Euhydric hypercapnia increases vasoreactivity of rat pulmonary arteries via HCO3- transport and depolarisation

Pathogenic Mechanisms of Pulmonary Arterial Hypertension: Homeostasis Imbalance of Endothelium-Derived Relaxing and Contracting Factors

Pulmonary arterial hypertension (PAH) is a progressive and fatal disease. Sustained pulmonary vasoconstriction and concentric pulmonary vascular remodeling contribute to the elevated pulmonary vascular resistance and pulmonary artery pressure in PAH. Endothelial cells regulate vascular tension by producing endothelium-derived relaxing factors (EDRFs) and endothelium-derived contracting factors (EDCFs). Homeostasis of EDRF and EDCF production has been identified as a marker of the endothelium integrity. Impaired synthesis or release of EDRFs induces persistent vascular contraction and pulmonary artery remodeling, which subsequently leads to the development and progression of PAH. In this review, the authors summarize how EDRFs and EDCFs affect pulmonary vascular homeostasis, with special attention to the recently published novel mechanisms related to endothelial dysfunction in PAH and drugs associated with EDRFs and EDCFs.

Hemodynamic and Metabolic Tolerance of Acetate-Free Biofiltration in Mechanically Ventilated Critically Ill Patients: A Real-Life Study

Intradialytic hypotension can lead to superimposed organ hypoperfusion and ultimately worsens long-term kidney outcomes in critically ill patients requiring kidney replacement therapy. Acetate-free biofiltration (AFB), an alternative technique to bicarbonate-based hemodialysis (B-IHD) that does not require dialysate acidification, may improve hemodynamic and metabolic tolerance of dialysis. In this study, we included 49 mechanically ventilated patients requiring 4 h dialysis (AFB sessions n = 66; B-IHD sessions n = 62). Whereas more AFB sessions were performed in patients at risk of hemodynamic intolerance, episodes of intradialytic hypotension were significantly less frequent during AFB compared to B-IHD, whatever the classification used (decrease in mean blood pressure ≥ 10 mmHg; systolic blood pressure decrease >20 mmHg or absolute value below 95 mmHg) and after adjustment on the use of vasoactive agent. Diastolic blood pressure readily increased throughout the dialysis session. The use of a bicarbonate zero dialysate allowed the removal of 113 ± 25 mL/min of CO₂ by the hemofilter. After bicarbonate reinjection, the global CO₂ load induced by AFB was +25 ± 6 compared to +80 ± 12 mL/min with B-IHD (p = 0.0002). Thus, notwithstanding the non-controlled design of this study, hemodynamic tolerance of AFB appears superior to B-IHD in mechanically ventilated patients. Its use as a platform for CO₂ removal also warrants further research.

The Interaction between Trolox and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic Acid on Hypoxic Pulmonary Vasoconstriction in the Isolated Rabbit Lung

BACKGROUND

The mechanism of hypoxic pulmonary vasoconstriction (HPV) is still debatable. It has been proposed that reactive oxygen species (ROS) might be involved in HPV. However, there is no special transporter for superoxide anion in the cell membrane and it may release from the cells via anion exchanger. Therefore, the aim of this study was to investigate the interaction of ROS and anion exchanger in acute HPV.

METHODS

The present study was performed in the isolated rabbit lung. After preparation, the lungs were divided into four hypoxic groups of control, Trolox (antioxidant)-treated, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS, anion exchanger inhibitor)-treated, and Trolox+DIDS-treated. Pulmonary artery pressure, left atrial pressure, and lung weight were continuously registered and PVR was then calculated. PO₂, PCO₂, HCO₃^-, pH, and NO metabolites of the perfusate were measured during steady-state and at the end of experiments (30 minutes). All data were compared with ANOVA and t-test and significance was considered when P<0.05.

RESULTS

Ventilation of the lungs with hypoxic gas induced HPV in the control group. DIDS did not have a further effect on HPV compared with the control group. The combination of Trolox and DIDS decreased HPV rather than Trolox per se at 5 minutes. Furthermore, HPV was abolished in both the Trolox and Trolox+DIDS groups at 30 minutes. Concentrations of NO metabolites in the Trolox+DIDS group were more than other groups.

CONCLUSION

The present study indicates a possible interaction between ROS and anion exchanger in acute HPV. It also suggests the modulatory effect of NO at above condition.

Precapillary Pulmonary Hypertension and Sleep-Disordered Breathing: Is There a Link?

Among patients with sleep apnea the reported prevalence of precapillary pulmonary hypertension (PH) has varied largely, depending on patient selection, disease definition, and associated conditions, in particular chronic pulmonary disease. However, in the absence of comorbidities, PH seems to be rare in patients with sleep apnea. Conversely, sleep-related breathing disorders have been commonly found in patients with PH and they have been associated with an impaired quality of life. Since sleep-related breathing disorders may affect the pulmonary circulation and vice versa, patients with sleep-related breathing disorders should be evaluated for risk factors, symptoms and clinical signs of PH and right ventricular heart failure and patients with PH should be evaluated for sleep apnea. Therapeutic options for patients with sleep apnea and PH may include supplemental oxygen, drugs and positive pressure ventilation. Both nocturnal oxygen administration and acetazolamide have been shown to improve sleep apnea in patients with PH. In addition, oxygen therapy also improved exercise performance. Further studies are needed to corroborate the efficacy of these and other treatments.

Extracellular HCO3- is sensed by mouse cerebral arteries: Regulation of tone by receptor protein tyrosine phosphatase γ

We investigate sensing and signaling mechanisms for H(+), [Formula: see text] and CO2 in basilar arteries using out-of-equilibrium solutions. Selectively varying pHo, [[Formula: see text]]o, or pCO2, we find: (a) lowering pHo attenuates vasoconstriction and vascular smooth muscle cell (VSMC) Ca(2+)-responses whereas raising pHo augments vasoconstriction independently of VSMC [Ca(2+)]i, (b) lowering [[Formula: see text]]o increases arterial agonist-sensitivity of tone development without affecting VSMC [Ca(2+)]i but c) no evidence that CO2 has direct net vasomotor effects. Receptor protein tyrosine phosphatase (RPTP)γ is transcribed in endothelial cells, and direct vasomotor effects of [Formula: see text] are absent in arteries from RPTPγ-knockout mice. At pHo 7.4, selective changes in [[Formula: see text]]o or pCO2 have little effect on pHi At pHo 7.1, decreased [[Formula: see text]]o or increased pCO2 causes intracellular acidification, which attenuates vasoconstriction. Under equilibrated conditions, anti-contractile effects of CO2/[Formula: see text] are endothelium-dependent and absent in arteries from RPTPγ-knockout mice. With CO2/[Formula: see text] present, contractile responses to agonist-stimulation are potentiated in arteries from RPTPγ-knockout compared to wild-type mice, and this difference is larger for respiratory than metabolic acidosis. In conclusion, decreased pHo and pHi inhibit vasoconstriction, whereas decreased [[Formula: see text]]o promotes vasoconstriction through RPTPγ-dependent changes in VSMC Ca(2+)-sensitivity. [Formula: see text] serves dual roles, providing substrate for pHi-regulating membrane transporters and modulating arterial responses to acid-base disturbances.

Effects of hypercapnia and NO synthase inhibition in sustained hypoxic pulmonary vasoconstriction

Background

Acute respiratory disorders may lead to sustained alveolar hypoxia with hypercapnia resulting in impaired pulmonary gas exchange. Hypoxic pulmonary vasoconstriction (HPV) optimizes gas exchange during local acute (0-30 min), as well as sustained (> 30 min) hypoxia by matching blood perfusion to alveolar ventilation. Hypercapnia with acidosis improves pulmonary gas exchange in repetitive conditions of acute hypoxia by potentiating HPV and preventing pulmonary endothelial dysfunction. This study investigated, if the beneficial effects of hypercapnia with acidosis are preserved during sustained hypoxia as it occurs, e.g in permissive hypercapnic ventilation in intensive care units. Furthermore, the effects of NO synthase inhibitors under such conditions were examined.

Method

We employed isolated perfused and ventilated rabbit lungs to determine the influence of hypercapnia with or without acidosis (pH corrected with sodium bicarbonate), and inhibitors of endothelial as well as inducible NO synthase on acute or sustained HPV (180 min) and endothelial permeability.

Results

In hypercapnic acidosis, HPV was intensified in sustained hypoxia, in contrast to hypercapnia without acidosis when HPV was amplified during both phases. L-N^G-Nitroarginine (L-NNA), a non-selective NO synthase inhibitor, enhanced acute as well as sustained HPV under all conditions, however, the amplification of sustained HPV induced by hypercapnia with or without acidosis compared to normocapnia disappeared. In contrast 1400 W, a selective inhibitor of inducible NO synthase (iNOS), decreased HPV in normocapnia and hypercapnia without acidosis at late time points of sustained HPV and selectively reversed the amplification of sustained HPV during hypercapnia without acidosis. Hypoxic hypercapnia without acidosis increased capillary filtration coefficient (Kfc). This increase disappeared after administration of 1400 W.

Conclusion

Hypercapnia with and without acidosis increased HPV during conditions of sustained hypoxia. The increase of sustained HPV and endothelial permeability in hypoxic hypercapnia without acidosis was iNOS dependent.

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.

Effects of hypercapnia with and without acidosis on hypoxic pulmonary vasoconstriction

Acute respiratory disorders and permissive hypercapnic strategy may lead to alveolar hypoxia and hypercapnic acidosis. However, the effects of hypercapnia with or without acidosis on hypoxic pulmonary vasoconstriction (HPV) and oxygen diffusion capacity of the lung are controversial. We investigated the effects of hypercapnic acidosis and hypercapnia with normal pH (pH corrected with sodium bicarbonate) on HPV, capillary permeability, gas exchange, and ventilation-perfusion matching in the isolated ventilated-perfused rabbit lung. No alteration in vascular tone was noted during normoxic hypercapnia with or without acidosis compared with normoxic normocapnia. Hypercapnia with normal pH resulted in a transient increase in HPV during the course of consecutive ventilation maneuvers, whereas hypercapnic acidosis increased HPV over time. Hypercapnic acidosis decreased exhaled NO during hypoxia more than hypercapnia with normal pH and normocapnia, whereas intravascular NO release was unchanged. However, inhibition of NO synthesis by nitro-L-arginine (L-NNA) resulted in a loss of the increased HPV caused by hypercapnic acidosis but not that caused by hypercapnia with normal pH. Furthermore, capillary permeability increased during hypoxic hypercapnia with normal pH but not hypoxic hypercapnic acidosis. This effect was NO-dependent because it disappeared during L-NNA administration. Ventilation-perfusion matching and arterial PO2 were improved according to the strength of HPV in hypercapnia compared with normocapnia during Tween nebulization-induced lung injury. In conclusion, the increased HPV during hypercapnic acidosis is beneficial to lung gas exchange by improving ventilation-perfusion matching and preserving the capillary barrier function. These effects seem to be linked to NO-mediated pathways.

Chronic hypercapnia downregulates arginase expression and activity and increases pulmonary arterial smooth muscle relaxation in the newborn rat

In rats, chronic hypercapnia has been reported to ameliorate hypoxia-induced pulmonary hypertension in newborn and adult and to enhance endothelium-dependent vasorelaxation in adult pulmonary arteries. The underlying mechanisms accounting for chronic hypercapnia-induced improvements in pulmonary vascular function are not understood. Hypothesizing that downregulation of arginase activity may be contributory, we examined relaxation responses and arginase activity and expression in pulmonary arteries from newborn rats that were exposed (from birth to 14 days) to either mild-to-moderate (5.5% inhaled CO(2)) or severe (10% CO(2)) hypercapnia with either normoxia or hypoxia (13% O(2)). Pulmonary arteries from pups exposed to normoxia and chronic hypercapnia (5.5 or 10% CO(2)) contracted less in response to a thromboxane A(2) analog, U-46619, and showed enhanced endothelium-dependent (but not independent) relaxation compared with arteries from normocapnic pups (P < 0.01). Parallel with these changes, arginase activity and arginase I (but not II) expression in lung and pulmonary arterial tissue were significantly decreased (P < 0.05). Exposure to 10% CO(2) significantly increased (P < 0.01) pulmonary arterial tissue nitric oxide (nitrite) generation. In pups chronically exposed to hypoxia (13% O(2)), severe hypercapnia (10% CO(2)) significantly (P < 0.05) enhanced endothelium-dependent relaxation, increased nitric oxide generation, and decreased arginase activity but not expression. We conclude that chronic hypercapnia-induced downregulation of lung arginase expression and/or activity may reduce pulmonary vascular resistance by enhancing nitric oxide generation and thus endothelium-dependent relaxation. This mechanism may explain some of the beneficial effects of chronic hypercapnia on experimental pulmonary hypertension.