Is the Ca2+-sensitive K+ channel under metabolic control in human red cells?

The Action of Red Cell Calcium Ions on Human Erythrophagocytosis in Vitro

In the present work we have studied in vitro the effect of increasing red cell Ca²⁺ ions on human erythrophagocytosis by peripheral monocyte-derived autologous macrophages. In addition, the relative contribution to phagocytosis of phosphatidylserine exposure, autologous IgG binding, complement deposition and Gárdos channel activity was also investigated. Monocytes were obtained after ficoll-hypaque fractionation and induced to transform by adherence to glass coverslips, for 24 h at 37°C in a RPMI medium, containing 10% fetal calf serum. Red blood cells (RBC) were loaded with Ca²⁺ using 10 μM A23187 and 1 mM Ca-EGTA buffers, in the absence of Mg²⁺. Ca²⁺-loaded cells were transferred to above coverslips and incubated for 2 h at 37°C under various experimental conditions, after which phagocytosis was assessed by light microscopy. Confirming earlier findings, phagocytosis depended on internal Ca²⁺. Accordingly; it was linearly raised from about 2–15% by increasing the free Ca²⁺ content of the loading solution from 0.5 to 20 μM, respectively. Such a linear increase was virtually doubled by the presence of 40% autologous serum. At 7 μM Ca²⁺, the phagocytosis degree attained with serum was practically equal to that obtained with either 2 mg/ml affinity-purified IgG or 40% IgG-depleted serum. However, phagocytosis was reduced to levels found with Ca²⁺ alone when IgG-depleted serum was inactivated by heat, implying an involvement of complement. On the other hand, phagocytosis in the absence of serum was markedly reduced by preincubating macrophages with phosphatidylserine-containing liposomes. In contrast, a similar incubation in the presence of serum affected it partially whereas employing liposomes made only of phosphatidylcholine essentially had no effect. Significantly, the Gárdos channel inhibitors clotrimazole (2 μM) and TRAM-34 (100 nM) fully blocked serum-dependent phagocytosis. These findings show that a raised internal Ca²⁺ promotes erythrophagocytosis by independently triggering phosphatidylserine externalization, complement deposition and IgG binding. Serum appeared to stimulate phagocytosis in a way dependent on Gárdos activity. It seems likely that Ca²⁺ promoted IgG-binding to erythrocytes via Gárdos channel activation. This can be an important signal for clearance of senescent human erythrocytes under physiological conditions.

Modulation of Ca2+-activated K+ channels of human erythrocytes by endogenous protein kinase C

Single IK(Ca) channels of human erythrocytes were studied with the patch-clamp technique to define their modulation by endogenous protein kinase C (PKC). The perfusion of the cytoplasmic side of freshly excised patches with the PKC activator, phorbol 12-myristate 13-acetate (PMA), inhibited channel activity. This effect was blocked by PKC(19-31), a peptide inhibitor specific for PKC. Similar results were obtained by perfusing the membrane patches with the structurally unrelated PKC activator 1-oleoyl-2-acetylglycerol (OAG). Blocking of this effect was induced by perfusion with PKC(19-31) or chelerythrine. Channel activity was not inhibited by the PMA analog 4alpha-phorbol 12,13-didecanoate (4alphaPDD), which has no effect on PKC. Activation of endogenous cAMP-dependent protein kinase (PKA), which is known to up-modulate IK(Ca) channels, restored channel activity previously inhibited by OAG. The application of OAG induced a reversible reduction of channel activity previously up-modulated by the activation of PKA, indicating that the effects of the two kinases are commutative, and antagonistic. Kinetic analysis showed that down-regulation by PKC mainly changes the opening frequency without significantly affecting mean channel open time and conductance. These results provide evidence that an endogenous PKC down-modulates the activity of native IK(Ca) channels of human erythrocytes. Our results show that PKA and PKC signal transduction pathways integrate their effects, determining the open probability of the IK(Ca) channels.

Caffeine activates a mechanosensitive Ca(2+) channel in human red cells

Caffeine is known to activate influx of both mono- and divalent cations in various cell types, suggesting that this xanthine opens non-selective cation channels at the plasma membrane. This possibility was investigated in human erythrocytes, studying the caffeine action on net Ca(2+), Na(+) and K(+) movements in ATP-depleted cells. Whole populations and subpopulations of young and old erythrocytes were employed. Caffeine was tested in the presence of known mechanosensitive channel blockers (Gd(3+), neomycin and amiloride) and ruthenium red as a possible inhibitor. Caffeine enhanced net cation fluxes in a concentration-dependent way. In whole populations, the Ca(2+) entry elicited by 20 mM caffeine was fully suppressed by Gd(3+) (5 microM), amiloride (250 microM) and ruthenium red (100 microM) and partially blocked by neomycin (100 microM). The above blockers also inhibited caffeine-dependent Na(+) entry whilst showing antagonistic effects on the corresponding K(+) efflux. These compounds fully suppressed hypotonically-induced (-35 mOsm/kg) Ca(2+) influx at nearly the same concentrations completely blocking caffeine-stimulated Ca(2+) entry. The effect of inhibitors on Ca(2+) influx in young cells exceeded that in old cells at similar concentrations. The results clearly show that caffeine stimulates a stretch-activated Ca(2+) channel in human red cells and that aged cells are less susceptible to mechanosensitive channel blockers.

Calmodulin antagonists do not inhibit IK(Ca) channels of human erythrocytes

Patch-clamp recordings were performed to study the effects of three calmodulin (CaM) antagonists on the gating of intermediate calcium-activated K(+) channels (IK(Ca)) of human erythrocytes. In the cell-attached configuration, both opening frequency and open probability of IK(Ca) channels were not significantly different in control cells and in those incubated with calmidazolium, trifluoperazine or W7. IK(Ca) channels in excised membrane patches, were normally activated by the calcium bathing the cytoplasmic side in the presence of CaM antagonists, at calcium concentrations ranging from 10(-7) to 10(-3) M. The activity of IK(Ca) channels, which had been previously up-modulated by an endogenous cAMP-dependent protein kinase, was not inhibited when perfused with CaM antagonists. The results presented in this study demonstrate that calmodulin antagonists do not inhibit the activity of native IK(Ca) channels of human erythrocytes. These data are in accordance with findings on the cloned IK(Ca) indicating that calmodulin is constitutively associated with these channels.

Properties of an inwardly rectifying ATP-sensitive K+ channel in the basolateral membrane of renal proximal tubule

The potassium conductance of the basolateral membrane (BLM) of proximal tubule cells is a critical regulator of transport since it is the major determinant of the negative cell membrane potential and is necessary for pump-leak coupling to the Na+,K+-ATPase pump. Despite this pivotal physiological role, the properties of this conductance have been incompletely characterized, in part due to difficulty gaining access to the BLM. We have investigated the properties of this BLM K+ conductance in dissociated, polarized Ambystoma proximal tubule cells. Nearly all seals made on Ambystoma cells contained inward rectifier K+ channels (gammaslope, in = 24.5 +/- 0.6 pS, gammachord, out = 3.7 +/- 0.4 pS). The rectification is mediated in part by internal Mg2+. The open probability of the channel increases modestly with hyperpolarization. The inward conducting properties are described by a saturating binding-unbinding model. The channel conducts Tl+ and K+, but there is no significant conductance for Na+, Rb+, Cs+, Li+, NH4+, or Cl-. The channel is inhibited by barium and the sulfonylurea agent glibenclamide, but not by tetraethylammonium. Channel rundown typically occurs in the absence of ATP, but cytosolic addition of 0. 2 mM ATP (or any hydrolyzable nucleoside triphosphate) sustains channel activity indefinitely. Phosphorylation processes alone fail to sustain channel activity. Higher doses of ATP (or other nucleoside triphosphates) reversibly inhibit the channel. The K+ channel opener diazoxide opens the channel in the presence of 0.2 mM ATP, but does not alleviate the inhibition of millimolar doses of ATP. We conclude that this K+ channel is the major ATP-sensitive basolateral K+ conductance in the proximal tubule.

Differences in Ca2+ pumping activity between sub-populations of human red cells

It has been shown recently that the free Ca2+ content of human red cells rises during ageing in vivo. With the aim of determining the mechanisms involved in such a change, we have investigated some aspects of Ca2+ homeostasis. Both the initial rate of Ca2+ influx and some kinetic parameters of the Ca2+ pump of human red cells were studied in light and dense sub-populations obtained through stringent, self-formed Percoll gradients. At 37 degrees C and pH 7.4, no differences in Ca2+ entry were found. By contrast, either at pH 7.0 or 7.4, the maximal Ca2+ extrusion rate of the approximately 10% heaviest cells was one-half of the corresponding lighter ones. The results demonstrate that the elevated free Ca2+ concentration distinctive of senescent cells, arises from a reduction in Ca2+ extrusion capacity during ageing. The possible physiological significance of this finding is discussed.

Activation of red cell Ca2(+)-activated K+ channel by Ca2+ involves a temperature-dependent step

We found that vanadate-induced 45Ca2+ uptake by red cells is maximal at 25 degrees C. At this temperature, the Cai-induced increase of the K+ permeability (the Gárdos effect) shows a lag (up to 8 min) which is not observed at 37 degrees C. This cannot be explained by the lack of availability of Ca2+ for the Ca2(+)-activated K+ channel, and suggests that its activation by Ca2+ is mediated by a temperature-dependent mechanism which remains unknown so far. The lag is not observed when the Gárdos effect was initiated by propranolol. This shows that the putative temperature-dependent step is different from chloride transport.

Metabolic control of the K+ channel of human red cells

The effects of cAMP, ATP and GTP on the Ca2(+)-dependent K+ channel of fresh (1-2 days) or cold-stored (28-36 days) human red cells were studied using atomic absorption flame photometry of Ca2(+)-EGTA loaded ghosts which had been resealed to monovalent cations in dextran solution. When high-K+ ghosts were incubated in an isotonic Na+ medium, the rate constant of Ca2(+)-dependent K+ efflux was reduced by a half on increasing the theophylline concentration to 40 mM. This effect was observed in ghosts from both fresh and stored cells, but only if they were previously loaded with ATP. The inhibition was more marked when Mg2+ was added together with ATP, and it was abolished by raising free Ca2+ to the micromolar level. Like theophylline, isobutyl methylxanthine (10 mM) also affected K+ efflux. cAMP (0.2-0.5 mM), added both internally and externally (as free salt, dibutyryl or bromide derivatives), had no significant effect on K+ loss when the ghost free-Ca2+ level was below 1 microM, but it was slightly inhibitory at higher concentrations. The combined presence of cAMP (0.2 mM) plus either theophylline (10 mM), or isobutyl methylxanthine (0.5 mM), was more effective than cAMP alone. This inhibition showed a strict requirement for ATP plus Mg2+ and it was not overcome by raising internal Ca2+. Ghosts from stored cells seemed more sensitive than those from fresh cells, to the combined action of cAMP and methylxanthines. Loading ATP into ghosts from fresh or stored cells markedly decreased K+ loss. Although this effect was observed in the absence of added Mg2+ (0.5 mM EDTA present), it was potentiated upon adding 2 mM Mg2+. The K+ efflux from ATP-loaded ghosts was not altered by dithio-bis-nitrobenzoic acid (10 mM) or acridine orange (100 microM), while it was increased two- to fourfold by incubating with MgF2 (10 mM), or MgF2 (10 mM) + theophylline (40 mM), respectively. By contrast, a marked efflux reduction was obtained by incorporating 0.5 mM GTP into ATP-containing ghosts. The degree of phosphorylation obtained by incubating membranes with (gamma-32P)ATP under various conditions affecting K+ channel activity, was in direct correspondence to their effect on K+ efflux. The results suggest that the K+ channel of red cells is under complex metabolic control, via cAMP-mediated and nonmediated mechanisms, some which require ATP and presumably, involve phosphorylation of the channel proteins.

Activation of potassium channels in renal epithelioid cells (MDCK) by extracellular ATP

Extracellular ATP has been shown to stimulate transepithelial chloride transport in confluent Madin-Darby canine kidney (MDCK) cell layers and to enhance potassium conductance in subconfluent MDCK cells. The present study has been performed to test for the effect of extracellular ATP on channel activity in patches from subconfluent MDCK cells. Within 8 s, addition of extracellular ATP (10 mumol/l) leads to a sustained, but fully reversible, appearance of potassium-selective channels in cell-attached patches [increase of open probability from 0.03 +/- 0.02 (n = 10) to 0.50 +/- 0.07 (n = 6)]. With the use of pipettes filled with 145 mmol/l KCl, inwardly rectifying property of the channels is disclosed with a single-channel conductance of 65.7 +/- 3.1 pS (n = 9) at zero potential difference between pipette and bath and with a reversal potential of 75.4 +/- 2.0 mV (n = 5; pipette negative vs. reference in the bath). The open probability of the channels is not significantly modified by altering pipette potential from -50 mV, pipette positive, to 50 mV, pipette negative. At extracellular calcium activities of less than 10 nmol/l, ATP leads to a transient activation of channels. In conclusion, extracellular ATP activates inwardly rectifying potassium channels in the cell membrane of subconfluent MDCK cells. A sustained activation of the channels requires the presence of extracellular calcium and is probably mediated by increases in intracellular calcium.

A stretch-activated K+ channel sensitive to cell volume

The role of K+ channels in cell osmoregulation was investigated by using the patch-clamp technique. In cell-attached patches from Necturus proximal tubule, the short-open-time K+ channel at the basolateral membrane could be stretch-activated by pipette suction, where a negative pressure of 6 cm H2O (588.6 Pa) was sufficient to increase the open probability of the channel by a factor of 4.0 +/- 0.8 (n = 7 tubules). A 50% reduction in bath osmolarity increased cell volume by 66 +/- 10% and increased the K+-channel open probability by a factor of 5.8 +/- 1.4 (n = 7) in the same cell-attached patches that were activated by pipette suction. A kinetic analysis indicates one open state and at least two closed states for this epithelial K+ channel. Both suction and swelling shorten the longest time constant of the closed-time distribution by a factor of 3, without significant effect on either the mean open time or the shorter closed-state time constant. The similar effect of suction and swelling is consistent with the hypothesis that stretch-activated K+ channels mediate the increase in macroscopic K+ conductance that occurs during osmoregulation of amphibian proximal tubules. Calculations based on a simple model indicate that small increments in cell volume could produce statistically significant increases in K+-channel activity.

Effect of extracellular adenosine triphosphate on electrical properties of subconfluent Madin-Darby canine kidney cells

1. The present study has been performed to test for an influence of extracellular ATP on the potential differences across the cell membrane (PD) in subconfluent MDCK cells utilizing conventional microelectrodes. 2. In the absence of ATP, the mean measured PD was -47.5 +/- 0.3 mV (+/- S.E.M., n = 320). Application of 10 mumol/l ATP leads to rapid (less than 2 s) hyperpolarization of the cell membrane by -18.5 +/- 0.4 mV (n = 221), reduction of input resistance by 14 +/- 1 M omega (n = 106) and increase of the sensitivity of PD to alterations of extracellular potassium. 3. The concentration needed for half-maximal effect (K1/2) of ATP is approximately 0.5 mumol/l. ATP-gamma-S (K1/2 approximately 0.4 mumol/l) aand ADP (K1/2 approximately 0.9 similarly effective, whereas up to 1 mmol/l AMP or adenosine does not significantly alter PD. Application of 10 mumol/l theophylline, 1 mumol/l phentolamine and 10 mumol/l indomethacin does not blunt the hyperpolarizing effect of ATP. 4. The ATP-induced hyperpolarization is completely abolished in the presence of 1 mmol/l quinidine but only incompletely by 0.1 mmol/l quinidine or 1 mmol/l barium. In calcium-free extracellular fluid (1 mmol/l EDTA added) PD is 18.5 +/- 1.7 mV (n = 18). With reduced extracellular calcium, the hyperpolarizing effect of ATP is blunted (-12.3 +/- 1.6 mV, n = 18) and only transient. 5. In conclusion, ATP hyperpolarizes MDCK cells by increasing the potassium conductance. The activation of potassium channels requires calcium.

The effect of cyanide on apparent potassium conductance across the peritubular cell membrane of frog proximal tubules

To test for the effect of cyanide on frog proximal renal tubules the potential difference across the peritubular cell membrane (PDpt) has been recorded continuously before and during peritubular application of 1 mmol/l cyanide using conventional microelectrodes. Before application of cyanide PDpt amounts to -61.5 +/- 2.2 mV in the absence of luminal substrate. Cyanide depolarizes the peritubular cell membrane by +18.8 +/- 2.3 mV/10 min in the presence and by +4.5 +/- 0.9 mV/10 min in the absence of luminal substrate. The rapid depolarization of the cell membranes to addition of glucose to luminal perfusate is not significantly influenced by exposure to cyanide, whereas the influence of altered peritubular potassium concentration (from 3 to 9 mmol/l) is significantly reduced from +15.2 +/- 1.7 mV to +8.7 +/- 1.8 mV. Following exposure to cyanide the lumped resistance of the luminal and peritubular cell membranes increases significantly by 36 +/- 7%/6 min, and the cellular core resistance significantly by 14 +/- 6%/6 min. As a result, cyanide markedly decreases the peritubular potassium conductance, depolarizes the cell membranes and reduces the driving force for sodium coupled transport processes. Thus cyanide fully mimics the effects of ouabain, although cyanide in contrast to ouabain is expected to deplete the cells from ATP. In conclusion ATP/ADP is not likely to play a major role in the regulation of sodium coupled transport processes and peritubular potassium conductance in amphibian proximal tubules.

Regulation of the basolateral potassium conductance of the Necturus proximal tubule

Two methods, the measurement of the response of the basolateral membrane potential (Vbl) of proximal tubule cells of Necturus to step changes in basolateral K+ concentration, and cellular cable analysis, were used to assess the changes in basolateral potassium conductance (GK) caused by a variety of maneuvers. The effects of some of these maneuvers on intracellular K+ activity (aiK) were also evaluated using double-barreled ion-selective electrodes. Perfusion with 0 mM K+ basolateral solution for 15 min followed by 45 min of 1 mM K+ solution resulted in a fall in basolateral potassium (apparent) transference number (tK), Vbl and aiK. Results of cable analysis showed that total basolateral resistance, Rb, rose. The electrophysiological effects of additional manipulations, known to inhibit net sodium reabsorption across the proximal tubular epithelium of Necturus, were also investigated. Ouabain caused a fall in tK accompanied by large decreases in aiK and Vbl. Lowering luminal sodium caused a fall in tK and a small reduction in Vbl. Selective reduction of peritubular sodium, a maneuver that has been shown to block sodium transport from lumen to peritubular fluid, also resulted in a significant decrease in tK. These results suggest that GK varies directly with rate of transport of the sodium pump, irrespective of the mechanism of change in pump turnover.

On the mechanisms of ATP-induced and succinate-induced redistribution of cations in isolated rat liver cells

1. The ability of external ATP to induce calcium uptake in isolated rat liver cells was further characterized. Stimulation of calcium uptake was specific for ATP, other nucleotides or ATP metabolites had no comparable effect. ATP was dephosphorylated while stimulating calcium uptake, but there was no stoichiometry between ATP hydrolysis and calcium uptake nor did dephosphorylation depend on calcium concentration. ATP acted from outside and was dephosphorylated by an ecto-ATPase of the cells. 2. In addition to its direct action, ATP enhanced succinate-dependent calcium uptake in a cooperative fashion. This is best explained by different sites of action. ATP increases cell membrane permeability while succinate stimulates uptake into mitochondria. 3. ATP was able to lower Na+ and K+ gradients and the pH gradient between cells and incubation medium. Increasing calcium concentration counteracted this effect though calcium uptake was then stimulated. 4. Succinate alone did not affect monovalent cation gradients but raised the pH gradient. It partially counteracted the ATP effects on these gradients. 5. Since catecholamine-like actions of ATP may be mediated by an increase in cytoplasmic calcium concentration, the action of extracellular ATP can be taken as a model to study the role of calcium as a transmitter of hormone actions. From interdependence between ATP-stimulated and succinate-stimulated calcium uptake, conclusions can be drawn on the resulting cytoplasmic calcium concentration and its effect on plasma membrane permeability.

Active calcium transport in red cell ghosts resealed in dextran solutions

1. Human erythrocytes when lysed and resealed to Ca in the presence of dextran can be readily separated from the suspending medium by low-speed centrifugation. 2. Ghosts trapped Ca and EGTA at the same ratio as present in the haemolytic medium and remained tight to Ca after washing and subsequent incubation for up to 90 min at 37 degrees C. 3. Ca extrusion could be promoted by substrates other than ATP only from ghosts that had been loaded with low free Ca concentrations (1--22 microM). The order of activation by the various substrates employed was ATP greater than adenine + inosine greater than inosine. 4. The kinetics of extrusion depended markedly on internal free Ca. The system showed a high affinity state (KCa about 3 microM; V = 0.34 mumol Ca/ml ghosts per min) at low concentrations (1--22 microM) and a low affinity state (KCa about 250 microM; V = 0.17 mumol Ca/ml ghosts per min) at high concentrations (0.2--4.0 mM). 5. Both at low and at high free Ca, La-sensitive ATP hydrolysis was closely correlated with La-dependent Ca efflux, in keeping with an stoichiometry of 1.6. The rate of extrusion was maximal in the presence of 160 mM KCl and decreased to various extents when K was fully replaced by different cations, following the order K greater than Na = choline greater than Mg. 7. The efflux rate of high-K ghosts, resealed to alkaline cations, was stimulated by external Na, whilst Mg and choline was practically without effect. 8. The results indicate that human red cells possess a powerful Ca extrusion mechanism, the activity of which can be modulated by alkaline cations.

Stimulation of monovalent cation fluxes by electron donors in the human red cell membrane

When human red cells are incubated at 37 degrees C with the artificial electron donor system ascorbate + phenazine methosulphate the fluxes of Rb+ (K+) through the cell membrane are increased. The effect of this donor system is much stronger in energy-depleted than in normal cells. The same effects are produced by HS-glutathione, NADH or NADPH loaded into resealed ghosts, but these electron donors were ineffective when added to the incubation medium. The Rb+ (K+) fluxes induced by electron donors resemble closely those induced by an increase of intracellular Ca2+ (Gardos effect). The electron donors require the presence of intracellular Ca2+ to be effective, but at levels that do not stimulate by themselves the fluxes of K+. Flavoenzyme inhibitors (atebrin and chlorpromazine), oligomycin and quinine prevented the effects of both electron donors and Ca2+ alone; antimycin, upcouplers and ethacrynic acid inhibited them partially; ouabain, furosemide, and rotenone had no effect. The results could be explained if the effect of electron donors is to bring about a change in the redox state of some membrane component(s) that makes intracellular Ca2+ more effective to elicit rapid K+ movements. Plasma membrane oxidoreductase activities could be engaged in this change.

Oscillations of membrane potential in L cells. IV. Role of intracellular Ca2+ in hyperpolarizing excitability

Effects of divalent cations on oscillations of membrane potentials (i.e., spontaneous repetitive hyperpolarizing responses) and on hyperpolarizing responses induced by electrical stimuli as well as on resting potentials were studied in large nondividing L cells. Deprivation of Ca2+ from the external medium inhibited these hyperpolarizing responses accompanying slight depolarization of the resting potential Sr2+ or Mn2+ applied to the external medium in place of Ca2+ was able to substitute for Ca2+ in the generation of hyperpolarizing responses, while Mg2+, Ba2+ or La3+ suppressed hyperpolarizing responses. The addition of A23187 to the bathing medium or intracellular injection of Ca2+, Sr2+, Mn2+ or La3+ induced membrane hyperpolarization. When the external Ca2+, Sr2+ or Mn2+ concentration was increased, the resting potential also hyperpolarized, in a saturating manner. The amplitude of maximum hyperpolarization produced by high external Ca2+ was of the same order of magnitude as those of hyperpolarizing responses and was dependent on the external K+ concentration. In the light of these experimental observations, it was deduced that the K+ conductance increase associated with the hyperpolarizing excitation is the result of an increase in the intracellular concentration of free Ca2+ mainly derived from the external solution.

Unmasking of a potassium leak in resealed human red blood cell ghosts

A selective potassium leak is observed in resealed, human red blood cell ghosts when hemolysis is performed with distilled water at pH 6.5, 0 degrees C. The leak, which has a maximum near pH 6.7, is suppressed when either magnesium or a chelating agent is present in the hemolysing medium. The potassium leak has the additional property that it can be suppressed after resealing by washing the ghost membranes in a medium containing a low concentration of ATP or EDTA. The data suggest that through the dilution of endogenous chelating agents at hemolysis a potassium leak may be unmasked.