Elevated CO2 modulates airway contractility

Carbon dioxide and MAPK signalling: towards therapy for inflammation

Inflammation, although necessary to fight infections, becomes a threat when it exceeds the capability of the immune system to control it. In addition, inflammation is a cause and/or symptom of many different disorders, including metabolic, neurodegenerative, autoimmune and cardiovascular diseases. Comorbidities and advanced age are typical predictors of more severe cases of seasonal viral infection, with COVID-19 a clear example. The primary importance of mitogen-activated protein kinases (MAPKs) in the course of COVID-19 is evident in the mechanisms by which cells are infected with SARS-CoV-2; the cytokine storm that profoundly worsens a patient's condition; the pathogenesis of diseases, such as diabetes, obesity, and hypertension, that contribute to a worsened prognosis; and post-COVID-19 complications, such as brain fog and thrombosis. An increasing number of reports have revealed that MAPKs are regulated by carbon dioxide (CO2); hence, we reviewed the literature to identify associations between CO2 and MAPKs and possible therapeutic benefits resulting from the elevation of CO2 levels. CO2 regulates key processes leading to and resulting from inflammation, and the therapeutic effects of CO2 (or bicarbonate, HCO3-) have been documented in all of the abovementioned comorbidities and complications of COVID-19 in which MAPKs play roles. The overlapping MAPK and CO2 signalling pathways in the contexts of allergy, apoptosis and cell survival, pulmonary oedema (alveolar fluid resorption), and mechanical ventilation-induced responses in lungs and related to mitochondria are also discussed. Video Abstract.

Molecular Mechanisms of Obesity-Induced Development of Insulin Resistance and Promotion of Amyloid-β Accumulation: Dietary Therapy Using Weak Organic Acids via Improvement of Lowered Interstitial Fluid pH

Insulin resistance is one of the etiologies of type 2 diabetes mellitus (T2DM) and has been suggested to contribute to the development of Alzheimer's disease by promoting amyloid-β accumulation. Various causes of insulin resistance have been suggested; however, mechanisms of insulin resistance development remain to be elucidated in many respects. Elucidating the mechanisms underlying the development of insulin resistance is one of the key factors in developing methods to prevent the onset of T2DM and Alzheimer's disease. It has been suggested that the body pH environment plays an important role in the control of cellular functions by regulating the action of hormones including insulin and the activity of enzymes and neurons, thereby maintaining homeostatic conditions of the body. This review introduces: (1) Mitochondrial dysfunction through oxidative stress caused by obesity-induced inflammation. (2) Decreased pH of interstitial fluid due to mitochondrial dysfunction. (3) Development of insulin resistance due to diminution of insulin affinity to its receptor caused by the lowered interstitial fluid pH. (4) Accelerated accumulation of amyloid-β due to elevated activities of β- and γ-secretases caused by the lowered interstitial fluid pH. (5) Diet therapies for improving insulin resistance with weak organic acids that act as bases in the body to raise the pH of lowered interstitial fluid and food factors that promote absorption of weak organic acids in the gut.

Hypercapnia in COPD: Causes, Consequences, and Therapy

Chronic obstructive pulmonary disease (COPD) is a progressive respiratory disorder that may lead to gas exchange abnormalities, including hypercapnia. Chronic hypercapnia is an independent risk factor of mortality in COPD, leading to epithelial dysfunction and impaired lung immunity. Moreover, chronic hypercapnia affects the cardiovascular physiology, increases the risk of cardiovascular morbidity and mortality, and promotes muscle wasting and musculoskeletal abnormalities. Noninvasive ventilation is a widely used technique to remove carbon dioxide, and several studies have investigated its role in COPD. In the present review, we aim to summarize the causes and effects of chronic hypercapnia in COPD. Furthermore, we discuss the use of domiciliary noninvasive ventilation as a treatment option for hypercapnia while highlighting the controversies within the evidence. Finally, we provide some insightful clinical recommendations and draw attention to possible future research areas.

Role of hyperpnea in the relaxant effect of inspired CO2 on methacholine-induced bronchoconstriction

Inhaling carbon dioxide (CO₂) in humans is known to cause inconsistent effects on airway function. These could be due to direct effects of CO₂ on airway smooth muscle or to changes in minute ventilation (e). To address this issue, we examined the responses of the respiratory system to inhaled methacholine in healthy and mild asthmatics while breathing air or gas mixtures containing 2% or 4% CO₂. Respiratory mechanics were measured by a forced oscillation technique at 5 Hz during tidal breathing. At baseline, respiratory resistance (R₅) was significantly higher in asthmatics (2.53±0.38 cm H₂O•L^-1•s) than healthy subjects (2.11±0.42 cm H₂O•L^-1•s) (p=0.008) with room air. Similar values were observed with CO₂ 2% or 4% in the two groups. e, tidal volume (V_T), and breathing frequency (BF) significantly increased with CO₂-containing mixtures (p<0.001) with insignificant differences between groups. After methacholine, the increase in R₅ and the decrease in respiratory reactance (X₅) were significantly attenuated up to about 50% with CO₂-containing mixtures instead of room air in both asthmatic (p<0.001) and controls (p<0.001). Mediation analysis showed that the attenuation of methacholine-induced changes in respiratory mechanics by CO₂ was due to the increase in e (p=0.006 for R₅ and p=0.014 for X₅) independently of the increase in V_T or BF, rather than a direct effect of CO₂. These findings suggest that the increased stretching of airway smooth muscle by the CO₂-induced increase in e is a mechanism through which hypercapnia can attenuate bronchoconstrictor responses in healthy and mild asthmatic subjects.

Air Pollution and the Airways: Lessons from a Century of Human Urbanization

Since the industrial revolution, air pollution has become a major problem causing several health problems involving the airways as well as the cardiovascular, reproductive, or neurological system. According to the WHO, about 3.6 million deaths every year are related to inhalation of polluted air, specifically due to pulmonary diseases. Polluted air first encounters the airways, which are a major human defense mechanism to reduce the risk of this aggressor. Air pollution consists of a mixture of potentially harmful compounds such as particulate matter, ozone, carbon monoxide, volatile organic compounds, and heavy metals, each having its own effects on the human body. In the last decades, a lot of research investigating the underlying risks and effects of air pollution and/or its specific compounds on the airways, has been performed, involving both in vivo and in vitro experiments. The goal of this review is to give an overview of the recent data on the effects of air pollution on healthy and diseased airways or models of airway disease, such as asthma or chronic obstructive pulmonary disease. Therefore, we focused on studies involving pollution and airway symptoms and/or damage both in mice and humans.

Carbon dioxide transport across membranes

Carbon dioxide (CO2) movement across cellular membranes is passive and governed by Fick's law of diffusion. Until recently, we believed that gases cross biological membranes exclusively by dissolving in and then diffusing through membrane lipid. However, the observation that some membranes are CO2 impermeable led to the discovery of a gas molecule moving through a channel; namely, CO2 diffusion through aquaporin-1 (AQP1). Later work demonstrated CO2 diffusion through rhesus (Rh) proteins and NH3 diffusion through both AQPs and Rh proteins. The tetrameric AQPs exhibit differential selectivity for CO2 versus NH3 versus H2O, reflecting physico-chemical differences among the small molecules as well as among the hydrophilic monomeric pores and hydrophobic central pores of various AQPs. Preliminary work suggests that NH3 moves through the monomeric pores of AQP1, whereas CO2 moves through both monomeric and central pores. Initial work on AQP5 indicates that it is possible to create a metal-binding site on the central pore's extracellular face, thereby blocking CO2 movement. The trimeric Rh proteins have monomers with hydrophilic pores surrounding a hydrophobic central pore. Preliminary work on the bacterial Rh homologue AmtB suggests that gas can diffuse through the central pore and three sets of interfacial clefts between monomers. Finally, initial work indicates that CO2 diffuses through the electrogenic Na/HCO3 cotransporter NBCe1. At least in some cells, CO2-permeable proteins could provide important pathways for transmembrane CO2 movements. Such pathways could be amenable to cellular regulation and could become valuable drug targets.