Hypercapnia in the critcally ill
Professor John Laffey, National University of Ireland, Galway, Ireland
Arterial CO2 tensions (PaCO2) represents a balance between CO2 production and elimination via the lungs, and in health is maintained within a tight range (3.5 – 4.5 kPa). Traditional approaches to CO2 management in the critically ill emphasized the use of higher tidal and minute ventilation to avoid hypercapnia and its associated the risks. The demonstration that high lung stretch directly injures the lungs heralded the use of more protective ventilatory strategies that reduce lung stretch, and have been proven to improve survival in patients with ARDS. Consequently, hypercapnia – and its associated hypercapnic acidosis (HCA) - is prevalent in the critically ill, ‘permitted’ order to realize the benefits of lower lung stretch. Experimental and clinical investigations have generated key advances in our understanding of the effects of hypercapnia. Hypercapnia to be a potent biologic agent, with the potential to exert both beneficial and potentially harmful effects. Hypercapnia modulates the innate immune response, with inhibition of nuclear factor kappa-B, is a key transcriptional protein in injury, inflammation and repair, mediating diverse effects of hypercapnia. Advances in extracorporeal technologies have made possible the direct removal of CO2 while maintaining lung protective ventilation, a promising, though as yet unproven, approach. Consequently, it is important to understand the biology of hypercapnia, in order to best understand when hypercapnia should be encouraged, tolerated or avoided in patients with ARDS.
CO2 sensing in the brain
Professor Nicholas Dale, University of Warwick, UK
The detection and regulated excretion of CO2, via breathing, is fundamental for homeostatic control of blood pH and preservation of life. Breathing is highly sensitive to the partial pressure of CO2 (PCO2) in blood. This vital physiological function was previously thought to depend exclusively upon the indirect changes in pH that follow the accumulation of CO2. However, evidence suggests that CO2 can have direct effects on breathing in addition to those of pH.
Connexins are large-conductance hexameric plasma membrane channels. They can dock together to form a passageway between adjacent cells, a gap junction, to permit transfer of ions and small molecules. Connexin channels not docked to those in neighbouring cells form “hemichannels” and open to the extracellular space. Chemosensory cells at the surface of the medulla oblongata use hemichannels of connexin26 (Cx26) to detect CO2 and effect adaptive changes in breathing. Physiological levels CO2 cause hemichannels of Cx26 to open, permitting the release of the neurotransmitter ATP and excitation of the neural networks controlling breathing. CO2 most likely binds to Cx26 by carbamylating Lys125, which forms a salt bridge to Arg104 on a neighbouring subunit to open the hemichannel.
Understanding the interaction of CO2 with Cx26 has enabled the rational design of a dominant negative subunit, dnCx26, which coassembles with wild type Cx26 to remove its CO2 sensitivity. Transduction with dnCx26 of the chemosensory cells in the medulla oblongata greatly reduces the chemosensitivity of breathing, thus directly linking the functional motif of CO2-binding to the physiological function of Cx26.
CO2 diffusion inside leaves during photosynthesis
Dr Tory Clarke, Australian National University, Australia
The rate of photosynthesis is very sensitive to the level of CO2 inside the chloroplasts, where photosynthesis takes place and CO2 is fixed by rubisco. CO2 first diffuses from the atmosphere through stomatal pores into the substomatal space. The CO2 must then cross internal airspace, cell walls, plasmalemma, cytoplasm, chloroplast envelope and part of the chloroplast stroma before it is fixed in the first step of the Calvin-Benson cycle. The combined conductance to CO2 transfer from substomatal cavities to the site of fixation is termed mesophyll conductance. In order to estimate mesophyll conductance, the exchange of CO2 from atmosphere into the leaf, together with the isotopic composition of the CO2 is measured. By exploiting Rubisco’s natural preference for 12CO2 over 13CO2, we can calculate mesophyll conductance from carbon isotopic discrimination. Mesophyll conductance is an important photosynthetic parameter that influences the amount of CO2 available for fixation and is a target for improving crop productivity. Understanding variations in mesophyll conductance across leaves, and what drives these changes, is essential for modelling how the manipulation of photosynthetic pathways may alter plant productivity. Using Nicotiana tabacum var. Samsun, we have investigated how mesophyll conductance, and other photosynthetic and leaf anatomy parameters, vary across leaf ages and throughout the canopy, with the goal of better informing plant productivity models.
Molecular and cellular mechanisms for CO2 sensing: lessons from aquatic organisms
Dr Martin Tresguerres, Scripps Institution of Oceanography, UC San Diego, USA
My laboratory uses aquatic animals as models to study the evolution of acid-base sensing mechanisms at the molecular, cellular, and organismal levels. Because the internal fluids of aquatic organisms have lower CO2 and HCO3- levels and higher pH (“acid-base” parameters) compared to air-breathing vertebrates, their underlying acid-base sensing mechanisms must be tuned to different and specific set points. Kinetic assays on the evolutionary conserved acid-sensing enzyme soluble adenylyl cyclase (sAC) demonstrate a species-specific responsiveness to [HCO3-] that in each case matches physiologically relevant levels. For example, sAC’s HCO3- half-maximal stimulation is ~5 mM in sharks, ~10 mM in bony fishes and coral, which are lower than the ~20 mM reported in mammals. Additionally, aquatic animals routinely experience metabolic and environmental acid-base disturbances that can span >1 pH unit and >10-fold changes in [HCO3-]. Physiologically, this implies acid-base sensing plays essential and multiple homeostatic roles. Experimentally, this is advantageous because it allows imposing extreme (but physiologically relevant) acid-base challenges (i.e. 0-100 mM bicarbonate, pH 6.0-9.0) that maximize the magnitude of physiological responses and facilitate their detection. These approaches have led to the discovery of several novel physiological functions under sAC modulation in aquatic animals, including base secretion in shark gill epithelial cells, salt and fluid absorption across fish intestine, heart beat rate in hagfish, and pHi regulation in corals. In addition to their implications for organismal, environmental, and evolutionary physiology, these results provide clues about similar processes that might be under sAC control in humans and therefore might have biomedical relevance.