The Ca2+-sensitive K+-conductance of the human red cell membrane is strongly dependent on cellular pH

Effect of cytosolic pH on epithelial Na+ channel in normal and cystic fibrosis sweat ducts

The activities of cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel and the amiloride-sensitive epithelial Na(+) channel (ENaC) are acutely coordinated in the sweat duct. However, the mechanisms responsible for cross-talk between these ion channels are unknown. Previous studies indicated that luminal pH of sweat ducts varies over 3 pH units and that the cytoplasmic pH affects both CFTR and ENaC. Therefore, using basolaterally alpha-toxin-permeabilized apical membrane preparations of sweat ducts as an experimental system, we tested the hypothesis that the cytosolic pH may mediate the cross-talk between CFTR and ENaC. We showed that while luminal pH had no effect, cytosolic pH acutely affected ENaC activity. That is, acidic pH inhibited, while basic pH activated, ENaC. pH regulation of ENaC appears to be independent of CFTR or endogenous kinase activities because basic pH independently stimulated ENaC (1) in normal ducts even when CFTR was deactivated, (2) in CF ducts that lack CFTR in the plasma membranes and (3) after blocking endogenous kinase activity with staurosporine. Considering the evidence of Na(+)/H(+) exchange (NHE) activity as shown by the expression of mRNA and function of NHE in the basolateral membrane of the sweat duct, we postulate that changes in cytosolic Na(+) ([Na(+)]( i )) may alter cytosolic pH (pH( i )) as salt loads into the cell during electrolyte absorption. These changes may play a role in coordinating the activities of ENaC and CFTR during transepithelial salt transport.

Volume control in sickle cells is facilitated by the novel anion conductance inhibitor NS1652

A low cation conductance and a high anion conductance are characteristic of normal erythrocytes. In sickle cell anemia, the polymerization of hemoglobin S (HbS) under conditions of low oxygen tension is preceded by an increase in cation conductance. This increase in conductance is mediated in part through Ca++-activated K+ channels. A net efflux of potassium chloride (KCl) leads to a decrease in intracellular volume, which in turn increases the rate of HbS polymerization. Treatments minimizing the passive transport of ions and solvent to prevent such volume depletion might include inhibitors targeting either the Ca++-activated K+ channel or the anion conductance. NS1652 is an anion conductance inhibitor that has recently been developed. In vitro application of this compound lowers the net KCl loss from deoxygenated sickle cells from about 12 mmol/L cells/h to about 4 mmol/L cells/h, a value similar to that observed in oxygenated cells. Experiments performed in mice demonstrate that NS1652 is well tolerated and decreases red cell anion conductance in vivo.

Cytosolic pH regulates GCl through control of phosphorylation states of CFTR

Our objective in this study was to determine the effect of changes in luminal and cytoplasmic pH on cystic fibrosis transmembrane regulator (CFTR) Cl- conductance (GCl). We monitored CFTR GCl in the apical membranes of sweat ducts as reflected by Cl- diffusion potentials (VCl) and transepithelial conductance (GCl). We found that luminal pH (5.0-8.5) had little effect on the cAMP/ATP-activated CFTR GCl, showing that CFTR GCl is maintained over a broad range of extracellular pH in which it functions physiologically. However, we found that phosphorylation activation of CFTR GCl is sensitive to intracellular pH. That is, in the presence of cAMP and ATP [adenosine 5'-O-(3-thiotriphosphate)], CFTR could be phosphorylated at physiological pH (6.8) but not at low pH (approximately 5.5). On the other hand, basic pH prevented endogenous phosphatase(s) from dephosphorylating CFTR. After phosphorylation of CFTR with cAMP and ATP, CFTR GCl is normally deactivated within 1 min after cAMP is removed, even in the presence of 5 mM ATP. This deactivation was due to an increase in endogenous phosphatase activity relative to kinase activity, since it was reversed by the reapplication of ATP and cAMP. However, increasing cytoplasmic pH significantly delayed the deactivation of CFTR GCl in a dose-dependent manner, indicating inhibition of dephosphorylation. We conclude that CFTR GCl may be regulated via shifts in cytoplasmic pH that mediate reciprocal control of endogenous kinase and phosphatase activities. Luminal pH probably has little direct effect on these mechanisms. This regulation of CFTR may be important in shifting electrolyte transport in the duct from conductive to nonconductive modes.

Proton modulation of outward K+ currents in interferon-gamma-activated microglia

Whole-cell outward potassium currents (IK) were measured in interferon (IFN)-gamma-activated cultured murine microglial cells. Acidification of the external milieu moved the threshold of activation of IK in a depolarizing direction, while alkalinization showed the opposite effect. A shift of more than 20 mV of the steady-state activation and inactivation curves of IK in hyperpolarizing direction was measured when pH was changed from 5.8 to 7.8. The time-dependent inactivation of IK was slower when superfusing cells with acid solutions than with alkaline ones. In contrast, variations in the pH of the intracellular solution did not alter kinetics of IK. However, alkalinization of the internal solution from a pH of 5.8 to 7.8 led to a two-fold increase in the current density of IK.

The pH-sensitivity of transepithelial K+ transport in vestibular dark cells

The pH-sensitivity of transepithelial K+ transport was studied in vitro in isolated vestibular dark cell epithelium from the gerbil ampulla. The cytosolic pH (pHi) was measured microfluorometrically with the pH-sensitive dye 2',7'-bicarboxyethyl-5(6)-carboxyfluorescein (BCECF) and the equivalent short-circuit current (Isc), which is a measure for transepithelial K+ secretion, was calculated from measurements of the transepithelial voltage (Vt) and the transepithelial resistance (Rt) in a micro-Ussing chamber. All experiments were conducted in virtually HCO3(-)-free solutions. Under control conditions, pHi was 7.01 +/- 0.04 (n = 18), Vt was 9.1 +/- 0.5 mV, Rt 16.7 +/- 0.09 omega cm2, and Isc was 587 +/- 30 microA/cm2 (n = 49). Addition of 20 mM propionate- caused a biphasic effect involving an initial acidification of pHi, increase in Vt and Isc and decrease in Rt and a subsequent alkalinization of pHi, decrease of Vt and increase of Rt. Removal of propionate- caused a transient effect involving an alkalinization of pHi, a decrease of Vt and Isc and an increase in Rt, pHi in the presence of propionate- exceeded pHi under control conditions. Effects of propionate- on Vt, Rt and Isc were significantly larger when propionate- was applied to the basolateral side rather than to the apical side of the epithelium. The pHi-sensitivity of Isc between pH 6.8 and 7.5 was -1089 microA/(cm2.pH-unit) suggesting that K+ secretion ceases at about pHi 7.6. Acidification of the extracellular pH (pHo) caused an increase of Vt and Isc and a decrease of Rt most likely due to acidification of pHi. Effects were significantly larger when the extracellular acidification was applied to the basolateral side rather than to the apical side of the epithelium. The pHo sensitivity of Isc between pH 7.4 and 6.4 was -155 microA/(cm2.pH unit). These results demonstrate that transepithelial K+ transport is sensitive to pHi and pHo and that vestibular dark cells contain propionate- uptake mechanism. Further, the data suggest that cytosolic acidification activates and that cytosolic alkalinization inactivates the slowly activating K+ channel (IsK) in the apical membrane. Whether the effect of pHi on the IsK channel is a direct or indirect effect remains to be determined.

Effect of NH4+/NH3 on cytosolic pH and the K+ channels of freshly isolated cells from the thick ascending limb of Henle's loop

The conductance properties of the luminal membrane of cells from the thick ascending limb of Henle's loop of rat kidney (TAL) are dominated by K+. In excised membrane patches the luminal K+ channel is regulated by pH changes on the cytosolic side. To examine this pH regulation in intact cells of freshly isolated TAL segments we measured the membrane voltage (Vm) in slow-whole-cell (SWC) recordings and the open probability (Po) of K+ channels in the cell-attached nystatin (CAN) configuration, where channel activity and part of Vm can be recorded. The pipette solution contained K+ 125 mmol/l and Cl- 32 mmol/l. Intracellular pH was determined by 2',7'bis(2-carboxyethyl)-5,(6)-carboxyfluorescein (BCECF) fluorescence. pH changes were induced by the addition of 10 mmol/l NH4+/NH3 to the bath. In the presence of NH4+/NH3 intracellular pH acidified by 0.53 +/- 0.11 units (n = 7). Inhibition of the Na+2Cl-K+ cotransporter by furosemide (0.1 mmol/l) reversed this effect and led to a transient alkalinisation by 0.62 +/- 0.14 units (n = 7). In SWC experiments Vm of TAL cells was -72 +/- 1 mV (n = 70). NH4+/NH3 depolarised Vm by 22 +/- 2 mV (n = 25). In 11 SWC experiments furosemide (0.1 mmol/l) attenuated the depolarising effect of NH4+ from 24 +/- 3 mV to 7 +/- 3 mV. Under control conditions the single-channel conductance of TAL K+ channels in CAN experiments was 66 +/- 5 pS and the reversal voltage for K+ currents was 70 +/- 2 mV (n = 35). The Po of K+ channels in CAN patches was reduced by NH4+/NH3 from 0.45 +/- 0.15 to 0.09 +/- 0.07 (n = 7). NH4+/NH3 exposure depolarised the zero current voltage of the permeabilised patches by -9.7 +/- 3.6 mV (n = 5). The results show that TAL K+ channels are regulated by cytosolic pH in the intact cell. The cytosolic pH is acidified by NH4+/NH3 exposure at concentrations which are physiologically relevant because Na+2Cl-K+(NH4+) cotransporter-mediated import of NH4+ exceeds the rate of NH3 diffusion into the TAL. K+ channels are inhibited by this acidification and the cells depolarise. In the presence of furosemide TAL cells alkalinise proving that NH4+ uptake occurs by the Na+2Cl-K+ cotransporter. The findings that, in the presence of NH4+/NH3 and furosemide, Vm is not completely repolarised and that K+ channels are not activated suggest that the respective K+ channels may in addition to their pH regulation be inhibited directly by NH4+/NH3.

Intracellular pH and its relationship to regulation of ion transport in normal and cystic fibrosis human nasal epithelia

1. Intracellular pH (pHi) of cultured human airway epithelial cells from normal and cystic fibrosis (CF) subjects were measured with double-barrelled pH-sensitive liquid exchanger microelectrodes. The cells, which were grown to confluence on a permeable collagen matrix support, were mounted in a modified miniature Ussing chamber. All studies were conducted under open circuit conditions. Values are given as means +/- S.E.M. and n refers to the number of preparations. 2. Normal preparations (n = 15) were characterized by a transepithelial potential difference (Vt) of -18 +/- 2 mV, an apical membrane potential (Va) of -19 +/- 2 mV, a basolateral membrane potential (Vb) of -37 +/- 2 mV, a transepithelial resistance (Rt) of 253 +/- 15 omega cm2, a fractional apical membrane resistance (fRa) of 0.40 +/- 0.04 and an equivalent short circuit current (Ieq) of -73 +/- 7 microA cm-2. 3. CF preparations (n = 13) were characterized by a Vt of -46 +/- 7 mV, a Va of 3 +/- 5 mV, a Vb of -43 +/- 3 mV, Rt of 373 +/- 47 omega cm2, fRa of 0.44 +/- 0.04 and an Ieq of -130 +/- 16 microA cm-2. All parameters except Vb and fRa were significantly different (P < 0.025) from those of normal preparations. 4. Despite large differences in electrochemical driving force for proton flow across the apical cell membranes between normal and CF preparations (-4 +/- 3 mV and 20 +/- 7 mV, respectively), pHi was similar (7.15 +/- 0.02 and 7.11 +/- 0.05, respectively). The driving force across the basolateral membrane was similar in normal and CF preparations (22 +/- 3 and 26 +/- 3 mV, respectively). 5. Intracellular alkalinization achieved by removal of CO2 from the luminal Ringer solution or by luminal ammonium prepulse led to stimulation of Ieq in both normal (from -58 to -70 microA cm-2, n = 4; P < 0.05) and CF (from -144 to -163 microA cm-2, n = 4; P < 0.005) preparations. The increase in Ieq was associated with a reduction of Rt, increase in fRa, and hyperpolarization of Vb. All changes in bioelectric properties in response to intracellular alkalinization were fully reversible. 6. Intracellular acidification achieved by serosal ammonium prepulse led to marked reductions of Ieq in both normal (from -95 to -31 microA cm-2, n = 6; P < 0.05) and CF (from -111 to -67 microA cm-2, n = 7; P < 0.005) preparations.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of ammonium chloride on membrane currents of acinar cells dispersed from the rat parotid gland

In acinar cells freshly dispersed from rat parotid glands, the effects of ammonium chloride (NH4Cl) on membrane currents were studied using the whole-cell clamp method. When membrane currents were recorded with command pulses to 0 mV, applied at 2-s intervals from a holding potential of -70 mV, NH4Cl (5-20 mM) transiently decreased outward currents and then slowly increased both outward and inward currents. After reaching a peak in about 40-50 s, both outward and inward currents gradually decreased in the presence of NH4Cl and, on its wash-out, the currents returned to the control level. Butyrate (5-20 mM) had little effect on the resting membrane currents, but markedly inhibited the response to NH4Cl. Tetraethylammonium (5 mM) strongly reduced both the resting and NH4Cl-induced outward currents, whereas it slightly potentiated the NH4Cl-induced inward current without affecting the membrane current at the holding potential. Without ATP in the patch pipettes, carbachol-induced membrane currents were relatively resistant to Ca2+ removal from the external medium, but NH4Cl-induced currents were quickly abolished in the absence of Ca2+. We conclude that intracellular alkalinization with NH4Cl increases Ca2+ influx and activates Ca(2+)-dependent outward K+ and inward Cl- currents.

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.

Fibronectin-integrin binding promotes hyperpolarization of murine erythroleukemia cells

The resting electrical potential (delta psi p) of murine erythroleukemia cells (MELC) was measured by the patch-clamp technique at different times after seeding onto culture surfaces enriched with bovine serum albumin (BSA) or Fibronectin (FN). While BSA did not produce significant changes of potential and cell shape, FN promoted a 15-20 mV hyperpolarization that preceded a marked cell spreading. This hyperpolarization was abolished by either treating cells with anti FN-receptor antibodies, or adding the RGDS tetrapeptide, suggesting that electric signals are elicited by the specific interaction of the FN cell binding domain with integrin receptors.

Effects of intracellular pH on calcium-activated potassium channels in rabbit tracheal smooth muscle

1. The effects of intracellular pH (pHi) on calcium-activated potassium channels (Ca2(+)-activated K+ channels) were studied in membrane patches of smooth muscle freshly dispersed from the rabbit trachea. Single-channel currents were recorded with an 'inside-out' patch clamp technique, mainly at 0 mV, with the external (electrode) medium containing 130 mM-K+ and the internal (bath) medium 6 mM-K+. 2. With an internal Ca2+ concentration ([Ca2+]i) of 1 microM, the fraction of time during which the channel was in an open state (the open probability, Po) was more than 0.8 at pHi 7.4. The channel activity nearly disappeared at pHi 7.0. The [Ca2+]i-Po relationship was shifted to higher [Ca2+]i by acidosis, the shift being approximately an 8-fold increase for a fall in pHi of 0.5 units. 3. The membrane potential and current intensity (V-I) relationship of single channels between +30 and -50 mV was shifted in a hyperpolarizing direction by intracellular acidosis. The shift was roughly 10 mV for 1 pH unit at 1 microM [Ca2+]i. At pHi 7.4 [Ca2+]i 1 microM, the V-Po relationship was shifted in a depolarizing direction by acidification. When [Ca2+]i was increased to 10 microM, V-Po relationship became less sensitive to V as well as pHi changes. 4. When Po was high, the probability density function of open and closed time distributions could be fitted by two exponentials. When Po was decreased to less than 0.3, either by reducing [Ca2+]i or by lowering pHi, another component having long closed times appeared. At similar Po values, the time constant of open time distribution was smaller with lower pHi. 5. It is concluded that the main effect of an increase in intracellular hydrogen ions is to decrease the open probability of the Ca2(+)-activated K+ channel, by reducing the sensitivity to Ca2+ and also shortening the open state.

Regulation of inwardly rectifying K+ channels by intracellular pH in opossum kidney cells

The effects of intracellular pH on an inwardly rectifying K+ channel ("Kin channel") in opossum kidney (OK) cells were examined using the patch-clamp technique. Experiments with inside-out patches were first carried out in Mg2(+)- and adenosine triphosphate (ATP)-free conditions, where Mg2(+)-induced inactivation and ATP-induced reactivation of Kin channels were suppressed. When the bath (cytoplasmic side) pH was decreased from 7.3 to either 6.8 or 6.3, Kin channels were markedly inhibited. The effect of acid pH was not fully reversible. When the bath pH was increased from 7.3 to 7.8, 8.3 or 8.8, the channels were activated reversibly. The channel activity exhibited a sigmoidal pH dependence with a maximum sensitivity at pH 7.5. Inside-out experiments were also carried out with a solution containing 3 mM Mg-ATP and a similar pH sensitivity was observed. However, in contrast with the results obtained in the absence of Mg2+ and ATP, the effect of acid pH was fully reversible. Experiments with cell-attached patches demonstrated that changes in intracellular pH, which were induced by changing extracellular pH in the presence of an H+ ionophore, could influence the channel activity reversibly. It is concluded that the activity of Kin channels can be controlled by the intracellular pH under physiological conditions.

Membrane sidedness and the interaction of H+ and K+ on Ca2(+)-activated K+ transport in human red blood cells

The sided effects of H+ on Ca2(+)-stimulated K+ transport (the Gardos channel) were studied in human red blood cells. Cells were loaded with Ca2+ during energy depletion with the internal pH adjusted to desired levels prior to treatment with the anion-exchange inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), which inhibits pH equilibration across the membrane. This treatment provides a "pH clamp" whereby the internal and external H+ (H+i and H+o) concentrations can be varied separately. Channel activity was evaluated by measuring either net K+ loss or unidirectional 42K+ efflux from cells where SO2(-4) replaced Cl- on both sides of the membrane. When pHi was set at 7.4, decreasing pHo from values of 8.0 to 5.0 inhibited K+ efflux. This effect of H+o could be overcome by increasing K+o at all values of pHo. In addition, this effect of K+o could be separated from its effects on altering the membrane potential, indicating an interaction between K+o and H+o on the channel. A similar interaction was shown to occur between H+i and K+i. K+o is known to be required for activation of Ca2(+)-stimulated K+ transport, since the channel in cells preincubated in the absence of K+o (prior to exposure to Ca+i) becomes refractory to subsequent activation by Ca2+i and K+o. We found that H+o would not substitute for K+o in this regard nor would H+o inhibit the protective effect of K+o; in addition, H+ was not transported inward in exchange for K+i. Thus it would appear that there are two external sites where K+o interacts with the channel. One site is antagonized by H+o, whereas the second site is required for channel activation independent of H+ in the range studied. The inside of the channel would have, by an analogous argument, at least one site where K+i and H+i interact.

Proton fluxes associated with the Ca pump in human red blood cells

Ca fluxes and H fluxes were measured in human red blood cells at 37 degrees C to characterize the effects of extracellular protons (Hout) on the Ca pump and to determine the stoichiometry of Ca-H exchange. A pH-stat technique was used to measure the rate of H influx, and 45Ca was used to determine the rate of Ca efflux. 4,4'-Diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) was used to reduce proton permeability. A La-sensitive H influx was observed in Ca-loaded cells (Ca = 2 mmol/l packed cells) and was not observed in the cells loaded with vanadate as well as Ca. Similar results were obtained in Ca-loaded ghosts. The La dose-response curves for H influx and for Ca efflux were similar [50% inhibitory concentration (IC50) = approximately 5 microM] in intact red blood cells. The stoichiometry of the La-sensitive fluxes among different experiments ranged from 1.7 to 2.1 H/Ca when extracellular pH (pHout) = 6.3. Thus the Ca pump in intact red blood cells mediates Ca-2H exchange at pHout = 6.3. A 100-fold decrease in Hout [from pH 6.5 to 8.5; intracellular pH (pHin) approximately 7.4] only decreased Ca efflux 1.5- to 3-fold, hence Hout had little effect on the overall rate under the conditions studied. The small effect of Hout was a surprising result for a Ca-H exchange system, since one would have expected a steep dependence of Ca pump on Hout at Hout less than the Michaelis constant (Km). However, no La-sensitive H influx was observed when pHout = 8. On the basis of these data, it is suggested that the Ca pump also mediates Ca efflux uncoupled from H influx (Ca2+/phi H+). Ca efflux in the presence of 11 mM extracellular Ca (Caout) was one-fifth the value obtained in the absence of Caout at pHout = 8.5; this inhibition was reversed by increasing Hout (to pH 6.1). These results are consistent with a model in which 1) the Ca pump mediates Ca2+/2H+ exchange at high Hout; 2) the Ca pump mediates Ca2+/phi H+ exchange at low pHout; 3) the rates of the two processes are less than or equal to 4-fold different; 4) Caout inhibits pump activity at low Hout; and 5) Caout competes with Hout for binding.

Ca2+-activated K+ conductance of human red cell membranes exhibits two different types of voltage dependence

The voltage dependence for outward-going current of the Ca-activated K+ conductance (gK(Ca] of the human red cell membrane has been examined over a wide range of membrane potentials (Vm at constant values of [K+]ex, [K+]c and pHc, the intact cells being preloaded to different concentrations of ionized calcium. Outward-current conductances were calculated from initial net effluxes of K+ and the corresponding (Vm - EK) values. The basic conductance, defined as the outward-current conductance at (Vm - EK) greater than or equal to 20 mV and [K+]ex greater than or equal to 3 mM (B. Vestergaard-Bogind, P. Stampe and P. Christophersen, J. Membrane Biol. 95:121-130, 1987) was found to be a function of cellular ionized Ca. At all degrees of Ca activation gK(Ca) was an apparently linear function of voltage (Vm range -40 to +70 mV), the absolute level as well as the slope decreasing with decreasing activation. In a simple two-state model the constant voltage dependence can, at the different degrees of Ca activation, be accounted for by a Boltzmann-type equilibrium function with an equivalent valence of approximately 0.4, assuming chemical equilibrium at Vm = 0 mV. Alternatively, the phenomenon might be explained by a voltage-dependent block of the outward current by an intracellular ion. Superimposed upon the basic conductance is the apparently independent inward-rectifying steep voltage function with an equivalent valence of approximately 5 and chemical equilibrium at the given EK value.

Role of Ca2+-activated K+ channel in regulation of NaCl reabsorption in thick ascending limb of Henle's loop

1. Reabsorption of NaCl in the thick ascending limb of Henle's loop involves the integrated function of the Na+,K+,Cl- -cotransport system and a Ca2+-activated K+ channel in the luminal membrane with the Na+,K+-pump and a net Cl- conductance in the basolateral membrane. 2. Assay of K+ channel activity after reconstitution into phospholipid vesicles shows that the K+ channel is stimulated by Ca2+ in physiological concentrations and that its activity is regulated by calmodulin and phosphorylation from cAMP dependent protein kinase. 3. For purification luminal plasma membrane vesicles are isolated and solubilized in CHAPS. K+ channel protein is isolated by affinity chromatography on calmodulin columns. The purified protein has high Ca2+-activated K+ channel activity after reconstitution into vesicles. 4. The purified K+ channel consists of two proteins of 51 and 36 kDa. Phosphorylation from cAMP dependent protein kinase stimulates K+ channel activity and labels the 51 kDa band. The 36 kDa band is rapidly cleaved by trypsin and may be involved in Ca2+ stimulation. 5. Opening of the K+ channel by Ca2+ in physiological concentrations and regulation by calmodulin and phosphorylation by protein kinase may mediate kinetic and hormonal regulation of NaCl transport across the tubule cells in TAL.

Commitment to differentiation of murine erythroleukemia cells involves a modulated plasma membrane depolarization through Ca2+-activated K+ channels

The role of the plasma membrane potential (delta psi p) in the commitment to differentiation of murine erythroleukemia (MEL) cells has been studied by analyzing the ionic basis and the time course of this potential in the absence or the presence of different types of inducers. delta psi p was determined by measuring the distribution of tetraphenylphosphonium (TPP+) across the plasma membrane and displayed a 22-hour depolarization phase (from -28 to +5 mV) triggered by factors contained in foetal calf serum (FCS) and followed by a nearly symmetrical repolarization phase. After measuring the electrochemical equilibrium potential of Na+, K+, and Cl-, the relative contribution of these ions to delta psi p was evaluated by means of ion substitution experiments and by the addition of ion flux inhibitors (tetrodotoxin [TTX], 4-acetoamide-4'-isothiocyanostilbene-2,2'-disulfonate [SITS]) and ionophores (Valinomycin, A23187). The Na+ contribution to delta psi p appeared negligible, the potential being essentially generated by K+ and Cl- fluxes. When evaluated by a new mathematical approach, the effects of Valinomycin and A23187 at different times of incubation provided evidence that both the depolarization and the repolarization phase were due to variations of the K+ permeability across the plasma membrane (PK) mediated by Ca2+-activated K+ channels. All the inducers tested (dimethylsulfoxide [DMSO], hexamethylen-bis-acetamide [HMBA], diazepam), although they did not modify the ionic basis of delta psi p, strongly attenuated the depolarization rate of this potential. This attenuation was not brought about when the inducers were added to noninducible MEL cell clonal sublines. Cell commitment occurred only during the depolarization phase and increased proportionally to the attenuation of this phase up to a threshold beyond which the further increase of the attenuation was associated with the inhibition of commitment. The major role of the inducers apparently consisted of the stabilization of the Ca2+-activated K+ channels, suggesting that a properly modulated delta psi p depolarization through these channels is primarily involved in the signal generation for MEL cell commitment to differentiation.

Reconstitution in phospholipid vesicles of calcium-activated potassium channel from outer renal medulla

A barium-sensitive Ca-activated K+ channel in the luminal membrane of the tubule cells in thick ascending limb of Henle's loop is required for maintenance of the lumen positive transepithelial potential and may be important for regulation of NaCl reabsorption. In this paper we examine if the K+ channel can be solubilized and reconstituted into phospholipid vesicles with preservation of its native properties. The K+ channel in luminal plasma membrane vesicles can be quantitatively solubilized in CHAPS at a detergent/protein ratio of 3. For reconstitution, detergent is removed by passage over a column of Sephadex G 50 (coarse). K+-channel activity is assayed by measurement of 86Rb+ uptake against a large opposing K+ gradient. The reconstituted K+ channel is activated by Ca2+ in the physiological range of concentration (K1/2 approximately 2 X 10(-7) M at pH 7.2) as found for the K+ channel in native plasma membrane vesicles and shows the same sensitivity to inhibitors (Ba2+, trifluoperazine, calmidazolium, quinidine) and to protons. Reconstitution of the K+ channel into phospholipid vesicles with full preservation of its native properties is an essential step towards isolation and purification of the K+-channel protein. Titration with Ca2+ shows that most of the active K+ channels in reconstituted vesicles have their cytoplasmic aspect facing outward in contrast to the orientation in plasma membrane vesicles, which requires also addition of Ca2+ ionophore in order to observe Ca2+ stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Voltage dependence of the Ca2+-activated K+ conductance of human red cell membranes is strongly dependent on the extracellular K+ concentration

The conductance of the Ca2+-activated K+ channel (gK(Ca)) of the human red cell membrane was studied as a function of membrane potential (Vm) and extracellular K+ concentration ([K+]ex). ATP-depleted cells, with fixed values of cellular K+ (145 mM) and pH (approximately 7.1), and preloaded with approximately 27 microM ionized Ca were transferred, with open K+ channels, to buffer-free salt solutions with given K+ concentrations. Outward-current conductances were calculated from initial net effluxes of K+, corresponding Vm, monitored by CCCP-mediated electrochemical equilibration of protons between a buffer-free extracellular and the heavily buffered cellular phases, and Nernst equilibrium potentials of K ions (EK) determined at the peak of hyperpolarization. Zero-current conductances were calculated from unidirectional effluxes of 42K at (Vm-EK) approximately equal to 0, using a single-file flux ratio exponent of 2.7. Within a [K+]ex range of 5.5 to 60 mM and at (Vm-EK) greater than or equal to 20 mV a basic conductance, which was independent of [K+]ex, was found. It had a small voltage dependence, varying linearly from 45 to 70 microS/cm2 between 0 and -100 mV. As (Vm-EK) decreased from 20 towards zero mV gK(Ca) increased hyperbolically from the basic value towards a zero-current value of 165 microS/cm2. The zero-current conductance was not significantly dependent on [K+]ex (30 to 156 mM) corresponding to Vm (-50 mV to 0). A further increase in gK(Ca) symmetrically around EK is suggested as (Vm-EK) becomes positive. Increasing the extracellular K+ concentration from zero and up to approximately 3 mM resulted in an increase in gK(Ca) from approximately 50 to approximately 70 microS/cm2. Since the driving force (Vm-EK) was larger than 20 mV within this range of [K+]ex this was probably a specific K+ activation of gK(Ca).

IN CONCLUSION

The Ca2+-activated K+ channel of the human red cell membrane is an inward rectifier showing the characteristic voltage dependence of this type of channel.

Flux ratio of valinomycin-mediated K+ fluxes across the human red cell membrane in the presence of the protonophore CCCP

The ratio of valinomycin-mediated unidirectional K+ fluxes across the human red cell membrane, has been determined in the presence of the protonophore carbonylcyanide m-chlorophenylhydrazone, CCCP, using the K+ net efflux and 42K influx. The driving force for the net efflux (Vm - EK+) has been calculated from the membrane potential, estimated by the CCCP-mediated proton distribution and the Nernst potential for potassium ions across the membrane. An apparent driving potential for the K+ net efflux has been calculated from the K+ flux ratio, determined in experiments where the valinomycin and CCCP concentrations were varied systematically. This apparent driving force, in conjunction with the actual driving force calculated on basis of the CCCP estimated membrane potential, is used to calculate a flux ratio exponent, which represents an estimate of the deviation of valinomycin-mediated K+ transport from unrestricted electrodiffusion, when protonophore is present. In the present work, the flux ratio exponent is found to be 0.90 when the CCCP concentration is 5.0 microM and above, while the exponent decreases to about 0.50 when no CCCP is present. The influence of CCCP upon the rate constants in the valinomycin transport cycle is discussed. The significance of this result is that red cell membrane potentials are overestimated, when calculated from valinomycin-mediated potassium isotope fluxes, using a constant field equation.

Modulation of K+ conductance by intracellular pH in pancreatic beta-cells

Measurements of the effects of NH3/NH4+ on glucose-induced electrical activity in beta-cells from microdissected mouse islets of Langerhans and on intracellular pH in single collagenase-isolated islets pre-loaded with a fluorescent pH probe were performed and are reported here. Application of NH3/NH4+ (15 mM) in the presence of glucose (11 mM) promptly hyperpolarized the beta-cell membrane, reduced input resistance by 60% and blocked electrical activity. These changes were paralleled by an increase in islet fluorescence indicative of a cytosolic pH increase. Removal of NH4Cl initially stimulated electrical activity, which returned to resting level with a time constant of 51 s. Concomitant with the removal of NH4Cl there was a drop in pHi followed by a slow return to resting level with a time constant of 83 s. The results suggest that the [Ca2+]-dependent K+ channel in the beta-cell membrane is activated by a rise in cytosolic pH.

Single-file diffusion through the Ca2+-activated K+ channel of human red cells

The ratio between the unidirectional fluxes through the Ca2+-activated K+-specific ion channel of the human red cell membrane has been determined as a function of the driving force (Vm-EK). Net effluxes and 42K influxes were determined during an initial period of approximately 90 sec on cells which had been depleted of ATP and loaded with Ca. The cells were suspended in buffer-free salt solutions in the presence of 20 microM of the protonophore CCCP, monitoring in this way changes in membrane potential as changes in extracellular pH. (Vm-EK) was varied at constant EK by varying the Nernst potential and the conductance of the anion and the conductance of the potassium ion. In another series of experiments EK was varied by suspending cells in salt solutions with different K+ concentrations. At high extracellular K+ concentrations both of the unidirectional fluxes were determined as 42K in- and effluxes in pairs of parallel experiments. Within a range of (Vm-EK) of -6 to 90 mV the ratio between the unidirectional fluxes deviated strongly from the values predicted by Ussing's flux ratio equation. The Ca2+-activated K+ channel of the human red cell membrane showed single-file diffusion with a flux ratio exponent n of 2.7. The magnitude of n was independent of the driving force (Vm-EK), independent of Vm and independent of the conductance gK.