Effects of electron donors on Ca2+-dependent K+ transport in one-step inside-out vesicles from the human erythrocyte membrane

Effect of Insulin on Ca2+-Dependent Hyperpolarization in Erythrocytes from Healthy Donors and Patients with Type 2 Diabetes Mellitus Accompanied by Arterial Hypertension

Insulin decreased A23187-induced hyperpolarization of the erythrocyte membrane in healthy donors. These data indicate that insulin plays a role in the regulation of Ca(2+)-activated potassium channels in human erythrocytes. However, insulin had little effect on hyperpolarization response of cells induced by artificial ascorbate--phenazine methosulfate donor-acceptor system. Addition of insulin to cell suspension from patients with type 2 diabetes mellitus did not modulate hyperpolarization of the erythrocyte membrane induced by A23187 or ascorbate-phenazine methosulfate, which reflects impairment of regulatory mechanisms for Ca(2+)-activated potassium channels in erythrocytes.

Membrane transport in sickle cell disease

We have reviewed here a number of membrane transport events in red cells from normal individuals and sickle cell patients which respond to changes in O(2) tension. Some deoxygenation-induced changes in membrane permeability are unique to HbS cells and contribute to their dehydration and subsequent sickling. Polymerization of HbS, or specific oxidant damage (or altered redox potential), is a likely factor underlying the abnormal behavior. The key regulatory sites within the membrane or associated proteins remain uncertain and their identity will form the focus of future research. A model for sickle cell dehydration is presented. Inhibition of these permeability changes represents possible avenues for future chemotherapy to ameliorate the condition.

The effects of adriamycin and adriamycin complexes with transitional metals on Ca(2+)-dependent K+ channels of human erythrocytes

The influence of adriamycin (ADR) and ADR complexes with transitional metals Fe2+, Cu2+ and Co2+ on Ca(2+)-dependent K+ channels of human erythrocytes was investigated. We show that the anthracycline moiety of ADR increases Ca(2+)-dependent K+ efflux from erythrocytes, induced by low concentrations of propranolol, while the whole molecule of ADR has not any effect on Ca(2+)-dependent K+ channels, induced by propranolol or A23187 and on Pb(2+)-dependent K+ efflux. Ethidium bromide, verapamil and trifluoroperazine inhibited Ca(2+)-dependent K+ efflux, induced by high doses of propranolol. The anthracycline moiety of ADR is able to abolish blocking effect of ethidium bromide and verapamil, but does not influence the blocking effect of trifluoroperazine. We further show that ADR complexes with Fe2+, Cu2+ and Co2+ are potent inhibitors of Ca(2+)-dependent K+ efflux, induced by propranolol, but not of Pb(2+)-dependent K+ efflux. On the contrary, ADR-Fe3+ complex activates K(+)-permeability of human red blood cell. It is suggested that opposite effects of anthracycline moiety of ADR and ADR complexes with transitional metals on Ca(2+)-dependent K+ channels, induced by propranolol is due to their influence on the pathways of Ca2+ transport into cells, rather than their action directly on K+ channels.

Effects of plasma membrane oxidoreductases on Ca2+ mobilization and protein phosphorylation in rat brain synaptosomes

We have investigated the possible role of plasma membrane oxidoreductases in the Ca2+ export mechanisms in rat brain synaptic membranes. Ca2+ efflux in nerve terminals is controlled both by a high-affinity/low capacity Mg-dependent ATP-stimulated Ca2+ pump and by a low affinity/high capacity ATP-independent Na(+)-Ca2+ exchanger. Both Ca2+ efflux mechanisms were strongly inhibited by pyridine nucleotides, in the order NADP greater than NAD greater than NADPH greater than NADH with IC50 values of ca. 10 mM for NADP and ca. 3 mM for the other agents in the case of the ATP-driven Ca2+ pump and with IC50 values between 8 and 10 mM for the Na(+)-Ca2+ exchanger. Oxidizing agents such as DCIP3 and ferricyanide inhibited the ATP-driven Ca2+ efflux mechanism but not the Na(+)-Ca2+ exchanger. In addition, full activation of plasma membrane oxidoreductases requires both an acceptor and an electron donor; therefore the combined effects of both substrates added together were also studied. When plasma membrane oxidoreductases of the synaptic plasma membrane were activated in the presence of both NADH (or NADPH) and DCIP or ferricyanide, the inhibition of the ATP-driven Ca2+ pump was optimal; by contrast, the pyridine nucleotide-mediated inhibition of the Na(+)-Ca2+ exchanger was partially released when both substrates of the plasma membrane oxidoreductases were present together. Furthermore, the activation of plasma membrane oxidoreductases also strongly inhibited intracellular protein phosphorylation in intact synaptosomes, mediated by either cAMP-dependent protein kinase, Ca2+ calmodulin-dependent protein kinases, or protein kinase C.

All or none cell responses of Ca2+-dependent K channels elicited by calcium or lead in human red cells can be explained by heterogeneity of agonist distribution

We have studied the all or none cell response of Ca2+-dependent K+ channels to added Ca in human red cells depleted of ATP by incubation with iodoacetate and inosine. A procedure was used which allows separation and differential analysis of responding and nonresponding cells. Responding (H for heavy) cells incubated in medium containing 5 mM K lose KCl and water and increase their density to the point of sinking on diethylphthalate (specific gravity = 1.12) on centrifugation. Nonresponding (L for light) cells do not lose KCl at all. There is no intermediate behavior. Increasing the Ca concentration in the medium increases the fraction of cells which become H. No differences in the sensitivity to Ca2+ of the individual K+ channels were detected in inside-out vesicles prepared either from H or from L cells. The Ca content of H cells was higher than that of L cells. Cells depleted of ATP by incubation with iodoacetate and inosine sustain pump-leak Ca fluxes of about 15 mumol/liter cells per hour. ATP seems to be resynthesized in these cells at the expense of cell 2,3-diphosphoglycerate stores at a rate of about 150 mumol/liter cells per hour. Inhibition of 2,3-diphosphoglycerate phosphatase by tetrathionate increased 6-8 times the measured rate of uptake of external 45Ca. This was accompanied by an increase in the fraction of H cells. All or none cell responses of Ca2+-dependent K channels have also been evidenced in intact human red cells on addition of Pb. They have the same characteristics as those in responding and nonresponding cells. The detailed study of the kinetics of Pb-induced shrinkage of red cells suspended in medium containing 5 mM K showed that changes of Pb concentration changed not only the fraction of H cells but also the rate of shrinkage of responding cells. H cells generated by Pb treatment contained significantly more lead than L cells. The above results suggest that the two all or none cell responses studied here can be explained by heterogeneity of agonist distribution among cells. Since pump-leak fluxes exist in both cases, differences of agonist distribution could be generated by heterogeneity of pumping among cells. This interpretation turns interest from K channels to Ca pumps to explain the heterogeneous behavior of red cells in response to a uniform stimulus.

An estimate of the number of Ca2+-dependent K+ channels in the human red cell

An original approach has been designed to count Ca2+-dependent K+ channels in the human red cell using a preparation of inside-out vesicles. The relative frequency of vesicles having no K+ channels is estimated from the fraction of 42K+ (or 86Rb+) which is not released from loaded vesicles on maximal stimulation with Ca2+. The mean number of channels per vesicle is then calculated from this figure assuming a Poisson distribution for the K+ channels. From this value and the mean vesicular radius, computed from the volume/surface ratio, the mean number of channels per cell can be estimated. A value of 142 +/- 27 (mean +/- S.E.) was obtained, which is well above that estimated by comparison of unitary conductance and tracer equilibration rate measurements (about 10 channels/cell, Grygorczyk, R. Schwarz, W. and Passow, H. (1984) Biophys. J. 45, 693-698), but compares favourably with the channel density inferred from comparison with the number of Na+ pumps in a similar preparation of inside-out vesicles (100-200/cell, Lew, V.L., Muallem, S. and Seymour, C.A. (1982) Nature 296, 742-744). The procedure described here can be considered for general application as an alternative to other known procedures.

Decrease of NADH in yeast cells by external ferricyanide reduction

Ferricyanide reduction catalyzed by vitamin K-3 was accompanied by the decrease in intracellular (NAD(P)H concentration of yeast cells, and the rate of ferricyanide reduction depended on intracellular concentration of NADH rather than NADPH. The addition of glucose to the cell suspensions enhanced both ferricyanide reduction and intracellular NADH concentration. The catalytic action of vitamin K-3 on ferricyanide reduction was observed in the presence of NADH and plasma membrane preparations. As the toxic action of vitamin K-3 on cell growth of yeast was enhanced by addition of ferricyanide, ferricyanide reduction catalyzed by vitamin K-3 may inhibit cell growth by decreasing intracellular NADH concentration.

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.

Analysis of the all or nothing behaviour of Ca-dependent K channels in one-step inside-out vesicles from human red cell membranes

The all or nothing behaviour of Ca2+-dependent K+ channels has been analyzed in one-step inside-out vesicles. There is a threshold for Ca2+ below which the K+ channels remain silent, and which ranges between the 10(-6) and 10(-8) M for different vesicles under the experimental conditions tested, in the absence of Mg2+. The increase of Ca2+ concentration within this range recruits a larger fraction of the vesicles to the active (permeable to 86Rb+) state. The apparent rate of 86Rb+ transport through each individual channel was found to increase, however, with Ca2+ concentration. This finding is not an artefact due to size heterogeneity of the vesicle population, and it is consistent with the variations of the mean open time of the channels with Ca2+ concentration reported previously in patch-clamp experiments. The electron donor system ascorbate + phenazine-methosulphate increases the rate of 86Rb+ transport through the channels whereas oxidized cytochrome c has the opposite effect.

Inhibition of Ca2+-dependent K+ channels by lead in one-step inside-out vesicles from human red cell membranes

Pb2+ modified the apparent threshold sensitivity to Ca2+ of individual K+ channels with a biphasic time-course. At first, the sensitivity to Ca2+ was lowered with the result of a decrease of the fraction of activated vesicles at a given Ca2+ concentration. Later, Pb2+ increased the sensitivity to Ca2+ and the fraction of activated vesicles. The increase of Pb2+ concentration increased the extent of the initial inhibition but decreased its duration. The inhibitory effect was not observed when the addition of Ca2+ preceded the addition of Pb2+. The presence of Mg2+ in the incubation medium was also required. In the absence of Mg2+, Pb2+ decreased the rate of uptake of 86Rb, but no decrease in the fraction of activated vesicles could be demonstrated.

Modulation of Ca2+-dependent K+ transport by modifications of the NAD+/NADH ratio in intact human red cells

The effects of variations of the NAD+/NADH quotient on the uptake of 86Rb by human red cells loaded by non-disruptive means with the chelator Benz2 and different amounts of 45Ca has been examined. The NAD+/NADH quotient was modified by the addition of pyruvate and/or lactate or xylitol. It was found that the uptake of 86Rb at a given intracellular Ca2+ concentrations was faster in the reduced state (lactate or xylitol added). Metabolic changes were associated with variations of the redox state. However, glycolitic intermediates did not significantly modify the apparent affinity for Ca2+ of the Ca2+-dependent K+ channel in one-step inside-out vesicles prepared from the erythrocyte membrane. Taken together, these results suggest that modifications of the cytoplasmic redox potential could modulate the sensitivity to Ca2+ of the Ca2+-dependent K+ channel in the human red cells under physiological conditions. This conclusion is consistent with previous findings in inside-out vesicles of human erythrocytes using artificial electron donors.

The effect of ETH 1001 on ion fluxes across red blood cell membranes

The calcium selective ionophore, ETH 1001, and the divalent cation ionophore, A23187, promoted Ca2+ flux across RBC membranes under various experimental conditions. ETH 1001 did not promote the passive movement of Mg2+ whereas A23187 did. The results confirm the potential application of ETH 1001 as a Ca2+ selective ionophore for biological membranes.

Evidence against involvement of the human erythrocyte plasma membrane Ca2+-ATPase in Ca2+-dependent K+ transport

Two tests were performed to assess the relationship between the Ca2+-activated K+ channel and the Ca2+-pumping ATPase in human erythrocytes. Antibodies against the purified ATPase inhibited the ATPase in resealed erythrocytes, but had no effect on the K+ channel (as assessed by Rb+ efflux). Reconstituted liposomes containing the purified active Ca2+-pumping ATPase showed no Ca2+-activated Rb+ influx. Both of these results suggest that some molecule other than the Ca2+-ATPase is responsible for the K+ channel.