Diamide stimulates calcium-sodium exchange in dog red blood cells

Na(+) and Ca(2+) pumps in the gills, epipodites and branchiostegites of the european lobster Homarus gammarus: effects of dilute sea water

Crude homogenates and plasma-membrane-enriched fractions were prepared from the epithelium of the gills, epipodites and branchiostegites of intermoult European lobsters Homarus gammarus, and Na(+)/K(+)-ATPase, Ca(2+)-ATPase and Na(+)/Ca(2+) exchange activities were quantified in these tissues. Lobsters were kept in sea water (salinity 35) or were adapted to dilute sea water (22.1). The lobster hyperregulates haemolymph osmolarity and Ca(2+) levels in both media. Homogenates of the podobranchs, arthrobranchs and pleurobranchs had comparable Na(+)/K(+)-ATPase specific activities, and mean activities increased significantly for all three types of gills when the animals were kept in dilute sea water. In the epipodites and branchiostegites, Na(+)/K(+)-ATPase specific activities exceeded those in the gills, and exposure to dilute sea water greatly enhanced these activities. In sea water, 80 % of the total Na(+)/K(+)-ATPase activity is associated with the gills and epipodites (each tissue containing 40 %) and 20 % with the branchiostegites; in dilute sea water, the gills contained approximately 25 %, the epipodites 40 % and the branchiostegites approximately 35 % of the total activity, indicating the relative importance of the epipodites and branchiostegites for ionic hyperregulation in dilute media. In plasma membrane vesicles isolated from the gills, epipodites and branchiostegites, Ca(2+) transport driven by ATP and by a Na(+)gradient was demonstrated. Exposure to dilute sea water enhanced Na(+)/Ca(2+)exchange and Ca(2+)-ATPase activities in the epipodites and branchiostegites; in the gills, however, Ca(2+) transport activities decreased. The role of these tissues and enzymes in Na(+) and Ca(2+) handling by the lobster is discussed.

Interaction of reactive oxygen species with ion transport mechanisms

The use of electrophysiological and molecular biology techniques has shed light on reactive oxygen species (ROS)-induced impairment of surface and internal membranes that control cellular signaling. These deleterious effects of ROS are due to their interaction with various ion transport proteins underlying the transmembrane signal transduction, namely, 1) ion channels, such as Ca2+ channels (including voltage-sensitive L-type Ca2+ currents, dihydropyridine receptor voltage sensors, ryanodine receptor Ca2+-release channels, and D-myo-inositol 1,4,5-trisphosphate receptor Ca2+-release channels), K+ channels (such as Ca2+-activated K+ channels, inward and outward K+ currents, and ATP-sensitive K+ channels), Na+ channels, and Cl- channels; 2) ion pumps, such as sarcoplasmic reticulum and sarcolemmal Ca2+ pumps, Na+-K+-ATPase (Na+ pump), and H+-ATPase (H+ pump); 3) ion exchangers such as the Na+/Ca2+ exchanger and Na+/H+ exchanger; and 4) ion cotransporters such as K+-Cl-, Na+-K+-Cl-, and Pi-Na+ cotransporters. The mechanism of ROS-induced modifications in ion transport pathways involves 1) oxidation of sulfhydryl groups located on the ion transport proteins, 2) peroxidation of membrane phospholipids, and 3) inhibition of membrane-bound regulatory enzymes and modification of the oxidative phosphorylation and ATP levels. Alterations in the ion transport mechanisms lead to changes in a second messenger system, primarily Ca2+ homeostasis, which further augment the abnormal electrical activity and distortion of signal transduction, causing cell dysfunction, which underlies pathological conditions.

Potassium transport in lamprey (Lampetra fluviatilis) erythrocytes: Evidence for K+ channels

1. Unidirectional K+ (86Rb) influx in lamprey red blood cells was studied under different conditions. 2. The influx of 86Rb was markedly inhibited by 1 mM Ba2+ when cells were incubated in saline containing 4 mM K+. In K(+)-free media, the influx rate constant of 86Rb was lower, and 1 mM Ba2+ had no blocking effect. 3. Treatment of the red cells with 0.1 mM ouabain in the absence of external K+ resulted in the appearance of the component of 86Rb influx inhibited by 1 mM Ba2+, quinine, TEA or amiloride. 4. Similar results were obtained in red cells incubated in Na(+)-free media MgCl2-sucrose. 5. The results obtained provide evidence for the existence of K+ channels in the red cell membrane of the lamprey. Under physiological conditions (in the presence of 4 mM K+) the total rate constant for the 86Rb influx in erythrocytes was about 1.9/hr, including ouabain-sensitive (0.6/hr), Ba(2+)-sensitive (1.1/hr) and residual (0.2/hr) components.

Activation of ion transport pathways by changes in cell volume

Swelling-activated K+ and Cl- channels, which mediate RVD, are found in most cell types. Prominent exceptions to this rule include red cells, which together with some types of epithelia, utilize electroneutral [K(+)-Cl-] cotransport for down-regulation of volume. Shrinkage-activated Na+/H+ exchange and [Na(+)-K(+)-2 Cl-] cotransport mediate RVI in many cell types, although the activation of these systems may require special conditions, such as previous RVD. Swelling-activated K+/H+ exchange and Ca2+/Na+ exchange seem to be restricted to certain species of red cells. Swelling-activated calcium channels, although not carrying sufficient ion flux to contribute to volume changes may play an important role in the activation of transport pathways. In this review of volume-activated ion transport pathways we have concentrated on regulatory phenomena. We have listed known secondary messenger pathways that modulate volume-activated transporters, although the evidence that volume signals are transduced via these systems is preliminary. We have focused on several mechanisms that might function as volume sensors. In our view, the most important candidates for this role are the structures which detect deformation or stretching of the membrane and the skeletal filaments attached to it, and the extraordinary effects that small changes in concentration of cytoplasmic macromolecules may exert on the activities of cytoplasmic and membrane enzymes (macromolecular crowding). It is noteworthy that volume-activated ion transporters are intercalated into the cellular signaling network as receptors, messengers and effectors. Stretch-activated ion channels may serve as receptors for cell volume itself. Cell swelling or shrinkage may serve a messenger function in the communication between opposing surfaces of epithelia, or in the regulation of metabolic pathways in the liver. Finally, these transporters may act as effector systems when they perform regulatory volume increase or decrease. This review discusses several examples in which relatively simple methods of examining volume regulation led to the discovery of transporters ultimately found to play key roles in the transmission of information within the cell. So, why volume? Because it's functionally important, it's relatively cheap (if you happened to have everything else, you only need some distilled water or concentrated salt solution), and since it involves many disciplines of experimental biology, it's fun to do.

Influence of Ca/Na exchange and diamide on membrane partitioning of glyceraldehyde-3-phosphate dehydrogenase in dog red cells

1. Ca/Na exchange in dog red cells is greatly stimulated by pretreatment of the cells with diamide, but only if the diamide preincubation is carried out in a Na-free, Ca-containing medium. 2. Membranes prepared from dog red cells that had been pre-exposed to diamide in solutions containing various combinations of Ca and Na were subjected to polyacrylamide gel electrophoresis and Western blotting. 3. The band representing glyceraldehyde-3-phosphate dehydrogenase was selectively increased under the very same ionic conditions that result in a diamide-induced stimulation of Ca/Na exchange. 4. Glyceraldehyde-3-phosphate dehydrogenase may participate in the modulation by diamide of Ca/Na exchange in dog red cells.

Stimulation of calcium-sodium exchange in dog red blood cells by hemolysis and resealing

Osmotic hemolysis and resealing greatly increase calcium influx in dog red blood cells. The resealed ghosts show a saturable calcium entry pathway with complex kinetics. As expected for a calcium-sodium exchanger, calcium uptake is stimulated by internal sodium and inhibited by external sodium. Compared to fresh, intact red cells the resealed ghost calcium-sodium exchanger is less responsive to quinidine and to alterations in medium tonicity. The differences in calcium uptake rate among cells from different donors are minimized in the ghost preparation. There are several ways to stimulate sodium-dependent calcium movements in these cells, of which hemolysis-resealing is the most potent. The results of these and previous studies suggest that dog red blood cells have a latent capacity for calcium-sodium exchange.

Volume-activated transport systems in dog red blood cells

1. When dog red blood cells are shrunken or swollen, transport pathways are activated that do not function discernibly when the cell is at normal volume. Swelling the cells turns on two pathways, a Ca-Na exchanger and a Cl-dependent K pathway. 2. Shrinking the cells activates a Na-H antiporter. 3. The passive net flow of ions through these transporters is in such a direction as to correct the perturbation of cell volume: when the cell water content has returned to normal, the transporters turn off. 4. Recently we have investigated agents that can lock or fix the volume-responsive transporters in the activated state. Na-H exchange, for example, can be fixed in the on position with either glutaraldehyde or N-phenylmaleimide. 5. Ca-Na exchange can be locked on by the sulfhydryl-oxidizing agent, diamide. We have used these effects to investigate the relationships between cell volume and the transport mechanisms. 6. It is possible, for instance, to distinguish whether certain inhibitors act on the transporters per se or on the apparatus that perceives cell volume and communicates with the transporters. 7. Furthermore, in the case of the Ca-Na exchanger some indication of the membrane polypeptides involved in volume regulation has been possible, using radioactive compounds that bind covalently to sulfhydryl groups.