Different O2-sensing mechanisms by different K+ channels

Carotid body chemoreceptors: physiology, pathology, and implications for health and disease

The carotid body (CB) is the main peripheral chemoreceptor for arterial respiratory gases O2 and CO2 and pH, eliciting reflex ventilatory, cardiovascular, and humoral responses to maintain homeostasis. This review examines the fundamental biology underlying CB chemoreceptor function, its contribution to integrated physiological responses, and its role in maintaining health and potentiating disease. Emphasis is placed on 1) transduction mechanisms in chemoreceptor (type I) cells, highlighting the role played by the hypoxic inhibition of O2-dependent K+ channels and mitochondrial oxidative metabolism, and their modification by intracellular molecules and other ion channels; 2) synaptic mechanisms linking type I cells and petrosal nerve terminals, focusing on the role played by the main proposed transmitters and modulatory gases, and the participation of glial cells in regulation of the chemosensory process; 3) integrated reflex responses to CB activation, emphasizing that the responses differ dramatically depending on the nature of the physiological, pathological, or environmental challenges, and the interactions of the chemoreceptor reflex with other reflexes in optimizing oxygen delivery to the tissues; and 4) the contribution of enhanced CB chemosensory discharge to autonomic and cardiorespiratory pathophysiology in obstructive sleep apnea, congestive heart failure, resistant hypertension, and metabolic diseases and how modulation of enhanced CB reactivity in disease conditions may attenuate pathophysiology.

Modulation of voltage-dependent Shaker family potassium channels by an aldo-keto reductase

The beta subunit (Kvbeta) of the Shaker family voltage-dependent potassium channels (Kv1) is a cytosolic protein that forms a permanent complex with the channel. Sequence and structural conservation indicates that Kvbeta resembles an aldo-keto reductase (AKR), an enzyme that catalyzes a redox reaction using an NADPH cofactor. A putative AKR in complex with a Kv channel has led to the hypothesis that intracellular redox potential may dynamically influence the excitability of a cell through Kvbeta. Since the AKR function of Kvbeta has never been demonstrated, a direct functional coupling between the two has not been established. We report here the identification of Kvbeta substrates and the demonstration that Kvbeta is a functional AKR. We have also found that channel function is modulated when the Kvbeta-bound NADPH is oxidized. Further studies of the enzymatic properties of Kvbeta seem to favor the role of Kvbeta as a redox sensor. These results suggest that Kvbeta may couple the excitability of the cell to its metabolic state and present a new avenue of research that may lead to understanding of the physiological functions of Kvbeta.

Increased oxidative stress is associated with chronic intermittent hypoxia-mediated brain cortical neuronal cell apoptosis in a mouse model of sleep apnea

Chronic intermittent hypoxia (CIH), as occurs in obstructive sleep apnea (SA), is associated with substantial cortico-hippocampal damage leading to impairments of neurocognitive, respiratory and cardiovascular functions. Previous studies in a rat model have shown that CIH increases brain cortical neuronal cell death. However, the molecular events leading to CIH-mediated neuronal cell death remain largely undefined. The oscillation of O2 concentrations during CIH remarkably mimics the processes of ischemia/re-oxygenation and could therefore increase cellular production of reactive oxygen species (ROS). We extended the CIH paradigm to a mouse model of SA to identify the molecular mechanisms underlying cortical neuronal cell death. A significant increase of ROS production in mouse brain cortex and cortical neuronal cells was detected by fluorescent oxidation assays upon exposure of mice to CIH, followed by increased expression of oxidative stress response markers, c-Fos, c-Jun and NF-kappaB in mouse brain cortex, as revealed by immunohistochemical and LacZ reporter assays respectively. Long-term exposure of mice to CIH increased the levels of protein oxidation, lipid peroxidation and nucleic acid oxidation in mouse brain cortex. Furthermore, exposure of mice to CIH induced caspase-3 activation and increased some cortical neuronal cell apoptosis. On the other hand, transgenic mice overexpressing Cu,Zn-superoxide dismutase exposed to CIH conditions had a lower level of steady-state ROS production and reduced neuronal apoptosis in brain cortex compared with that of normal control mice. Taken together, these findings suggest that the increased ROS production and oxidative stress propagation contribute, at least partially, to CIH-mediated cortical neuronal apoptosis and neurocognitive dysfunction.

Large conductance Ca2+-activated K+ channels sense acute changes in oxygen tension in alveolar epithelial cells

The rise in alveolar oxygen tension (PO(2)) that occurs as the newborn infant takes its first breaths induces removal of liquid from the lung lumen due to ion transport across the alveolar epithelium and the activity of alveolar Na(+) channel (ENaC). In the present study, we have aimed to identify an ion conductance in alveolar epithelial A549 cells that responds to acute changes in PO(2). Variation in PO(2) did not affect single-channel ENaC activity. However, in these cells we have detected single-channel conductance having properties similar to those of large conductance Ca(2+)-activated K(+) (BK(Ca)) channels. Reverse transcriptase-polymerase chain reaction and Western blotting demonstrated presence of alpha-BKCa channel subunit and iberiotoxin, a blocker of BK(Ca) channels, inhibited whole cell K(+) current. Chronic changes in PO(2) did not affect expression, recruitment, or function of BK(Ca) channels in A549 cells. In contrast, acute changes of PO(2) regulated the BK(Ca) channel activity by controlling the channel mean open time. This effect of PO(2) was insensitive to inhibitor of flavoproteins, diphenylene iodinium. In addition, decrease in PO(2) and iberiotoxin induced membrane depolarization and Ca(2+) oscillations in A549 cells. We conclude that BK(Ca) channels serve as oxygen sensors in human alveolar A549 epithelial cells.

Dual action of cytosolic calcium on calcium channel activity during hypoxia in hippocampal neurones

The role of cytoplasmic calcium level (Ca(i)) in regulation of Ca channel activity during hypoxia was studied in hippocampal neurones from rats. Whole-cell patch clamp recordings in combination with measurements of O(2) partial pressure (pO(2)) were used. Lowering of pO(2) induced a potentiation of HVA Ca channel activity by 25.7% at Ca(i) = 75 nM in comparison with Ca(2+)-free solution. Increase of Ca(i) up to 410 nM slightly increased the effect and significantly slowed the Ca(2+) current run-down. On the other hand, hypoxia increased a steady-state channel inactivation and speeded up the kinetics of Ca(2+) current decay by about 30%. We conclude that moderate hypoxia induces dual action on Ca channels: intracellularly mediated augmentation of Ca influx via Ca channels and their Ca(2+)-dependent inactivation.

Selective cardiorespiratory and catecholaminergic areas express the hypoxia-inducible factor-1alpha (HIF-1alpha) under in vivo hypoxia in rat brainstem

Under severe oxygen deprivation, all cells are able to express the transcription factor HIF-1, which activates a wide range of genes. Under tolerable hypoxia, chemosensory inputs are integrated in brainstem areas, which control cardiorespiratory responses. However, the molecular mechanisms of this functional acclimatization are unknown. We investigated when and where the inducible HIF-1alpha subunit is expressed in the rat brainstem in vivo, under physiological hypoxia. The regional localization of HIF-1alpha mRNA and protein was determined by in situ hybridization and immunocytochemistry in adult male rats exposed to moderate hypoxia (10% O2) for 1-6 h. HIF-1alpha protein was found in cell types identified by immunocytochemistry as catecholaminergic neurons. Hypoxia induced HIF-1alpha mRNA and protein in only some parts of the brainstem located dorsomedially and ventrolaterally, which are those involved in the cardiorespiratory control. No labelling was detected under normoxia. The protein was detected in glia and neurons after 1 and 6 h of hypoxia, respectively. A subset of A2C2 and A1C1 catecholaminergic neurons colocalized tyrosine hydroxylase and HIF-1alpha proteins under hypoxia, but no HIF-1alpha was detected in more rostral catecholaminergic areas. In contrast to cardiorespiratory areas, HIF-1alpha protein was already present under normoxia in glial cells of brainstem tracts but was not overexpressed under hypoxia, although HIF-1alpha mRNA was up-regulated. In conclusion, there appear to be two regulatory mechanisms for HIF-1alpha expression in the brainstem: hypoxic induction of HIF-1alpha protein in cardiorespiratory-related areas and constitutive protein expression unaffected by hypoxia in brainstem tracts.

Role of prostanoids and 20-HETE in mediating oxygen-induced constriction of skeletal muscle resistance arteries

This study determined the contribution of cytochrome P450 (CP450) 4A enzyme metabolites of arachidonic acid in mediating the constriction of isolated rat skeletal muscle resistance arteries in response to elevated PO2. Gracilis arteries (GA) were viewed via television microscopy and constrictor responses to elevated PO2 were measured with a video micrometer. Endothelium removal and treatment of GA with 17-octadecynoic acid (17-ODYA; suicide substrate inhibitor of CP450 4A enzymes) impaired oxygen-induced constriction of the vessels; treatment of endothelium-denuded GA with 17-ODYA eliminated responses to elevated PO2. NOS inhibition and inhibition of EET production had no effect on oxygen-induced constriction of the vessels, although cyclooxygenase inhibition with indomethacin impaired GA responses to elevated PO2. Treatment of GA with dibromododecenyl methylsulfimide (DDMS; inhibitor of 20-hydroxyeicosatetraenoic acid (20-HETE) production) or 6(Z),15(Z)-20-HEDE (antagonist for 20-HETE receptors) mimicked the effects of 17-ODYA on GA responses to elevated PO2. Treatment of vessels with iberiotoxin or glibenclamide reduced the constriction of the vessels in response to elevated PO2 while treatment with both K+ channel blockers eliminated oxygen-induced constriction of the vessels. Following treatment of GA with indomethacin and 20-HETE, the vessels failed to respond to elevated PO2. These results suggest that oxygen-induced constriction of skeletal muscle resistance arteries represents the combined effects of reduced prostanoid release from the vascular endothelium and enhanced 20-HETE production in vascular smooth muscle cells.