clearly understood. The components of the cholinergic system-ACh, choline acetyltransferase, acetylcholinesterase, butyrylcholinesterase, muscarinic receptors, and nicotinic receptors--in human placenta have been demonstrated by unequivocal methods. Primate placentae store and release ACh by mechanisms similar to those of nervous tissue. However, there are many gaps in our knowledge, which include: (a) endogenous quaternary ammonium compounds other than ACh in human placental extracts; (b) the specificity of placental enzymes; (c) the subtypes and structures of placental muscarinic and nicotinic receptors; and (d) the significance of placental alpha-bungarotoxin binding proteins, ACh receptor stimulation-cellular signaling by second messengers, and activation of immediate early target genes (C-fos, C-jun) encoding transcription factors. Several hypothetical non-neuronal functions of ACh in placenta have been postulated based upon available experimental evidence. These include: (a) regulation of blood flow and fluid volume in placental vessels; (b) opening and closing of trophoblastic channels; (c) induction of contractile properties to myofibroblasts; (d) facilitation of amino acid transport necessary for fetal growth across placenta; (e) release of placental hormones; and (f) modulation of the formation of myometrial and placental prostaglandins in human parturition. All of these roles are reasonable, and some of these roles mav turn out to be linked to one another to influence or maintain placental function.

Developmental changes in endothelium-dependent relaxation of pulmonary arteries: role of EDNO and prostanoids

We hypothesized that maturational changes in both prostaglandin and endothelium-derived nitric oxide (EDNO) activity contribute to developmental changes in endothelium-dependent relaxation of newborn pulmonary arteries. Responses to endothelium-dependent vasodilators acetylcholine, bradykinin, and calcium ionophore A-23187 were determined in phenylephrine-constricted third- and fourth-generation (1- to 2-mm-diameter) pulmonary artery rings from 2-day (2d)- and 1-mo (1m)-old lambs under control conditions (Con), after inhibition of EDNO synthesis with N omega-nitro-L-arginine (L-NNA), after inhibition of prostanoid synthesis with meclofenamate (Mec), or both modulators with both inhibitors. Endothelium-independent responses to sodium nitroprusside (SNP) were also measured in Con rings. Endothelium-dependent relaxation was greater in 2d than 1m Con rings, particularly at high concentrations when an increase in tension occurred in 1m rings. L-NNA attenuated endothelium-dependent relaxation more in 2d rings, and SNP caused greater relaxation in 2d rings. However, Mec abolished all age-related differences by attenuating relaxation in 2d rings and constriction in 1m rings. These data suggest that developmental changes in endothelium-dependent responses of ovine pulmonary artery rings reflect both a decrease in EDNO activity and maturational differences in the relative influence of dilator and constrictor prostanoids.

Studies on the characterization of the subtype(s) of muscarinic receptor involved in prostacyclin synthesis in rabbit cardiomyocytes

The present study was conducted to localize and characterize the subtype(s) of muscarinic receptor involved in prostacyclin (PGI2) production elicited by the cholinergic transmitter acetylcholine (ACh) in various cell types in the rabbit heart. ACh increased PGI2 synthesis measured as 6-keto-PGF1 alpha, in cultured coronary endothelial cells and freshly dissociated ventricular myocytes in a dose dependent manner but not in cultured coronary smooth muscle cells of rabbit heart. McN-A-343, a partially selective M1 muscarinic ACh receptor (mAChR) agonist, did not alter 6-keto-PGF1 alpha synthesis in these cell types. ACh induced 6-keto-PGF1 alpha synthesis in coronary endothelial cells and ventricular myocytes was not altered by a low concentration (10(-8) M) of pirenzipine, an M1 mAChR antagonist but was reduced by a higher concentration (10(-6) M). In coronary endothelial cells ACh induced 6-keto-PGF1 alpha production was reduced by hexahydro-sila-difendial (HHSiD), an M3 mAChR antagonist, and in ventricular myocytes by both 11-¿2-[(di-ethylamino) methyl]-1-piperidinyl]acetyl-5,11-dihydro-6-H-pyrido-[2,3-b]-benzodiazep ine-6 one¿ (AF-DX 116), an M2 receptor antagonist, and HHSiD. The decrease by ACh of isoporterenol stimulated cAMP accumulation was minimized by AF-DX 116 but not by HHSiD or pirenzipine. Pertussis toxin treatment minimized ACh induced decrease in isoproterenol stimulated rise in cAMP and ATP release, but not ACh induced 6-keto-PGF1 alpha synthesis. These data suggest that ACh stimulates prostacyclin production in coronary endothelial cells via M3 mAChR and in ventricular myocytes M2 and M3 mAChR. Moreover, ACh induced decrease in cAMP, but not the increase in 6-keto-PGF1 alpha production, is mediated by pertussis toxin sensitive G alpha i proteins in these cells.

Prostacyclin release in experimental diabetes: effects of evening primrose oil

Alterations in release of endothelium-derived vasomotor agents could underlie microvascular and neuropathic complications in diabetes. This study examined release of the potent vasodilator prostacyclin, measured as immunoreactive 6-keto prostaglandin F1 alpha, from rat lung, kidney and peripheral nerve. Tissues were taken from control and streptozotocin-diabetic rats which had been treated for 8 weeks with either evening primrose oil (EPO) or, as a control for lipid intake, coconut oil (CO). Lung and kidney slices were incubated in the presence of acetylcholine (ACh), the calcium ionophore 4-Br-A23187, arachidonic acid (AA) or without agonist (basal). Segments of sciatic nerve, with their epineuria punctured, were incubated with or without 4-Br-A23187. Basal prostacyclin release from the lung was significantly higher in rats treated with EPO irrespective of diabetic state (increased by 60% in controls and by 77% in diabetics). Levels were reduced in CO-diabetics compared to EPO-controls (53% reduction) and CO-controls (30% reduction), although this did not reach statistical significance in the latter. Basal prostacyclin release was also significantly reduced in the kidney from CO-diabetics (40% reduction compared to CO-controls and 56% reduction compared to EPO-controls). In the presence of AA, lung prostacyclin release was significantly lower in CO-diabetic rats compared to all other groups (40% reduction compared to EPO-diabetics and 60% compared to both control groups) but there were no differences in renal release between any group. Prostacyclin release by nerves from CO-diabetic rats was significantly reduced (by 91-93%) compared to all other groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental changes in endothelium-dependent pulmonary vasodilatation in pigs

1. We compared in vitro endothelium-dependent vasorelaxant responses to acetylcholine (ACh) and the endothelium-independent vasodilator response to sodium nitroprusside (SNP) in prostaglandin F2 alpha (PGF2 alpha)-precontracted muscular pulmonary arteries (PA) from pigs aged 5 min to 2 h (neonatal), 3-10 days, 3-8 weeks and adults. 2. In the pulmonary artery (PA) rings from neonatal animals, the vasodilator response to ACh was negligible. However, responses to ACh were present in all PA rings from older animals, being greatest at 3-10 days and then decreasing with age (P less than 0.001, ANOVA). ACh (30 microM) induced a 1 +/- 1%, 92 +/- 9%, 62 +/- 5% and 51 +/- 6% reduction of the PGF2 alpha-generated tension in neonatal, 3-10 days, 3-8 weeks and adult groups, respectively. 3. The relaxant response to SNP was present in the PA rings from all age groups and increased with age (P less than 0.001, ANOVA). SNP (1 microM)-induced relaxation was 55 +/- 9%, 73 +/- 7%, 97 +/- 5% and 93 +/- 6% in neonatal, 3-10 days, 3-8 week and adult groups, respectively. 4. Removal of the vascular endothelium abolished the relaxant response to ACh but had no effect on the response to SNP in any groups. 5. NG-monomethyl-L-arginine (30 microM), a nitric oxide synthesis inhibitor, inhibited the response to ACh but not to SNP. The lipoxygenase inhibitor, nordihydroguaiaretic acid, had no significant effect on responses to ACh or SNP in any group.6. These findings suggest that the nitric oxide pathway may not play a part in dilating the pig pulmonary arteries at birth, but may be important during the transitional period of establishing a stable post-natal pulmonary circulation. The increase in response to SNP with age parallels the increase in smooth muscle cell myofilaments to which it may be related.

Measurement of endothelial cytosolic calcium concentration and nitric oxide production reveals discrete mechanisms of endothelium-dependent pulmonary vasodilatation

Nitric oxide is an endothelium-derived relaxing factor. Conversion of L-arginine to nitric oxide follows mediator-induced elevation of endothelial cytosolic calcium concentration. However, not all endothelium-dependent vasodilatation is caused by endothelium-derived relaxing factor, and few studies have correlated changes in vascular tone with measurement of free cytosolic calcium concentration or nitric oxide. The effects of three endothelium-dependent vasodilators (acetylcholine, bradykinin, and A23187) on vascular tone and nitric oxide production were studied in proximal rat pulmonary artery rings. Changes in free cytosolic calcium concentration and nitric oxide production were also studied in bovine pulmonary artery endothelial cells. A23187 and bradykinin caused pulmonary vasodilatation, nitric oxide production, and elevation of endothelial calcium concentrations. Although acetylcholine caused endothelium-dependent vasodilatation, it reduced free cytosolic calcium concentration and failed to increase nitric oxide levels. Acetylcholine-induced dilatation was partially inhibited by meclofenamate but was unaffected by ouabain. Acetylcholine, unlike bradykinin and A23187, does not act through a nitric oxide-dependent mechanism in the rat pulmonary vasculature.

Augmentation of hypoxic pulmonary vasoconstriction in the isolated perfused rat lung by in vitro antagonists of endothelium-dependent relaxation

The role of the endothelium in hypoxic constriction of the intact pulmonary vascular bed has not been clearly elucidated. To test for a possible role for endothelium-derived relaxing factor(s) (EDRF) in the hypoxic pressor response, isolated, whole blood-perfused rat lungs from male Sprague-Dawley rats treated with meclofenamate were prepared. Three protocols were performed, including: (a) normal saline (control); (b) the putative EDRF inhibitors, eicosatetraynoic acid (ETYA, 1 X 10(-4) M) or nordihydroguaiaretic acid (NDGA, 1 X 10(-4) M) versus vehicle DMSO; and (c) the putative EDRF inhibitor hydroquinone (HQ, 1 X 10(-4) M) versus vehicle ethyl alcohol (ETOH). The pulmonary pressor response to angiotensin II (Ang II, 0.25 micrograms) injections alternated with 6-min periods of hypoxic ventilation (3% O2, 5% CO2) was measured before and after the administration of saline, inhibitors, or vehicles. The administration of the EDRF inhibitors ETYA, NDGA, and HQ resulted in a marked accentuation of the hypoxic pressor response that was not seen in the controls (P less than 0.05). In separate experiments, lungs precontracted with norepinephrine (1 X 10(-6) M) were pretreated with edrophonium (1 X 10(-4) M) and then observed for endothelium-dependent vasodilator responses to acetylcholine at increasing doses (1 X 10(-7)-1 X 10(-4) M). Administration of ETYA, NDGA, or HQ abrogated the observed vasodilatation to acetylcholine, which was not seen with vehicles alone (P less than 0.01). These studies suggest an important role for the endothelium in pulmonary vascular responsiveness to alveolar hypoxia through possible release of a relaxing factor(s) that attenuates the degree of pulmonary arterial constriction.

Stimulation of arachidonic acid release and inhibition of mitogenesis by cloned genes for muscarinic receptor subtypes stably expressed in A9 L cells

A family of genes encoding four distinct muscarinic receptors (designated m1-m4) has been cloned and stably expressed in A9 L cells. When the m1 and m3 receptors were stimulated with carbachol, there was a rapid rise of liberated arachidonic acid, inositol phosphates, and cAMP, while m2 and m4 receptor stimulation had no detectable stimulation of these second messengers. Pretreatment with phorbol 12-myristate 13-acetate (PMA) caused a marked acceleration and amplification of m1 and m3 receptor-mediated arachidonic acid release. In contrast, m1- and m3-mediated inositol phosphate formation was inhibited by the same PMA pretreatment. Arachidonic acid release was unaffected by manipulations of cAMP levels. Arachidonic acid production was inhibited by calcium-free medium and 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester (TMB-8; an inhibitor of cytosolic calcium mobilization) yet was unaffected by verapamil, a calcium-channel blocker. These experiments show that arachidonic acid release induced by the m1 and m3 receptors is regulated independently of phospholipase C and cAMP accumulation. Carbachol stimulation of the m1 and m3 cAMP accumulation. Carbachol stimulation of the m1 and m3 receptors also markedly decreased mitogenesis as measured by thymidine incorporation. The m1 receptor-mediated inhibition of mitogenesis could be partially blocked by indomethacin, a cyclooxygenase inhibitor. The inhibition of mitogenesis could be mimicked by cAMP elevation.

Acetylcholine dramatically increases prostanoid synthesis in piglet parietal cortex

We investigated effects of exogenous acetylcholine on prostanoid synthesis by parietal cortex in neonatal pigs. Cerebrospinal fluid (CSF) with no drug, and CSF containing acetylcholine at 10(-6) to 10(-3) M was injected under a 'closed' cranial window, and after 5 min the CSF was collected and analyzed by radioimmunoassay for prostaglandin (PG) E2, PGF2 alpha, PGD2, 6-keto-PGF1 alpha (the hydrolysis product of prostacyclin), and thromboxane (TX) B2 (the hydrolysis product of TXA2). PGE2 and PGF2 alpha were the predominant prostanoids in CSF under control conditions. Levels of all CSF prostanoids increased after topical application of acetylcholine, with the largest increases being for PGE2 and PGF2 alpha. During control conditions, levels were 1294 +/- 170 (mean +/- S.E.M.) pg/ml for PGE2 (n = 16), 1032 +/- 143 pg/ml for PGF2 alpha (n = 3), 659 +/- 92 pg/ml for 6-keto-PGF1 alpha (n = 15), 141 +/- 44 pg/ml for TXB2 (n = 12), and were below detectable levels for PGD2. Following application of 10(-3) M acetylcholine, levels were 34,535 +/- 5438 pg/ml for PGE2, 15,539 +/- 2772 pg/ml for PGF2 alpha, 2967 +/- 547 pg/ml for 6-keto-PGF1 alpha, 580 +/- 105 pg/ml for TXB2, and 556 +/- 221 pg/ml for PGD2. These results suggest that prostanoids could play a role in mediating effects of acetylcholine in the brain, or in modulating acetylcholine release via a negative feedback mechanism.