AMPK facilitates the hypoxic ventilatory response through non-adrenergic mechanisms at the brainstem.

We recently demonstrated that the hypoxic ventilatory response (HVR) is facilitated by the AMP-activated protein kinase (AMPK) in catecholaminergic neural networks that likely lie downstream of the carotid bodies within the caudal brainstem. Here, we further subcategorise the neurons involved, by cross-comparison of mice in which the genes encoding the AMPK-α1 (Prkaa1) and AMPK-α2 (Prkaa2) catalytic subunits were deleted in catecholaminergic (TH-Cre) or adrenergic (PNMT-Cre) neurons. As expected, the HVR was markedly attenuated in mice with AMPK-α1/α2 deletion in catecholaminergic neurons, but surprisingly was modestly augmented in mice with AMPK-α1/α2 deletion in adrenergic neurons when compared against a variety of controls (TH-Cre, PNMT-Cre, AMPK-α1/α2 floxed). Moreover, AMPK-α1/α2 deletion in catecholaminergic neurons precipitated marked hypoventilation and apnoea during poikilocapnic hypoxia, relative to controls, while mice with AMPK-α1/α2 deletion in adrenergic neurons entered relative hyperventilation with reduced apnoea frequency and duration. We conclude, therefore, that AMPK-dependent modulation of non-adrenergic networks may facilitate increases in ventilatory drive that shape the classical HVR, whereas AMPK-dependent modulation of adrenergic networks may provide some form of negative feedback or inhibitory input to moderate HVR, which could, for example, protect against hyperventilation-induced hypocapnia and respiratory alkalosis.

Regulation of gene expression by carbon dioxide.

Carbon dioxide (CO(2)) is a physiological gas found at low levels in the atmosphere and produced in cells during the process of aerobic respiration. Consequently, the levels of CO(2) within tissues are usually significantly higher than those found externally. Shifts in tissue levels of CO(2) (leading to either hypercapnia or hypocapnia) are associated with a number of pathophysiological conditions in humans and can occur naturally in niche habitats such as those of burrowing animals. Clinical studies have indicated that such altered CO(2) levels can impact upon disease progression. Recent advances in our understanding of the biology of CO(2) has shown that like other physiological gases such as molecular oxygen (O(2)) and nitric oxide (NO), CO(2) levels can be sensed by cells resulting in the initiation of physiological and pathophysiological responses. Acute CO(2) sensing in neurons and peripheral and central chemoreceptors is important in rapidly activated responses including olfactory signalling, taste sensation and cardiorespiratory control. Furthermore, a role for CO(2) in the regulation of gene transcription has recently been identified with exposure of cells and model organisms to high CO(2) leading to suppression of genes involved in the regulation of innate immunity and inflammation. This latter, transcriptional regulatory role for CO(2), has been largely attributed to altered activity of the NF-B family of transcription factors. Here, we review our evolving understanding of how CO(2) impacts upon gene transcription.

Hypocapnic hypothesis of Leigh disease.

Leigh syndrome (LS) is a neurogenetic disorder of children caused by mutations in at least 75 genes which impair mitochondrial bioenergetics. The changes have typical localization in basal ganglia and brainstem, and typical histological picture of spongiform appearance, vascular proliferation and gliosis. ATP deprivation, free radicals and lactate accumulation are suspected to be the causes. Hypocapnic hypothesis proposed in the paper questions the energy deprivation as the mechanism of LS. We assume that the primary harmful factor is hypocapnia (decrease in pCO2) and respiratory alkalosis (increase in pH) due to hyperventilation, permanent or in response to stress. Inside mitochondria, the pH signal of high pH/low bicarbonate ion (HCO-3) is transmitted by soluble adenyl cyclase (sAC) through cAMP dependent manner. The process can initiate brain lesions (necrosis, apoptosis, hypervascularity) in OXPHOS deficient cells residing at the LS area of the brain. The major message of the article is that it is not the ATP depletion but intracellular alkalization (and/or hyperoxia?) which seem to be the cause of LS. The paper includes suggestions concerning the methodology for further research on the LS mechanism and for therapeutic strategy.

Effects of severe hypocapnia on expression of bax and bcl-2 proteins, DNA fragmentation, and membrane peroxidation products in cerebral cortical mitochondria of newborn piglets.

BACKGROUND: Hypocapnia occurs in the newborn infant inadvertently or as a therapeutic modality and may result in neuronal and mitochondrial alterations in the newborn brain. Since mitochondria regulate apoptosis, these alterations may initiate a cascade of reactions that lead to apoptotic cell death. OBJECTIVES: This study tests the hypothesis that hypocapnia results in increased expression of the pro-apoptotic protein Bax, fragmentation of DNA and membrane lipid peroxidation in cerebral cortical mitochondria (mt) of newborn piglets. METHODS: Studies were performed in three groups of anesthetized normoxic newborn piglets: hypocapnic (H, n = 5), ventilated at a PaCO(2) of 11-15 mm Hg; normocapnic (N, n = 5), ventilated at a PaCO(2) of 40 mm Hg; and corrected normocapnic (CN, n = 4), ventilated as H with CO(2) added to maintain normocapnia. Tissue ATP and phosphocreatine levels were determined. Mitochondrial membrane proteins were separated, transblotted and probed with antibodies to Bax and Bcl-2. Bands were detected by enhanced chemiluminescence and analyzed by imaging densitometry. mtDNA was isolated. Cell and mitochondrial membrane lipid peroxidation products were measured spectrofluorometrically. RESULTS: ATP and PCr concentrations were similar in the 3 groups. The ratio of Bax/Bcl-2 increased significantly in H compared to N and CN. mtDNA fragmentation was also significantly greater in H compared to N or CN. Membrane lipid peroxidation was higher in H than in N or CN; and in CN compared to N. CONCLUSIONS: The data demonstrate that severe hypocapnia results in increased Bax expression, DNA fragmentation, and membrane lipid peroxidation in mitochondria of cerebral cortical neurons of newborn piglets, and may result in apoptotic cell death.

Compulsory hyperventilation and hypocapnia of patients with Leigh syndrome associated with SURF1 gene mutations as a cause of low serum bicarbonates.

Experimental data show that elevation of intracellular pH leads to severe lesions of brain cells. Acidification of intracellular fluid by accumulation of lactate may compensate the effect of respiratory alkalosis. Increased serum pH, and low PCO2, associated with hyperlactataemia (sometimes incorrectly called 'acidosis') have been reported in children with Leigh syndrome (LS). The aim of the study was to determine whether respiratory alkalosis is characteristic of patients with LS due to SURF1 mutations. All venous blood gas data (88 samples) of 18 spontaneously breathing LS patients with recently established SURF1 mutations, hospitalized during 1986-2000, were retrospectively reviewed. The data of an affected boy who survived on a respirator for more than 3 months (79 daily samples) were analysed separately. In spontaneously breathing patients, the data indicated that the patients had compensated or partially compensated respiratory alkalosis (pH 7.388+/-0.060, Pco2 29.2+/-5.7 mmHg, HCO3- 17.4+/-3.0 mmol/L, BE -6.7+/-3.2 mmol/L). Bicarbonate excretion was detected in urine of two examined LS cases in spite of decreased serum HCO3-. In the affected child maintained on a respirator, simple manipulation of the inspired CO2 tension to establish a normal pressure of 35-45 mmHg automatically caused an increase of serum HCO3- concentration to a normal value of 26.3+/-2.9 mmol/L (and BE to +2.2+/-3.1 mmol/L), in spite of cytochrome oxidase (COX) deficiency due to a confirmed SURF1 mutation. We suggest that respiratory alkalosis (hypocapnia) of Leigh syndrome patients with SURF1 mutations results from compulsory hyperventilation and speculate that hypocapnia may contribute to Leigh-like brain damage in the SURF1-deficient patients as well as in other patients presenting with Leigh-like syndrome. The supposition that accumulation of lactate may protect the brain of LS patients from alkalosis-related damage requires further study. Avoidance of any factors stimulating hyperventilation of LS patients and caution when attempting to correct low plasma bicarbonate are suggested.