Effect of hypoxia on energy and calcium metabolism in airway smooth muscle

Mitochondria signaling pathways in allergic asthma

Mitochondria, as the powerhouse organelle of cells, are greatly involved in regulating cell signaling pathways, including those related to the innate and acquired immune systems, cellular differentiation, growth, death, apoptosis, and autophagy as well as hypoxic stress responses in various diseases. Asthma is a chronic complicated airway disease characterized by airway hyperresponsiveness, eosinophilic inflammation, mucus hypersecretion, and remodeling of airway. The asthma mortality and morbidity rates have increased worldwide, so understanding the molecular mechanisms underlying asthma progression is necessary for new anti-asthma drug development. The lung is an oxygen-rich organ, and mitochondria, by sensing and processing O2, contribute to the generation of ROS and activation of pro-inflammatory signaling pathways. Asthma pathophysiology has been tightly associated with mitochondrial dysfunction leading to reduced ATP synthase activity, increased oxidative stress, apoptosis induction, and abnormal calcium homeostasis. Defects of the mitochondrial play an essential role in the pro-remodeling mechanisms of lung fibrosis and airway cells' apoptosis. Identification of mitochondrial therapeutic targets can help repair mitochondrial biogenesis and dysfunction and reverse related pathological changes and lung structural remodeling in asthma. Therefore, we here overviewed the relationship between mitochondrial signaling pathways and asthma pathogenic mechanisms.

The early asthmatic response is associated with glycolysis, calcium binding and mitochondria activity as revealed by proteomic analysis in rats

Background

The inhalation of allergens by allergic asthmatics results in the early asthmatic response (EAR), which is characterized by acute airway obstruction beginning within a few minutes. The EAR is the earliest indicator of the pathological progression of allergic asthma. Because the molecular mechanism underlying the EAR is not fully defined, this study will contribute to a better understanding of asthma.

Methods

In order to gain insight into the molecular basis of the EAR, we examined changes in protein expression patterns in the lung tissue of asthmatic rats during the EAR using 2-DE/MS-based proteomic techniques. Bioinformatic analysis of the proteomic data was then performed using PPI Spider and KEGG Spider to investigate the underlying molecular mechanism.

Results

In total, 44 differentially expressed protein spots were detected in the 2-DE gels. Of these 44 protein spots, 42 corresponded to 36 unique proteins successfully identified using mass spectrometry. During subsequent bioinformatic analysis, the gene ontology classification, the protein-protein interaction networking and the biological pathway exploration demonstrated that the identified proteins were mainly involved in glycolysis, calcium binding and mitochondrial activity. Using western blot and semi-quantitative RT-PCR, we confirmed the changes in expression of five selected proteins, which further supports our proteomic and bioinformatic analyses.

Conclusions

Our results reveal that the allergen-induced EAR in asthmatic rats is associated with glycolysis, calcium binding and mitochondrial activity, which could establish a functional network in which calcium binding may play a central role in promoting the progression of asthma.

Iodoacetic acid-induced rigor in ileal longitudinal smooth muscle of guinea-pig

1. Ileal tensions to iodoacetic acid (IAA) develop when tissue ATP concentration falls below approximately 60-55% of the control. 2. As the IAA concentration is increased (0.1-10 mM), the ATP concentrations decrease rapidly, and both the time of the onset and the duration of the contraction shorten. 3. In the presence of lactate, IAA failed to decrease the tissue ATP concentration and did not develop tension. 4. The contraction to IAA developed in the presence of Ca2+ antagonist, D-600 or in Ca2(+)-free solution, however, onset time was prolonged. 5. These results suggest that the contraction to IAA is referred to as 'rigor' because it increases with decreasing tissue ATP concentration in ileum.

Effects of oxygen and glucose deprivation on vasoactivity in isolated bovine middle cerebral arteries

The effects of oxygen and glucose deprivation on vasoactivity were investigated using helical strips of bovine middle cerebral artery. Hypoxia, created by reducing the PO2 of the bath, or oxidative inhibition with 2,4 dinitrophenol (DNP) or sodium azide, significantly reduced contractions induced by serotonin. Normal tonic contractions induced with fresh and aged whole blood, or 5-HT became phasic and quickly relaxed to baseline in a hypoxic environment. Glucose elimination from the Krebs medium, or the inhibition of the glycolytic pathway with iodoacetic acid (IAA), did not significantly reduce serotonin-induced contractions. However, contractions were inhibited more with the combination of oxygen and glucose deprivation, or DNP + IAA, than with oxygen deprivation alone. Efforts to produce rigor in this preparation by oxygen/substrate reduction or metabolic inhibition were unsuccessful. Tonic contractions induced by 70 mM potassium became phasic as the Ca++ concentration was reduced. Contractions resulting from the readdition of Ca++ to arteries exposed to calcium-free high potassium solution were significantly reduced in the presence of oxidative and/or glycolytic inhibitors. The uptake of 45Ca++, as measured by the lanthanum technique, decreased as the bath PO2 was reduced in both serotonin stimulated and unstimulated arteries. Glucose deprivation alone did not affect 45Ca++ uptake. This study suggests that hypoxia has a direct inhibitory affect on cerebral vasoactivity mediated by reductions in sarcoplasmic Ca++ uptake.

Time course of pulmonary vasoconstriction with repeated hypoxia and glucose depletion

To examine the effect of hypoxia on pulmonary vascular smooth muscle, rabbit lobar pulmonary artery was suspended in a glucose free solution and both chronologic changes in tension and ATP content were determined together at 30 min intervals after repeated hypoxic challenge (PO2 = 11 +/- 2 mm Hg). The pulmonary artery contracted and its ATP content decreased with hypoxia. This contraction was not inhibited by nifedipine, Ca++ -free EGTA, procaine, phentolamine, isoproterenol, diphenhydramine, prostaglandin E1, atropine or nitroglycerin. Upon reoxygenation (PO2 = 104 +/- 3 mm Hg), the elevated resting tension decreased in a biphasic fashion and the ATP content of the lobar pulmonary artery increased. When hypoxic challenges were repeated, the rate of constriction on hypoxia increased, while the relaxation rate on reoxygenation, tension developed by 30 min of hypoxia and the total amount of ATP decreased. These results suggest that the ATP content in the lobar pulmonary artery is very sensitive to in vitro acute hypoxia and that the Ca++ transport process is more easily impaired by reduction in ATP levels than is the contractile machinery.

Effect of dinitrophenol and anoxia on isometric tension in rabbit colon smooth muscle

In a previous study, it was suggested that inhibition of oxidative phosphorylation in smooth muscle is accompanied by a release of Ca2+ from mitochondria (Pettersson 1983); in this investigation, the effects of 2,4-dinitrophenol (DNP) and anoxia on isometric tension in pieces of smooth muscle from rabbit colon were studied. Addition of DNP to the bathing medium elicited a transient and dose-dependent contraction with an ED50 value of 2 X 10(-4) M. The contraction was not inhibited by pretreatment with atropine (1 X 10(-6) M). The removal of external Ca2+ did not reduce the contracting action of DNP. Addition of the local anaesthetic D-mepivacaine (1 X 10(-3) M), which reduces the Ca2+ efflux through the plasma membrane of the smooth muscle cells, augmented the response to DNP. Gassing the bathing medium with 95% N2 + 5% CO2 (anoxia) for 60 min. did not affect the basal tension, but markedly reduced the contractile response to DNP. The results indicate that DNP increased cytoplasmic Ca2+ significantly enough to induce a contraction of rabbit colon smooth muscle by a large release from an intracellular store, probably from the mitochondria. Since anoxia did not mimic the contracting action of DNP, it is suggested that a high concentration of DNP would be a more effective stimulus for mitochondrial Ca2+ release than anoxia.

Effects of dinitrophenol on phosphorylase a activity, adenine nucleotide levels and tension in rabbit colon smooth muscle

Glycogen phosphorylase a activity, the contents of adenine nucleotides and isometric tension were measured in rabbit colon smooth muscle after exposure to 2,4-dinitrophenol (DNP). DNP caused a dose-dependent increase in phosphorylase a activity, with an ED50 value of 1 X 10(-4)M. Since adenine nucleotides, especially 5-AMP by inhibiting the phosphorylase a to b conversion, might increase the phosphorylase a activity, a time-response study was undertaken in order to analyse the time course of changes in the adenine nucleotide content and phosphorylase a activity after addition of DNP. The latter activity was increased after only 1 min. of incubation. At this time no changes were found in the contents of ATP, ADP, 5-AMP or CP. Not until the phosphorylase a activity had reached its maximum after 5-7.5 min. did the 5-AMP content increase. The phosphorylase a activity then started to decline, but the 5-AMP content continued to rise. The effect of DNP on isometric tension was also studied to test whether the cytoplasmic Ca2+ concentration was increased. An increase in tension was observed 5 min. after administration of the drug and was maximal at 15 min. The results seem to dispute against 5-AMP as being a mediator for the DNP-stimulated increased in phosphorylase a activity in smooth muscle. The finding that DNP elicited a contraction of rabbit colon supports the earlier suggestion that the cytoplasmic Ca2+ concentration was increased (Pettersson 1983).

Mechanics and energetics of lengthening of active airway smooth muscle

For smooth muscle in general there appears only one report dealing with force-velocity (FV) relationships of active muscle subjected to forcible elongation by application of loads (P) greater than its maximum isometric tetanic tension (Po); for airway smooth muscle (ASM) there is none. Since ASM may be subjected to increasing stretch during inspiration, the relationship is important and was therefore studied with canine tracheal smooth muscle (TSM) as a model. FV data for P less than Po could be fitted by Hill's hyperbolic equation. For P greater than Po, lengthening velocity was greater than predicted by the equation. However at equivalent velocities, the muscle during elongation could support a load three times greater than during shortening; in this it resembled skeletal muscle. From this it may be speculated that distension of the airway during inspiration would not be associated with mechanical instability. With reference to energy requirements of the elongating TSM it was shown, as has been for skeletal muscle, that the net rate of energy liberation (assessed by measuring tissue levels of adenosine triphosphate and creatine phosphate) in an elongating active muscle is less than that of a muscle contracting isometrically.

Role of glucose in contractility of airway smooth muscle

This study investigated the response of isolated tracheal preparation (ITP) from guinea pigs to three bronchoconstrictive drugs: histamine (H), carbachol (CAR), and acetylcholine (ACH). Cumulative dose-response curves were obtained in substrate-rich (5.50 mM of glucose) and substrate-free physiological salt solutions under aerobic conditions (PO2 = 95 +/- 0.5 Torr) or in acute 30-min hypoxia (PO2 = 30 +/- 2 Torr). The reactivity of ITP was measured by ED50 (effective dose developing 50% of maximal response); and the contractility of ITP was measured by a maximal developed isometric tension. This study showed that the contractile response of isolated segments of tracheas to H, CAR, and ACH was significantly decreased (P less than 0.05) when the experimental medium contained no glucose, or when glucose was replaced with metabolic inhibitor 2-deoxyglucose. This data contrasted with findings obtained with main pulmonary arteries (MPAs) isolated from the same species, where the absence of glucose in the experimental medium did not affect patterns of histamine cumulative dose-response curves. It was also shown that pretreatment of isolated tracheas with a small dose of insulin significantly decreased (P less than 0.05) the contractile response of airway smooth muscle to histamine. In both experiments, either with or without glucose in the experimental medium, acute hypoxia (30 min) had a significantly greater effect (P less than 0.05) on the development of isometric tension of MPA compared with that on ITP. Both removal of glucose from the experimental medium and/or acute hypoxia significantly decreased (P less than 0.001) adenosin triphosphate and creatine phosphate contents of tracheal segments but did not influence the high-energy phosphate content of main pulmonary arteries exposed to similar conditions. In addition to pointing out the physiological and metabolic differences between pulmonary vascular and airway smooth muscle, this study showed the important dependence of airway smooth muscle on glucose as a substrate.

Intracellular pH in hypoxic smooth muscle

Hypoxia impairs contractility in canine tracheal smooth muscle (TSM). This is attributed to intracellular lactacidosis. The present studies were undertaken to confirm this. Lactate was found to be significantly increased in hypoxic TSM (65.36 +/- 7.37 mg/100 g wet tissue), compared to normoxic (29.83 +/- 5.05). Intracellular pH (pHi) was, however, significantly increased in hypoxic active TSM to 7.71 +/- 0.05 as compared to 7.30 +/- 0.03 in normoxic active muscle. pHi of resting normoxic muscle (7.20 +/- 0.04) was statistically not different from that of resting hypoxic muscle. The pHi's of resting normoxic and active hypoxic muscles were significantly different. These results show that under in vitro, hypoxic conditions: 1) an increase in glycolysis in TSM is indicated by the increased lactate production, 2) there is a surprising, concomitant rise in pHi rather than a decrease as previously expected, and 3) it is mechanical activity of the muscle which leads to this paradoxical result, inasmuch as pHi is unaltered in the resting hypoxic muscle.