Effects of changes in extra- and intracellular K+ on the endothelial production of prostacyclin

Chronic ouabain treatment increases the contribution of nitric oxide to endothelium-dependent relaxation

The aim of this study was to analyze the contribution of nitric oxide, prostacyclin and endothelium-dependent hyperpolarizing factor to endothelium-dependent vasodilation induced by acetylcholine in rat aorta from control and ouabain-induced hypertensive rats. Preincubation with the nitric oxide synthase inhibitor N-omega-nitro-L-arginine methyl esther (L-NAME) inhibited the vasodilator response to acetylcholine in segments from both groups but to a greater extent in segments from ouabain-treated rats. Basal and acetylcholine-induced nitric oxide release were higher in segments from ouabain-treated rats. Preincubation with the prostacyclin synthesis inhibitor tranylcypromine or with the cyclooxygenase inhibitor indomethacin inhibited the vasodilator response to acetylcholine in aortic segments from both groups. The Ca2+-dependent potassium channel blocker charybdotoxin inhibited the vasodilator response to acetylcholine only in segments from control rats. These results indicate that hypertension induced by chronic ouabain treatment is accompanied by increased endothelial nitric oxide participation and impaired endothelium-dependent hyperpolarizing factor contribution in acetylcholine-induced relaxation. These effects might explain the lack of effect of ouabain treatment on acetylcholine responses in rat aorta.

Does cold blood cardioplegia solution cause deterioration in clinical pulmonary function following coronary artery bypass graft surgery?

OBJECTIVE

Deterioration in pulmonary function is a common complication following coronary artery bypass graft surgery and there is still speculation to the precise causative factors thereof. Cardioplegia solution not drained by the atriocaval cannula enters the lung parenchyma unless removed by a pulmonary artery (PA) vent. The hypothesis of the present study was that cold blood cardioplegia solution damages the lung parenchyma, resulting in an observed deterioration of clinical lung function.

METHODS

A prospective, double-blind, randomised trial was conducted on 142 patients. The study group of 71 patients had a PA vent inserted at the time of cannulation, preventing cardioplegia from going through the lungs. In addition, positive end expiratory pressure (PEEP) was applied and low-volume lung ventilation carried out during cardiopulmonary bypass (CPB). The control group (n =71) had cardioplegia enter the lung parenchyma during cardiopulmonary bypass. Clinical parameters of arterial blood gases, including estimated shunt fraction, spirometry tests and radiographic analysis was made preoperatively and at set times through the postoperative period.

RESULTS

Baseline demographics and intraoperative and postoperative management was the same in both groups, thus, yielding a homogenous sample for analysis. Significant changes were noted in arterial blood gases, spirometry, and radiographic analysis of effusion and atelectasis over the time periods studied (p<0.001). There was, however, no significant difference between the study and control groups at any point (p > 0.05).

CONCLUSIONS

The data, therefore, suggest that allowing cold blood cardioplegia solution to circulate the lungs during cardiopulmonary bypass does not have any (beneficial or detrimental) effect on clinical lung function postoperatively.

Ouabain-induced hypertension alters the participation of endothelial factors in alpha-adrenergic responses differently in rat resistance and conductance mesenteric arteries

1. This study compares the role of endothelial factors in alpha-adrenoceptor contractile responses in mesenteric resistance (MRA) and superior (SMA) mesenteric arteries from ouabain-treated (8.0 microg day(-1), 5 weeks) and untreated rats. The role of the renin-angiotensin system was also evaluated. 2. Ouabain treatment increased systolic blood pressure. In addition, ouabain reduced the phenylephrine response in SMA but did not alter noradrenaline responses in MRA. 3. Endothelium removal or the nitric oxide synthase (NOS) inhibitor (l-NAME, 100 microm) increased the responses to alpha-adrenergic agonists in both vessels. After ouabain treatment, both endothelial modulation and the l-NAME effect were increased in SMA, while only the l-NAME effect was increased in MRA. Endothelial NOS expression remained unaltered after ouabain treatment. 4. Indomethacin (10 microm) similarly reduced the noradrenaline contraction in MRA from both groups; in contrast, in SMA, indomethacin only reduced phenylephrine-induced contractions in segments from untreated rats. Co-incubation of l-NAME and indomethacin leftward shifted the concentration-response curves for noradrenaline more in MRA from ouabain-treated rats; tetraethylammonium (2 mm) shifted the noradrenaline curves further leftward only in MRA from untreated rats. 5.Losartan treatment prevents the development of hypertension but not all vascular changes observed after ouabain treatment. 6. In conclusion, a rise in endothelial NO and impaired prostanoid participation might explain the reduction in phenylephrine-induced contraction in SMA after ouabain treatment. An increase in the modulatory effect of endothelial NO and impairment of endothelium-dependent hyperpolarizing factor effect might explain why the ouabain treatment had no effect on noradrenaline responses in MRA.

Involvement of nitric oxide in amiodarone- and dronedarone-induced coronary vasodilation in guinea pig heart

Amiodarone, a powerful antiarrhythmic compound, possesses coronary and peripheral vasodilator properties. The mechanisms responsible for these effects remain incompletely understood. In the present study, the coronary effects of amiodarone and dronedarone, a non-iodinated amiodarone-like compound, were investigated in isolated guinea pig hearts perfused at constant flow with high K+ solution (40 mM). Amiodarone (0.01-10 microM), dronedarone (0.01-1 microM) and verapamil (0.01-10 microM) induced concentration-dependent decreases in coronary perfusion pressure. Amiodarone- and dronedarone-mediated reductions in coronary perfusion pressure were not modified by a cyclooxygenase inhibitor, indomethacin (3 microM). L-Nitro-L-arginine (L-NOARG; 3-100 microM) caused a rightward shift of concentration-response curves to amiodarone and dronedarone in a dose-dependent way; L-arginine, a nitric oxide (NO) precursor, reversed this effect. Furthermore, when guinea pigs were treated with NG-nitro-L-arginine methyl ester (L-NAME; 20 mg/kg), amiodarone could not reduce coronary perfusion pressure. NO synthase inhibition did not affect the coronary perfusion pressure response to verapamil. 1H-[1,2,4]Oxadiazole (4,3-a) quinoxalin-1-one (ODQ), a specific inhibitor of the guanylyl cyclase, inhibited the effects of amiodarone but not those of verapamil. In the presence of L-NOARG and ODQ, and in hearts from animals treated with L-NAME, a decrease in coronary perfusion pressure was still observed at the highest concentration of dronedarone. These results show that, in guinea pig hearts, the coronary vasodilation induced by amiodarone highly depends on nitric oxide. Dronedarone differs from amiodarone by a remaining relaxant effect, refractory to inhibition of NO synthase pathway, probably due to its Ca+ antagonist properties.

Involvement of K+ channel permeability changes in the L-NAME and indomethacin resistant part of adenosine-5'-O-(2-thiodiphosphate)-induced relaxation of pancreatic vascular bed

1. We have previously demonstrated that adenosine-5'-O-(2-thiodiphosphate) (ADPbetaS), a potent P2Y-purinoceptor agonist, relaxed pancreatic vasculature not only through prostacyclin (PGI2) and nitric oxide (NO) release from the endothelium but also through other mechanism(s). In this study, we investigated the effects of an inhibitor of the Na+/K+ pump, of ATP-sensitive K+ (K(ATP)) channels and of small (SK(Ca)) or large (BK(Ca)) conductance Ca2+-activated K+ channels. Experiments were performed at basal tone and during the inhibition of NO synthase and cyclo-oxygenase. 2. In control conditions, ADPbetaS (15 microM) induced an initial transient vasoconstriction followed by a progressive and sustained vasodilatation. In the presence of N(omega)-nitro-L-arginine methyl ester (L-NAME, 200 microM) the transient vasoconstriction was reversed into a one minute vasodilator effect, which was then followed by a progressive and sustained vasodilatation similar to that observed with ADPbetaS alone. The addition of indomethacin (10 microM) did not significantly modify the profile of ADPbetaS-induced vasodilatation. 3. Ouabain (100 microM) decreased basal pancreatic flow rate and did not modify ADPbetaS-induced relaxation. This inhibitor of the Na+/K+ pump increased the pancreatic vasoconstriction induced by L-NAME or by the co-administration of L-NAME and indomethacin. Ouabain did not modify either the L-NAME or the L-NAME/indomethacin resistant part of the ADPbetaS vasodilatation. 4. The K(ATP) inhibitor tolbutamide (185 microM) did not significantly modify basal pancreatic flow rate and ADPbetaS-induced relaxation. This inhibitor which did not change L-NAME-induced vasoconstriction, significantly diminished the L-NAME resistant part of ADPbetaS-induced vasodilatation. Tolbutamide intensified the vasoconstriction induced by the co-administration of L-NAME and indomethacin. In contrast, the L-NAME/indomethacin resistant part of ADPbetaS vasodilatation was not changed by the closure of K(ATP). 5. The SK(Ca) inhibitor apamin (0.1 microM) did not significantly change pancreatic vascular resistance whatever the experimental conditions (in the absence or in presence of L-NAME or L-NAME/indomethacin). In the presence of L-NAME, the closure of SK(Ca) channels changed the one minute vasodilator effect of ADPbetaS into a potent vasoconstriction and thereafter modified only the beginning of the second part of the L-NAME-resistant part of the ADPbetaS-induced vasodilatation. In contrast, the L-NAME/indomethacin resistant part of ADPbetaS-induced relaxation remained unchanged in the presence of apamin. 6. Charybdotoxin (0.2 microM), an inhibitor of BK(Ca), increased pancreatic vascular resistance in the presence of L-NAME/indomethacin. In the presence of L-NAME, the closure of BK(Ca) channels reversed the one minute vasodilator effect of ADPbetaS into a potent vasoconstriction and drastically diminished the sustained vasodilatation. In contrast the L-NAME/indomethacin resistant part of ADPbetaS-induced relaxation was not modified by the presence of charybdotoxin. Under L-NAME/indomethacin/charybdotoxin/apamin infusions, ADPbetaS evoked a drastic and transient vasoconstriction reaching a maximum at the second minute, which was followed by a sustained increase in the flow rate throughout the ADPbetaS infusion. The maximal vasodilator effect of ADPbetaS observed was not modified by the addition of apamin. 7. The results suggest that the L-NAME-resistant relaxation induced by ADPbetaS in the pancreatic vascular bed involves activation of BK(Ca), K(ATP) and to a lesser extent of SK(Ca) channels, but the L-NAME/indomethacin resistant part of ADPbetaS-induced relaxation is insensitive to the closure of K(ATP), SK(Ca) and BK(Ca) channels.

RU 38486 inhibits intracellular calcium mobilization and PGI2 release from human myometrium: mechanisms of action

We previously demonstrated that the antiprogestogen RU 486, when superfused on myometrial strips, induces a rapid decrease in spontaneous uterine contractile frequency, an increase in amplitude and duration of contractions, and a concomitant decrease in 6-keto PGF(1alpha) release. In this study, we present further work on the role of calcium transients and the involvement of the PLC/PKC pathway in mediating RU 486 effects. We found no clear causal relationship between the spontaneous contractility controlled by external Ca++ concentration and 6-keto PGF(1alpha) release depending mostly on intracellular Ca++ mobilization. We show that RU 486 strengthened the inhibitory effect of TMB8, a potent inhibitor of internal calcium, on both spontaneous contractility and 6-keto PGF(1alpha), release and antagonized the stimulatory action of thapsigargin, a toxin blocking the endoplasmic reticulum calcium pump (ER Ca++ ATPase). These data indicate that RU 486 could act as an inhibitor of intracellular Ca++ mobilization. A slight but significant decrease of the prostanoid liberation was observed in the presence of U73122, an inhibitor of PLC, but not in the presence of neomycin, another PLC inhibitory compound. PKC inhibitors, staurosporine and H7 did not significantly affect spontaneous 6-keto PGF1alpha release, showing that PIP2 hydrolysis and PKC pathway were not involved in the basal release of the prostacyclin metabolite. Vasopressin (AVP), an agent known to induce contractility of the non-pregnant human uterus, markedly increased 6-keto PGF(1alpha) release in a dose-dependent manner. Stimulation of GTP-regulated proteins (G proteins) by ALF4 was accompanied by a rise in 6-keto PGF(1alpha) liberation and a high contractile activity. The effects of both vasopressin and ALF4- were not significantly opposed by RU 486, indicating that other sources of Ca++, not controlled by the steroid, were involved in the agonist-stimulated prostanoid release. Studies with structurally related RU 486 analogues showed that the steroid effects were not dependent on their antihormonal activity, but rather on a specific 11beta arylsubstitution and a 17beta-hydroxy-13beta-methyl configuration of the 4,9-estradien-3-one molecule.

Flow- and bradykinin-induced nitric oxide production by endothelial cells is independent of membrane potential

The objective of this study was to evaluate the role transmembrane potential plays in flow-induced nitric oxide (NO) production in endothelial cells (EC). NO production was monitored by measuring intracellular guanosine 3',5'-cyclic monophosphate (cGMP) and extracellular nitrite plus nitrate (NOx). Primary human umbilical vein endothelial cells (HUVEC) were exposed to laminar flow (22 dyn/cm2) of medium with 5.4 mM KCl (control medium) with or without 3 mM tetraethylammonium chloride (TEA) or 90 mM KCl (K(+)-rich medium). Bradykinin (BK) was added to time-matched stationary cultures to give a final concentration of 5 nM. With control medium, 30 s, 2 min, and 3 h of treatment with flow or 2 min of treatment with BK resulted in an approximately threefold increase in cGMP over stationary cultures. Depolarization with KCl or TEA did not influence cGMP production in flow-treated or stationary cultures. Flow of either control or potassium-rich medium resulted in an approximately 10-fold increase in average NOx production rate over 3 h compared with stationary cultures. Taken together these data indicate that neither membrane hyperpolarization nor normal membrane potential is necessary for flow- or BK-induced NO production by HUVEC.

The endothelium: a key to the future

The vascular endothelium is a complex modulator of a variety of biological systems and may well be the key to definitive success in the treatment of cardiovascular disorders. Surgically-induced endothelial injury may occur preoperatively during cardiac catheterization and intraoperatively from mechanical manipulation, ischemia, hypothermia, and exposure to cardioplegic solutions. The normal endothelium is antithrombogenic and yet promotes platelet aggregation and coagulation if injured. Vasospasm, occlusive intimal hyperplasia, and accelerated arteriosclerosis can also all occur as a result of endothelial injury. Furthermore, endothelial injury is harmful even in the absence of disruption of its monolayer integrity. Thus, preservation of the endothelium should be an additional objective for all cardiovascular surgeons. Synthetic vascular grafts, cardiac valves, and artificial ventricles do not spontaneously endothelialize and thus usually require some form of anticoagulation to maintain patency. Hence, endothelialization of prosthetic implants became an attractive concept. A number of different methods of obtaining an endothelial lining of prosthetic material has since been developed; these include facilitated endothelial cell migration, and endothelial cell seeding by using either venous or microvascular endothelial cells. Manipulating the endothelium might well provide the next major advancement for therapeutic and preventive measures for cardiovascular disease.

Contractions induced by potassium-free solution and potassium relaxation in vascular smooth muscle of hypertensive and normotensive rats

1. Vascular contractions induced by K(+)-free solution and relaxation responses following the return of K+ to the organ bath were studied in mesenteric arterial rings from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) with particular focus on the role of vascular adrenergic nerve-endings and endothelium. 2. In endothelium-denuded rings the omission of K+ from the incubation medium resulted in gradual contractions, the rate of which was slower in SHR than WKY. Nifedipine (1 microM) inhibited the contractions more effectively in SHR than WKY. 3. Adrenergic denervation in vitro with 6-hydroxydopamine reduced the contractions induced by the K(+)-free medium in endothelium-denuded rings. The remaining contractions after denervation were markedly greater in SHR than WKY. 4. The presence of intact vascular endothelium attenuated the K(+)-free contractions in both strains, the attenuation being smaller in SHR than WKY. NG-nitro-L-arginine methyl ester (L-NAME, 0.1 mM) and methylene blue (10 microM), but not indomethacin (10 microM), abolished the attenuating effect of endothelium on the K(+)-free contractions. L-Arginine (1 mM) reversed the effect of L-NAME in WKY but not in SHR. 5. The re-addition of K+ after full K(+)-free contractions dose-dependently relaxed the rings. The rate of this K(+)-induced relaxation was significantly slower in SHR than WKY at all K+ concentrations (0.1-5.9 mM) studied, whether the endothelium or functioning adrenergic nerve-endings were present or not. Ouabain (1 mM) totally inhibited the K+ relaxation in SHR but only partially in WKY.6. Vascular smooth muscle contractions induced by high concentrations of potassium were comparable between the strains. The EC50 for noradrenaline-induced contractions was lower in SHR than WKY, but the maximal forces did not differ significantly.7. In conclusion, the contractile response in K+-free solution more clearly differentiates vascular rings from SHR and WKY than the responses induced by the classical contractile agents noradrenaline and high concentrations of potassium. The depressant effect of the presence of intact endothelium on the K+-free contractions, which was smaller in SHR than WKY, is mediated via the endothelium-derived relaxing factor. Neurotransmitter release from vascular adrenergic nerve-endings participates less in the K+-free contractile response in SHR than WKY. Moreover, the contractile response is more dependent on calcium entry through nifedipine-sensitive calcium channels in SHR than WKY. The greater K+-free contractions of denervated endothelium-denuded rings and the reduced K+ relaxation rate in SHR when compared to WKY suggest increased cell membrane permeability and decreased activity of vascular Na+, K+-ATPase, respectively, in this type of genetic hypertension.

Activation and pulmonary toxicity of pyrrolizidine alkaloids

Pyrrolizidine alkaloids unsaturated in the 1,2 position are hepatotoxins. Certain of them, such as monocrotaline, are also pneumotoxins, producing pulmonary arterial hypertension and right ventricular hypertrophy as a delayed response two weeks after administration. Pneumotoxicity is the result of hepatic metabolism, the lung itself being unable to bioactivate pyrrolizidine alkaloids. The changes produced in the lung following exposure to pneumotoxic pyrrolizidine alkaloids are reviewed, together with the factors and interventions which modify or influence these changes. In the main, the earliest changes are seen in vascular smooth muscle and in the interactions between the smooth muscle and the endothelium. The search to identify the pneumotoxic metabolite is reviewed. It is generally accepted that pyrroles, or dehydroalkaloids, are responsible for the toxicity of pyrrolizidines. However, the primary pyrroles are intensely reactive, hydrolyzing and polymerizing within seconds in aqueous solution. Evidence for and against the pneumotoxin being a primary pyrrole or a stabilized secondary conversion product of a primary pyrrole is discussed.

P2 purinoceptors on vascular endothelial cells: physiological significance and transduction mechanisms

The characterization of P2 gamma purinoceptors on vascular endothelial cells has progressed rapidly since their existence was first demonstrated in 1983. They transduce the actions of extracellular ATP and ADP--endothelium-dependent relaxation, prostacyclin synthesis, endothelial cell mitogenesis--which play a vital role in the interaction between platelets (a rich source of extracellular adenine nucleotides) and the vessel wall. Release of prostacyclin limits the extent of intravascular platelet aggregation following vascular damage and platelet stimulation, while the mitogenic effect may accelerate the repair of a lesion. P2 gamma receptors on endothelial cells are coupled to a phospholipase C by a GTP-binding protein. Jean-Marie Boeynaems and Jeremy Pearson explain how the increases in cytoplasmic Ca2+ and diacylglycerol resulting from this initial event mediate several further effects. In particular, activation of a Ca2(+)-sensitive phospholipase A2 explains the increased synthesis of prostacyclin, while the phosphorylation of several proteins by calmodulin-dependent kinases modulates other endothelial cell functions.