Relative roles of sodium/calcium exchange and ATP-fueled calcium transport in the control of cell calcium

Na+/Ca2+ Exchanger 1 in Airway Smooth Muscle of Allergic Inflammation Mouse Model

Cytosolic free Ca²⁺ ([Ca²⁺]_cyt) is essential for airway contraction, secretion and remodeling. [Ca²⁺]_cyt homeostasis is controlled by several critical molecules, one of which is the Na⁺/Ca²⁺ exchanger 1 (NCX1) in the plasma membrane. Since little is currently known about NCX1 in the airway smooth muscle and its involvement in airway diseases, the present study was designed to investigate the expression and function of NCX1 in normal airway smooth muscle and its relevance to airway inflammation. Western blot analysis, tracheal smooth muscle contraction, and [Ca²⁺]_cyt measurements were performed in mouse tracheal smooth muscle tissues and primary airway smooth muscle cell cultures. Additional studies were performed in a mouse model of allergic airway inflammation. Our data showed that NCX1 proteins were expressed in the human bronchial smooth muscle cells (HBSMCs), murine airway and whole lung. Carbachol raised [Ca²⁺]_cyt in mouse tracheal smooth muscle cells and induced murine tracheal contraction, all of which were significantly attenuated by KB-R7943, a selective NCX inhibitor. Removal of extracellular Na⁺ increased [Ca²⁺]_cyt in HBSMCs and mouse tracheal SMCs, which was dependent on extracellular Ca²⁺ and sensitive to KB-R7943. TNF-α treatment of HBSMCs significantly upregulated mRNA and protein expression of NCX1 and enhanced NCX activity. Finally, KB-R7943 abolished the airway hyperresponsiveness to methacholine in an ovalbumin-induced mouse model of allergic airway inflammation. Together, these findings indicate that NCX1 in airway smooth muscle may play an important role in the development of airway hyperresponsiveness, and downregulation or inhibition of NCX1 may serve as a potential therapeutic approach for asthma.

Redox active calcium ion channels and cell death

Calcium plays a key role in both apoptotic and necrotic cell death. Emptying of intracellular calcium stores and/or alteration in intracellular calcium levels can modulate cell death in almost all cell types. These calcium fluxes are determined by the activity of membrane channels normally under tight control. The channels may be ligand activated or voltage dependent as well as being under the control of affector molecules such as calmodulin. It has become increasingly apparent that many calcium channels are affected by reactive oxygen or reactive nitrogen species; ROS/RNS. This may be part of the normal signaling pathways in the cell or by the action of exogenously generated ROS or RNS often by toxins. This review covers the recent literature on the activity of these redox active channels as related to cell death.

Inhibition of Ca2+-pump ATPase and the Na+/K+-pump ATPase by iron-generated free radicals. Protection by 6,7-dimethyl-2,4-DI-1- pyrrolidinyl-7H-pyrrolo[2,3-d] pyrimidine sulfate (U-89843D), a potent, novel, antioxidant/free radical scavenger

Preincubation of red blood cell (RBC) membranes with a model system known to generate reactive oxygen species (ROS) and free radicals (200 microM ferrous sulfate and 200 microM EDTA, Fe2+/EDTA) resulted inhibition of the Na+/K+ -pump ATPases was also associated with membrane protein crosslinking and lipid peroxidation, the latter as monitored by the formation of thiobarbituric acid reactive substances (TBARS). Inhibition of the ion transport ATPases, protein cross-linking and formation of TBARS were prevented by U-89843D in a concentration-dependent manner, with half-maximal protection seen at 0.3 microM. U-89843D was more potent than the classical antioxidant butylated hydroxytoluene. Neither U-89843D nor the solvent DMSO had any effect on the assay of TBARS. U-89843D exerted only minimal inhibitory activity on ATPase activities. Thus, U-89843D was potent in vitro in preventing a variety of membrane-damaging reactions mediated by ROS. It is suggested that protection of membranes from ROS-mediated damage is of potential usefulness in the prevention and treatment of certain disease processes.

Use of thermal analysis to distinguish magnesium and calcium stimulated ATPase activity in isolated transverse tubules from skeletal muscle

The presence of calcium stimulated adenosine triphosphatase (Ca2+,Mg(2+)-ATPase) activity in isolated transverse tubule (t-tubule) membranes is distinguished from magnesium adenosine triphosphatase (Mg(2+)-ATPase) activity on the basis of differing thermal stabilities. The Mg(2+)-ATPase is the major protein component of the t-tubule membrane, and it can be difficult to discriminate between the low levels of Ca2+ stimulated ATPase activity found in isolates of t-tubules compared to the much higher Mg(2+)-ATPase activity. Thermal analysis reveals different inactivation temperatures (Ti) for the proteins responsible for ATP dependent calcium transport (Ti = 49 degrees C) and Mg(2+)-ATPase activity (Ti = 57 degrees C) in isolated t-tubule membranes. The differential scanning calorimetry profile of t-tubule membranes consists of three major components with transition temperatures (Tm) of 51 degrees C, 57 degrees C and 63 degrees C. Denaturation of the component with Tm = 57 degrees C correlates with inactivation of Mg(2+)-ATPase activity, and denaturation of the Tm = 51 degrees C component correlates with the inactivation of Ca2+,Mg(2+)-ATPase activity and calcium transport. The functions of the t-tubule membrane component or components that denature with Tm = 63 degrees C have yet to be identified. The lack of stimulation of calcium transport in isolated t-tubules by oxalate, the impermeability of isolated t-tubules to oxalate, and experiments performed on t-tubules with defined amounts of sarcoplasmic reticulum (SR) added suggest that contamination of the isolated t-tubules by SR is unlikely to account for the level of Ca2+,Mg(2+)-ATPase activity detected. The presence of a Ca2+,Mg(2+)-ATPase in the t-tubule membrane would provide a mechanism that may be involved in the partial removal of calcium that is accumulated in the junctional space during muscle relaxation or calcium that is released from the terminal cisternae of sarcoplasmic reticulum during excitation-contraction coupling.

Inhibition of the Ca pump of intact red blood cells by t-butyl hydroperoxide: importance of glutathione peroxidase

Incubation of human red blood cells (RBCs) with t-butyl hydroperoxide (tBHP) resulted in inhibition of the Ca-pump ATPase. This was demonstrated using an assay of the Ca-pump ATPase activity in intact RBCs. In this assay, activity of the Ca-pump ATPase is expressed as the rate constant of the initial loss of ATP in RBCs exposed to Ca and A23187. Pseudo-first-order rate constants (Ca-pump ATPase rate constants) were lower in the presence of tBHP versus controls. Incubation of RBCs with tBHP resulted in both a time- and concentration-dependent inhibition of the Ca-pump ATPase (IC50 approximately 1 mM). Incubation of RBCs with tBHP also resulted in decreased oxyhemoglobin, increased methemoglobin and increased thiobarbituric acid reactive substances (TBARS). GSH levels were significantly lower in the presence of tBHP. GSH fell from a control value of 2.2 mmol/l RBC to 0.46 mmol/l RBC after incubation with 0.25 mM tBHP for 15 min. Both butylated hydroxytoluene and stobadine prevented the formation of TBARS and were partially effective in protecting the Ca-pump ATPase from tBHP-induced inhibition. Dithiothreitol was completely effective in preventing the tBHP-induced formation of TBARS as well as inhibition of the Ca-pump ATPase. However, when added after exposure to tBHP, dithiothreitol was unable to restore Ca-pump ATPase activity completely. An activity of dithiothreitol independent of enzymic thiol group reduction was apparent. In the presence of mercaptosuccinate, a potent inhibitor of glutathione peroxidase, the ability of dithiothreitol to protect the Ca-pump ATPase from tBHP-induced inhibition was abolished. Therefore, protection by dithiothreitol may be afforded by its ability to replenish GSH from oxidized glutathione, thus allowing glutathione peroxidase to metabolize tBHP. These results may be interpreted to suggest that inhibition of the Ca-pump ATPase in intact RBCs occurs as a result of tBHP-induced oxidant stress and subsequent lipid peroxidation which can be prevented by certain antioxidants including butylated hydroxytoluene, stobadine, and thiol-containing compounds such as dithiothreitol. These findings provide further insight into the mode of action of hydroperoxides and certain reactive oxygen species that have been implicated in oxidative stress associated with various pathological conditions. The importance of the GSH/glutathione peroxidase system in metabolizing organic hydroperoxides is also demonstrated.

Ion transport ATPases as targets for free radical damage. Protection by an aminosteroid of the Ca2+ pump ATPase and Na+/K+ pump ATPase of human red blood cell membranes

Preincubation of red blood cell membranes in the presence of ferrous sulfate and EDTA resulted in both a concentration- and time-dependent inhibition of the Na+/K+ pump ATPase, basal Ca2+ pump ATPase, and the calmodulin- (CaM) activated Ca2+ pump ATPase. The IC50 for all three ATPases was approximately 2.5 x 10(-5) M iron. The addition to membranes of ferrous iron and EDTA in an approximately 1:1 ratio resulted in conversion to the ferric iron form in several minutes. However, inhibition of the ion pump ATPases and cross-linking of membrane proteins occurred over the course of several hours. The time course of formation of thiobarbituric acid-reactive substances (TBARS) closely paralleled inhibition of the ion pump ATPases. Inhibition of the ion pump ATPases was prevented by the addition of deferoxamine or superoxide dismutase but not by mannitol, or catalase. Both butylated hydroxytoluene and tirilazad mesylate (U74006F) prevented the formation of TBARS, limited the inhibition of the ion pump ATPases, and reduced cross-linking of membrane proteins. These data may be interpreted to suggest that inhibition of ion pump ATPases in plasma membranes may occur as a result of iron-promoted formation of superoxide and subsequent lipid peroxidation, which can be prevented by free-radical scavengers including butylated hydroxytoluene and U74006F.

Na/Ca exchange and tension development in vascular smooth muscle: effect of amiloride

1. The potassium-sparing diuretic, amiloride, has been shown to inhibit the Na/Ca exchange system in various preparations. The effects of this drug have been investigated on the contractions of guinea-pig aortic strips elicited by reduction of external K, by addition of ouabain and by removal of external Na. 2. Amiloride (5 X 10(-6) M-5 X 10(-4) M) inhibited the mechanical responses when it was added before giving the stimulus for contractions, but was not effective in relaxing the contracted strips. The drug shifted to the right the dose-response curve for Ca in low K solution. 3. The calcium antagonist diltiazem had no effect on the ouabain-, low K- and Na-free-induced contractions. 4. Amiloride decreased the rate of relaxation of aortic strips induced by removal of the low K solution. 5. The pattern of amiloride action on ouabain-, low K- and Na-free-induced contractions suggests that the drug interferes with Ca influx. The effect of amiloride on the relaxation rate of low K-contracted aortic strips is consistent with an interference with Ca efflux. 6. It is suggested that amiloride prevents Ca fluxes through the Na/Ca exchange system of guinea-pig aortic strips.