Calcium-induced potassium pathway in sided erythrocyte membrane vesicles

Nigericin-stimulated ATPase activity in microsomal vesicles of tobacco callus

The effect of ionophores on K(+)-stimulated adenosinetriphosphatase (ATPase; ATP phosphohydrolase, EC 3.6.1.3) activity of microsomal vesicles from tobacco callus was investigated. A nigericin-stimulated K(+)-ATPase activity was enriched in a purified microsomal fraction, which was obtained from the interphase of a dextran density gradient between 1.03 and 1.06 g/ml. The purified microsomal fraction was free of mitochondrial membranes and was composed partly of tightly sealed vesicles as indicated by the low K(+) permeability coefficient. The K(+)-dependent ATPase of this fraction was stimulated slightly by either carbonyl cyanide m-chlorophenylhydrazone (CCCP) (29%) or valinomycin (31%) alone; this ATPase was significantly stimulated by a combination of CCCP and valinomycin (73%) or by nigericin alone (80%). The K(+)-ATPase activity was stimulated by nigericin at pH 6.5 but not at pH 8.5. At pH 6.5, the K(+)-ATPase was inhibited by N,N'-dicyclohexylcarbodiimide but not by oligomycin. Nigericin stimulated the ATPase activity in the absence of initial KCl or pH gradients across the vesicle membrane. These results suggest that nigericin stimulates the ATPase activity by dissipating the H(+) or K(+) gradient or both, and support the hypothesis that the K(+)-ATPase mediates a H(+)/K(+) transport.

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.

Mechanism of spontaneous inside-out vesiculation of red cell membranes

In certain conditions, human red cell membranes spontaneously form inside out vesicles within 20 min after hypotonic lysis. Study of the geometry of this process now reveals that, contrary to earlier views of vesiculation by endocytosis or by the mechanical shearing of cytoskeleton-depleted membrane, lysis generates a persistent membrane edge which spontaneously curls, cuts, and splices the membrane surface to form single or concentric vesicles. Analysis of the processes by which proteins may stabilize a free membrane edge led us to formulate a novel zip-type mechanism for membrane cutting-splicing and fusion even in the absence of free edges. Such protein-led membrane fusion represents an alternative to mechanisms of membrane fusion based on phospholipid interactions, and may prove relevant to processes of secretion, endocytosis, phagocytosis, and membrane recycling in many cell types.

The role of calmodulin on Ca2+ -dependent K+ transport regulation in the human red cell

Several lipophilic calmodulin antagonists (phenotiazines, butyrophenones and diphenylbutylpiperidines) inhibited Ca2+-induced loss of KC1 from human red cells. However, the Ki values for this effect did not bear good correlation with the Ki values reported for well-known calmodulin-dependent systems. In addition, the inhibition was strongly dependent on the haematocrit and valinomycin-induced KC1 fluxes were also affected. Added calmodulin did not have any effect on Ca2+-dependent 86Rb uptake by inside-out vesicles derived from red cell membranes whereas stimulation of Ca2+-dependent ATPase was apparent. Lipophilic anticalmodulins at high doses had all kinds of effects on 86Rb uptake by inside-out vesicles: increase, decrease or no change of the fraction of activated vesicles reached at submaximal Ca2+ concentrations, with or without modification of the relative rate of 86Rb uptake. The hydrophylic compound 48/80 decreased the fraction of activated vesicles reached at submaximal Ca2+ concentrations without affecting the relative rate of 86Rb uptake, but this effect took place only at concentrations 10-fold higher than the reported Ki for calmodulin-dependent systems. These results suggest that Ca2+-dependent K+ channels of red cells are not regulated by calmodulin.

The pathogenesis of cystic fibrosis: a proposal for calmodulin as the basic biochemical defect

In cystic fibrosis (CF) there are two major defects which lead to most clinical manifestations of the disease. These are the electrolyte sweat defect and the abnormality of mucous secretions. Both may be satisfactorily explained by increased intracellular Ca++. In Na+ reabsorbing cells, such as exocrine sweat glands, increased Ca++ inhibits transepithelial Na+ transport. In mucus-secreting cells, high Ca++ levels may lead to physiochemical changes in secreted mucus. Increased intracellular Ca++ levels are hypothesized to be caused either by a defective Ca++ efflux cellular mechanism, or by increased intracellular binding of Ca++.

A calmodulin activated Ca2+-dependent K+ channel in human erythrocyte membrane inside-out vesicles

The role of calmodulin in stimulating active calcium transport in the human red cell membrane is well documented. In contrast, efforts to characterize the effect of calmodulin on the Ca2+-dependent K+ channel in erythrocyte membranes have given rise to conflicting reports. These studies have indicated that experimental conditions may play a critical role in preserving the Ca2+-dependent K+ channels in erythrocyte inside-out vesicles. With these observations in mind, a double-labelling study of simultaneous active Ca2+ and passive Rb+ uptake in red-cell inside-out vesicles was undertaken. Addition of calmodulin and ATP to a suspension of inside-out vesicles containing 1 mM K+ caused a Ca2+-dependent increase in both the rate of active calcium transport and Rb+ uptake. The initial Rb+ isotope flux was increased 3-fold over the rate observed in the absence of calmodulin. The k1/2 for activation of K+ permeability was approx. 5 X 10(-7) M Ca2+ as compared to 10(-6) M Ca2+ for active Ca2+ transport. Addition of the calmodulin antagonists pimozide and chlorpromazine blocked calmodulin activation of the Ca2+-dependent K+ channel. The observation that activation of the K+ channel occurs at Ca2+ concentrations which are lower than those required for maximum stimulation of the calcium pump suggests that these processes are dependent on two states of the calmodulin molecule, characterized by a lower or higher amount of Ca2+ bound to calmodulin.

A Ca2+-refractory state of the Ca-sensitive K+ permeability mechanism in sickle cell anaemia red cells

Simultaneous measurements of Ca content and 42K+ influx in sickle cell anaemia red cells confirm predictions from earlier data in the literature that the increased Ca content of sickle cell anaemia cells which are not metabolically depleted does not cause a quinine-sensitive increase in K+ permeability. It is shown that the ionophore, A23187, can cause the Ca contained inside sickle cell anaemia cells to activate the quinine-sensitive K+-permeability mechanism. This demonstrates the existence of a Ca2+-refractory state of the K+ channel in sickle cell anaemia cells and a direct stimulatory effect of the ionophore A23187 on its Ca sensitivity.