The effect of the nitric oxide synthase inhibitor N-gamma-nitro-L-argine methyl ester on hypoxic pulmonary vasoconstriction

Role of endothelial nitric oxide in pulmonary and systemic arteries during hypoxia

Our aim was to investigate the role played by endothelial nitric oxide (NO) during acute vascular response to hypoxia, as a modulator of both vascular tone (through guanylate cyclase (sGC) activation) and mitochondrial O2 consumption (through competitive inhibition of cytochrome-c-oxydase (CcO)). Organ bath experiments were performed and O2 consumption (Clark electrode) was determined in isolated aorta, mesenteric and pulmonary arteries of rats and eNOS-knockout mice. All pre-contracted vessels exhibited a triphasic hypoxic response consisting of an initial transient contraction (not observed in vessels from eNOS-knockout mice) followed by relaxation and subsequent sustained contraction. Removal of the endothelium, inhibition of eNOS (by L-NNA) and inhibition of sGC (by ODQ) abolished the initial contraction without altering the other two phases. The initial hypoxic contraction was observed in the presence of L-NNA+NO-donors. L-NNA and ODQ increases O2 consumption in hypoxic vessels and increases the arterial tone in normoxia but not in hypoxia. When L-NNA+mitochondrial inhibitors (cyanide, rotenone or myxothiazol) were added, the increase in tone was similar in normoxic and hypoxic vessels, which suggests that inhibition of the binding of NO to reduced CcO restored the action of NO on sGC.

CONCLUSION

A complex equilibrium is established between NO, sGC and CcO in vessels in function of the concentration of O2: as O2 falls, NO inhibition of mitochondrial O2 consumption increases and activation of sGC decreases, thus promoting a rapid increase in tone in both pulmonary and systemic vessels, which is followed by the triggering of NO-independent vasodilator/vasoconstrictor mechanisms.

Hypoxic pulmonary vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Angiotensin II stimulates nitric oxide production in pulmonary artery endothelium via the type 2 receptor

We previously reported that angiotensin II stimulates an increase in nitric oxide production in pulmonary artery endothelial cells. The aims of this study were to determine which receptor subtype mediates the angiotensin II-dependent increase in nitric oxide production and to investigate the roles of the angiotensin type 1 and type 2 receptors in modulating angiotensin II-dependent vasoconstriction in pulmonary arteries. Pulmonary artery endothelial cells express both angiotensin II type 1 and type 2 receptors as assessed by RT-PCR, Western blot analysis, and flow cytometry. Treatment of the endothelial cells with PD-123319, a type 2 receptor antagonist, prevented the angiotensin II-dependent increase in nitric oxide synthase mRNA, protein levels, and nitric oxide production. In contrast, the type 1 receptor antagonist losartan enhanced nitric oxide synthase mRNA levels, protein expression, and nitric oxide production. Pretreatment of the endothelial cells with either PD-123319 or an anti-angiotensin II antibody prevented this losartan enhancement of nitric oxide production. Angiotensin II-dependent enhanced hypoxic contractions in pulmonary arteries were blocked by the type 1 receptor antagonist candesartan; however, PD-123319 enhanced hypoxic contractions in angiotensin II-treated endothelium-intact vessels. These data demonstrate that angiotensin II stimulates an increase in nitric oxide synthase mRNA, protein expression, and nitric oxide production via the type 2 receptor, whereas signaling via the type 1 receptor negatively regulates nitric oxide production in the pulmonary endothelium. This endothelial, type 2 receptor-dependent increase in nitric oxide may serve to counterbalance the angiotensin II-dependent vasoconstriction in smooth muscle cells, ultimately regulating pulmonary vascular tone.

Hypoxic pulmonary vasoconstriction in cardiothoracic surgery: basic mechanisms to potential therapies

Hypoxic pulmonary vasoconstriction is postulated to be an adaptive mechanism to match lung perfusion with ventilation; however, the consequences of the maladaptive effects of pulmonary vasoconstriction represent formidable therapeutic challenges. Understanding the basic mechanisms of hypoxic pulmonary vasoconstriction will enhance the assimilation of translational research into clinical practice. The purposes of this review are to (1) define basic mechanisms of pulmonary vasoconstriction and vasorelaxation; (2) delineate the biphasic contractile response to hypoxia; (3) critically examine data that support the mediator hypothesis versus the ion channel hypothesis; and (4) explore potential mechanistic-based therapies for hypoxic pulmonary vasoconstriction.

The contractile response of the mesenteric resistance arteries to prostaglandin F2alpha; effects of simultaneous hyperlipemia-diabetes

The effect of hyperlipemia associated with diabetes on the contractility of resistance arteries to prostaglandin F2alpha (PGF2alpha) was investigated employing 4 weeks simultaneously hyperlipemic-diabetic (HD), hyperlipemic (H), diabetic (D) and normal hamsters (controls, C). The isometric force produced by explanted arteries in the presence of 10(-8) to 10(-5) M PGF2alpha was recorded by the myograph technique. The results showed that compared with controls, the contractile response to 10(-5) M PGF2alpha was approx. 2 fold increased in HD group, and approx. 1.75 and 1.62-fold enhanced in H and D groups, respectively. Activation of protein kinase C with 10(-6) M phorbol 12-myristate 13-acetate increased the contractility to PGF2alpha in all groups and particularly in HD hamsters (approx. 10.16-fold). Inhibition of cyclooxygenase by indomethacin increased approx. 1.81-fold the arterial contractility to PGF2alpha in C group, whereas in H, D and HD hamsters had no effect. Blockage of Ca(2+)-activated K(+)-channels with 10(-3) M tetraethylammonium augmented the contraction to PGF2alpha approx. 6.43-fold in C group, and at significantly lower levels in H, D and HD groups, i.e. approx. 3.84, 3.72 and 3.33-fold, respectively. The results validate two conclusions: (i) simultaneous insult of hyperlipemia-hyperglycemia is associated with the highest contractility of the resistance arteries to PGF2alpha; the highest circulating glucose and cholesterol levels, and the enhancement in the protein kinase C pathway underlay the augmented contractility; (ii) no matter the pathology induced (hyperlipemia, diabetes or both simultaneously) a common dysfunctional response to PGF2alpha was installed; this consists in a reduced effect of cyclooxygenase inhibition, and a altered activity of Ca(2+) dependent K(+) channels.

Augmented Pulmonary Vascular and Venous Constrictions to NG-Nitro-L-Arginine Methyl Ester in Rats with Monocrotaline-Induced Pulmonary Hypertension

The hemodynamic effects of N(G)-nitro-L-arginine methyl ester (L-NAME, inhibitor of nitric oxide (NO) synthase) were examined in thiobutabarbital-anesthetized control-rats and rats with monocrotaline-induced pulmonary hypertension. L-NAME (1-16 mg/kg i.v.) increased mean arterial pressure, systemic vascular resistance, venous resistance and pulmonary vascular resistance, and decreased cardiac output in both the control and pulmonary hypertensive rats. Relative to the controls, L-NAME (16 mg/kg) caused a smaller increase (approximately 50% of control) in mean arterial pressure in the pulmonary hypertensive rats, but greater increases in venous (approximately 200%) as well as pulmonary vascular (approximately 400%) resistances and a greater decrease in cardiac output (approximately 140%). The results show that NO is an important dilator within the arterial, venous and pulmonary circulation; its pulmonary and venous dilator roles are augmented in pulmonary hypertension.

Endothelium-derived mediators and hypoxic pulmonary vasoconstriction

The vascular endothelium synthesises, metabolises or converts a multitude of vasoactive mediators, and plays a vital role in the regulation of pulmonary vascular resistance. Its role in hypoxic pulmonary vasoconstriction (HPV) is however controversial. Although HPV has been demonstrated in both pulmonary arteries where the endothelium has been removed and isolated pulmonary artery smooth muscle cells, many reports have shown either partial or complete dependence on an intact endothelium for sustained HPV (> approximately 20 min). However, despite many years of study no known endothelium-derived mediator has yet been unequivocally shown to be essential for HPV, although several may either facilitate the response or act as physiological brakes to limit the extent of HPV. In this article we review the evidence for and against the role of specific endothelium-derived mediators in HPV. We make the case for a facilitatory or permissive function of the endothelium, that in conjunction with a rise in smooth muscle intracellular Ca(2+) initiated by a mechanism intrinsic to smooth muscle, allows the development of sustained HPV. In particular, we propose that in response to hypoxia the pulmonary vascular endothelium releases an as yet unidentified agent that causes Ca(2+) sensitisation in the smooth muscle.

Pulmonary vasodilatory action of testosterone: evidence of a calcium antagonistic action

Recent evidence supports a beneficial effect of testosterone on the cardiovascular system. Testosterone acts as a coronary vasodilator and reduces myocardial ischemia in men with coronary heart disease. The aim of the current study was to determine whether testosterone has a similar vasodilatory action in the pulmonary circulation and to characterize the underlying mechanism of action. The vasodilatory action of testosterone was studied in pulmonary arteries (n = 132, mean internal diameter = 344 +/- 8 microm) isolated from male rats (n = 48, mass = 396 +/- 7 g) mounted in a small vessel wire myograph and loaded to a tension equivalent to 17.5 mm Hg. Micromolar concentrations of testosterone induced dilatation in pulmonary arteries preconstricted with prostaglandin F2alpha (100 microM) within seconds of application. Dilatation to testosterone was similar in vessels treated with N-gamma-nitro-l-arginine methyl ester (l-NAME) (10 microM) or vehicle (5 microl distilled water), -38.2 +/- 2.9%, and -38.1 +/- 3.4%, respectively, and in vessels treated with indomethacin (10 microM), flutamide (10 microM), or vehicle (5 microl ethanol), -35.5 +/- 2.8%, -43.2 +/- 3.6%, and -35.7 +/- 4.6%, respectively (all p > 0.05). Maximal dilatation to testosterone occurred following preconstriction with agents that activated voltage-gated calcium channels such as prostaglandin F2alpha (-34.6 +/- 5.0%), BAY K8644 (-32.9 +/- 8.7), or potassium chloride (-26.7 +/- 1.5%), compared with calcium-independent protein kinase C activation by phorbol dibutyrate (-14.7 +/- 1.6%) or capacitative calcium entry via thapsigargin (-5.1 +/- 0.9%). This study demonstrates that testosterone induces pulmonary dilatation via a mechanism that is independent of the classic androgen receptor and also of the release of nitric oxide or dilator prostaglandins. The data support a calcium antagonistic action for testosterone in the pulmonary circulation, primarily against voltage-gated calcium channels.

Hypoxic vasoconstriction of rat main pulmonary artery: role of endogenous nitric oxide, potassium channels, and phosphodiesterase inhibition

This study investigated the influence of NO, potassium (K+) channel blockade, and the phosphodiesterase inhibitors (PDEIs) theophylline (non-selective PDEI), siguazodan (PDE3I), rolipram (PDE4I), and zaprinast (PDE5I) on rat isolated main pulmonary artery hypoxic (95% N2 and 5% CO2) vasoconstriction. Hypoxic vasoconstriction increased by 27% (p < 0.01) in the presence of the NO synthase inhibitor L-NAME (10(-4) M), and by 15% (p < 0.05) in the presence of the K(ATP) channel blocker glibenclamide (10(-6) M), without potentiation by the combination of these two drugs. Hypoxic vasoconstriction decreased by 28% (p < 0.01) in presence of the Kv,-voltage-dependent channel blocker 4-aminopyridine (10(-3) M), whereas the other K+ channel blockers, charybdotoxin (BKCa, large-conductance Ca2+-sensitive K+ channels) and apamin (SKCa, small-conductance Ca2+-sensitive K+ channels) had no effect. The nonselective PDEI theophylline induced a concentration-dependent relaxation (pD2 = 4.05, Emax = 90% [expressed as a percentage of maximal relaxation induced by papaverine 10(-4) M]). Among the selective PDEIs, siguazodan was significantly (p < 0.01) more efficient than rolipram and zaprinast (Emax values were 84%, 67%, and 58%, respectively) and significantly (p < 0.05) more potent than zaprinast (pD2 values were 6.48, 6.34, and 6.16 for siguazodan, rolipram, and zaprinast). Glibenclamide and L-NAME significantly (p < 0.05) shifted the concentration-response curve (CRC) for zaprinast to the right, and L-NAME shifted the CRC significantly to the right for siguazodan. In the presence of L-NAME, glibenclamide had no effect on the CRC of zaprinast. We conclude that (a) NO exerts a permanent inhibitory effect against hypoxic vasoconstriction that might be mediated in part by an activation of K(ATP) channels; (b) a 4-aminopyridine-sensitive K+ channel is involved in vasoconstriction under hypoxic conditions; (c) PDEs 3 and 5 are the predominant PDE isoforms in rat pulmonary artery relaxation; and (d) NO and K(ATP), but neither BK(Ca), SK(Ca), nor Kv channels, are involved in the relaxant effect of PDEIs.