Ion channel-mediated fluxes in membrane vesicles: selective amplification of isotope uptake by electrical diffusion potentials

Iron transport by proteoliposomes containing mitochondrial F(1)F(0) ATP synthase isolated from rat heart

In this work, we present evidence of Fe(2+) transport by rat heart mitochondrial F(1)F(0) ATP synthase. Iron uptake by the vesicles containing the enzyme was concentration- and temperature-dependent, with an optimum temperature of 37 degrees C. Both ATP and ADP stimulated iron uptake in a concentration-dependent manner, whereas AMP, AMPPCP, and mADP did not. Inhibitors of the enzyme, oligomycin, and resveratrol similarly blocked iron transport. The iron uptake was confirmed by inhibition using specific antibodies against the alpha, beta, and c subunits of the enzyme. Interestingly, slight transport of common divalent and trivalent metal ions such as Mg(+2), Ca(+2), Mn(+2), Zn(+2), Cu(+2), Fe(+3), and Al(+3) was observed. Moreover, Cu(+2), even in the nM range, inhibited iron uptake and attained maximum inhibition of approximately 56%. Inorganic phosphate (Pi) in the medium exerted an opposite effect depending on the type of adenosine nucleotide, which was suppressed with ATP, but enhanced with ADP. A similarly stimulating effect of ATP and ADP with an inverse effect of Pi suggests that the activity of ATPase and ATP synthase may be associated with iron uptake in a different manner, probably via antiport of H(+).

The heterogeneity of ion channels in chromaffin granule membranes

Chromaffin granules are involved in catecholamine synthesis and traffic in the adrenal glands. The transporting membrane proteins of chromaffin granules play an important role in the ion homeostasis of these organelles. In this study, we characterized components of the electrogenic (86)Rb(+) flux observed in isolated chromaffin granules. In order to study single channel activity, chromaffin granules from the bovine adrenal medulla were incorporated into planar lipid bilayers. Four types of cationic channel were found, each with a different conductance. The unitary conductances of the potassium channels are 360 +/- 10 pS, 220 +/- 8 pS, 152 +/- 8 pS and 13 +/- 3 pS in a gradient of 450/150 mM KCl, pH 7.0. A multiconductance potassium channel with a conductivity of 110 +/- 8 pS and 31 +/- 4 pS was also found. With the exception of the 13 pS conductance channel, all are activated by depolarizing voltages. One type of chloride channel was also found. It has a unitary conductance of about 250 pS in a gradient of 500/150 mM KCl, pH 7.0.

A radioactive uptake assay to measure ion transport across ion channel–containing liposomes

Here we describe a procedure for incorporating ion channels into lipid vesicles (liposomes) and functional characterization of the channel population by assaying radioactive isotope uptake into these proteoliposomes. The technique as described will work only for potassium channels but can be easily modified, as suggested in the text, for other ion channels and transporters. Purified ion channel proteins in detergent micelles are combined with solubilized lipids. Detergent is subsequently removed from protein-lipid complexes by gel filtration or dialysis into high potassium (high [K+]) buffer. After freezing-thawing and sonication, the resultant larger liposomes are passed over another gel-filtration column to exchange an extraliposomal high [K+] to a low [K+] buffer, thus establishing a large K+ gradient across the liposomal membrane. Trace 86Rb is then added to the extraliposomal space and the reaction begins. If the ion channel is permeable to K+, the K+ inside exits the liposomes down its concentration gradient and the 86Rb outside accumulates in the intraliposomal space until equilibrium is reached. The reaction time course is monitored by measurement of accumulated 86Rb after removal of external 86Rb over an ion-exchange column. The 86Rb flux assay takes 2-5 hours depending on the reaction rate and the number of desired time points.

Quantification and distribution of big conductance Ca2+-activated K+ channels in kidney epithelia

Big conductance Ca2+ activated K+ channels (BK channels) is an abundant channel present in almost all kind of tissue. The accurate quantity and especially the precise distribution of this channel in kidney epithelia are, however, still debated. The aim of the present study has therefore been to examine the presence of BK channels in kidney epithelia and determine the actual number and distribution of these channels. For this purpose, a selective peptidyl ligand for BK channels called iberiotoxin or the radiolabeled double mutant analog 125I-IbTX-D19Y/Y36F has been employed. The presence of BK channels were determined by a isotope flux assay where up to 44% of the total K+ channel activity could be inhibited by iberiotoxin indicating that BK channels are widely present in kidney epithelia. Consistent with these functional studies, 125I-IbTX-D19Y/Y36F binds to membrane vesicles from outer cortex, outer medulla and inner medulla with Bmax values (in fmol/mg protein) of 6.8, 2.6 and 21.4, respectively. These studies were performed applying rabbit kidney epithelia tissue. The distinct distribution of BK channels in both rabbit and rat kidney epithelia was confirmed by autoradiography and immunohistochemical studies. In cortical collecting ducts, BK channels were exclusively located in principal cells while no channels could be found in intercalated cells. The abundant and distinct distribution in kidney epithelia talks in favor for BK channels being important contributors in maintaining salt and water homeostasis.

pH modulation of large conductance potassium channel from adrenal chromaffin granules

We report here that large conductance K(+) selective channel in adrenal chromaffin granules is controlled by pH. We measured electrogenic influx of (86)Rb(+) into chromaffin granules prepared from bovine adrenal gland medulla. The (86)Rb(+) influx was inhibited by acidic pH. Purified chromaffin granule membranes were also fused with planar lipid bilayer. A potassium channel with conductance of 432+/-9 pS in symmetric 450 mM KCl was observed after reconstitution into lipid bilayer. The channel activity was unaffected by charybdotoxin, a blocker of the Ca(2+)-activated K(+) channel of large conductance. It was observed that acidification to pH 6.4 cis side of the membrane lowered the channel open probability and single channel conductance. Whereas only weak influence on the single channel current amplitude and open probability were observed upon lowering of the pH at the trans side. We conclude that a pH-sensitive large conductance potassium channel operates in the chromaffin granule membrane.

Quinine inhibits mitochondrial ATP-regulated potassium channel from bovine heart

The mitochondrial ATP-regulated potassium (mitoK(ATP) channel has been suggested as trigger and effector in myocardial ischemic preconditioning. However, molecular and pharmacological properties of the mitoK(ATP) channel remain unclear. In the present study, single-channel activity was measured after reconstitution of the inner mitochondrial membrane from bovine ventricular myocardium into bilayer lipid membrane. After incorporation, a potassium-selective current was recorded with mean conductance of 103 +/- 9 pS in symmetrical 150 mM KCl. Single-channel activity of this reconstituted protein showed properties of the mitoK(ATP) channel: it was blocked by 500 microM ATP/Mg, activated by the potassium-channel opener diazoxide at 30 microM, inhibited by 50 microM glibenclamide or 150 microM 5-hydroxydecanoic acid, and was not affected by the plasma membrane ATP-regulated potassium-channel blocker HMR1098 at 100 microM. We observed that the mitoK(ATP) channel was blocked by quinine in the micromolar concentration range. The inhibition by quinine was additionally verified with the use of 86Rb+ flux experiments and submitochondrial particles. Quinine inhibited binding of the sulfonylurea derivative [3H]glibenclamide to the inner mitochondrial membrane. We conclude that quinine inhibits the cardiac mitoK(ATP) channel by acting on the mitochondrial sulfonylurea receptor.

Quantification and distribution of Ca(2+)-activated maxi K(+) channels in rabbit distal colon

The Ca(2+)-activated maxi K(+) channel is an abundant channel type in the distal colon epithelium, but nothing is known regarding the actual number and precise localization of these channels. The aim of this study has therefore been to quantify the maxi K(+) channels in colon epithelium by binding of iberiotoxin (IbTX), a selective peptidyl ligand for maxi K(+) channels. In isotope flux measurements 75% of the total K(+) channel activity in plasma membranes from distal colon epithelium is inhibited by IbTX (K(0.5) = 4.5 pM), indicating that the maxi K(+) channel is the predominant channel type in this epithelium. Consistent with the functional studies, the radiolabeled double mutant (125)I-IbTX-D19Y/Y36F binds to the colon epithelium membranes with an equilibrium dissociation constant of approximately 10 pM. The maximum receptor concentration values (in fmol/mg protein) for (125)I-IbTX-D19Y/Y36F binding to colon epithelium are 78 for surface membranes and 8 for crypt membranes, suggesting that the maxi K(+) channels are predominantly expressed in the Na(+)-absorbing surface cells, as compared with the Cl(-)-secreting crypt cells. However, aldosterone stimulation of this tissue induced by a low-Na(+) diet does not change the total number of maxi K(+) channels.

Retinoic acid induction of calcium channel expression in human NT2N neurons

Ca2+ channel expression and regulation of intracellular Ca2+ homeostasis were studied during retinoic acid (RA)-induced differentiation of the human teratocarcinoma cell line Ntera 2/C1.D1 (NT2- cells) into NT2N neurons, a unique model of human neurons in culture. The cytosolic Ca2+ level of undifferentiated NT2- cells was low (75 +/- 5 nM) and stable under basal conditions, and it was only marginally decreased (by 9%) upon removal of extracellular Ca2+. After 10 microM RA treatment, NT2- cells were irreversibly differentiated into a phenotype of neuron-like NT2N cells. Cytosolic Ca2+ level of NT2N neurons was higher (106 +/- 14 nM) than that of NT2- cells and spontaneously fluctuated (0.208 +/- 0.038 transients/min) under basal conditions. Although K+ increased 86Rb fluxes in both NT2- cells and NT2N neurons, it only increased cytosolic Ca2+ level in NT2N neurons. The K+-induced increase in cytosolic Ca2+ in NT2N neurons was antagonized by 0.1-10 microM nifedipine or verapamil, 5 microM omega-CgTx GVIA, but not by 1 microM omega-agatoxin IVA, 1 microM omega-agatoxin TK, 1 microM FTX-3.3, or 100 microM Ni+ implicating L- and N-type voltage-dependent Ca2+ channels. In L- and N-type channels, but not in P- and Q-types, mRNAs were expressed in NT2N neurons as well as NT2- cells. Quantitative analysis of L- and N-type Ca2+ protein levels showed major differences between NT2- cells and NT2N neurons. In NT2- cells, N-type Ca2+ channels were undetectable while L-type channels levels were fivefold lower compared to NT2N neurons. Our findings show that L- and N-type channels are expressed during differentiation of NT2- cells into neurons, and that these voltage-dependent Ca2+ channels have a major role in regulating intracellular Ca2+ homeostasis and neuronal excitability.

The intracellular potassium and chloride channels: properties, pharmacology and function (review)

Channels selective for potassium or chloride ions are present in membranes of intracellular organelles such as sarcoplasmic (endoplasmic) reticulum, mitochondria, nucleus, synaptic vesicles, and chromaffin, and zymogen granules. They probably play an important role in cellular events such as compensation of electrical charges during transport of Ca2+, delta pH formation in mitochondria or V-ATPase containing membrane granules, and regulation of volume changes, due to potassium and chloride transport into intracellular organelles. Intracellular potassium and chloride channels could also be the target for pharmacologically active compounds. This mini-review describes the basic properties, pharmacology, and current hypotheses concerning the functional role of intracellular potassium and chloride channels.

An aldosterone regulated chicken intestine protein with high affinity to amiloride

The pattern of chicken intestine amiloride-binding proteins was determined using the photoreactive amiloride analogue 2'-methoxy-5'-nitrobenzamil (NMBA) and a polyclonal anti-amiloride antibody. At 10(-7)M, NMBA inhibits approximately 62% of the Na+ channel activity. At this concentration the amiloride analogue labels a number of membrane proteins, and in particular a 40-45 kDa polypeptide denoted ABP40. Incorporation of NMBA into ABP40 could be prevented by a 100-fold excess of benzamil, but not by a 1000-fold excess of 5-(N-ethyl-N-isopropyl)-amiloride. Labeling of ABP40 was intense in membranes derived from salt-deprived chickens and approximately 5-fold weaker in membranes from salt-repleted animals. Because of its small size, ABP40 is not likely to be an avian Na+ channel subunit, yet this amiloride-binding protein could be involved in the response to aldosterone.

Inhibition of amiloride-sensitive Na+ channel by isothiouronium derivatives

The effects on the amiloride-blockable Na+ channel of a family of recently synthesized isothiouronium derivatives were measured in plasma membrane vesicles from rat distal colon. Some of these derivatives act as high-affinity Na(+)-like antagonists on the Na(+)-K(+)-adenosinetriphosphatase. One of the reagents tested, 1-bromo-2,4,6-tris(isothiouronium methyl)-benzene tribromide (Br-TITU), was found to be a potent blocker of the Na+ channel. At neutral pH, Br-TITU rapidly inhibits the channel mediated 22Na+ uptake, with an inhibition constant of 94 +/- 39 nM. The inhibition observed is specific and reversible. 1,3-Dibromo-2,4,6-tris(isothiouronium methyl)benzene tribromide and Br-TITU derivatives with methyl and phenyl substitutions on the isothiouronium moiety were much less effective blockers. Incubation of cells with Br-TITU at alkaline (but not neutral) pH produces irreversible inactivation of channels, possibly due ot covalent modification of a lysine residue. This inactivation can be attenuated by amiloride but not by Na+. Thus Br-TITU may be a useful reagent in identifying essential residues of the channel protein.

Functional reconstitution of P-glycoprotein reveals an apparent near stoichiometric drug transport to ATP hydrolysis

We have recently described an ATP-driven, valinomycin-dependent 86Rb+uptake into proteoliposomes reconstituted with mammalian P-glycoprotein (Eytan, G. D., Borgnia, M. J., Regev, R., and Assaraf, Y. G. (1994) J. Biol. Chem. 269, 26058-26065). P-glycoprotein mediated the ATP-dependent uptake of 86Rb+-ionophore complex into the proteoliposomes, where the radioactive cation was accumulated, thus, circumventing the obstacle posed by the hydrophobicity of P-glycoprotein substrates in transport studies. Taking advantage of this assay and of the high levels of P-glycoprotein expression in multidrug-resistant Chinese hamster ovary cells, we measured simultaneously both the ATPase and transport activities of P-glycoprotein under identical conditions and observed 0.5-0.8 ionophore molecules transported/ATP molecule hydrolyzed. The amount of 86Rb+ ions transported within 1 min via the ATP- and valinomycin-dependent P-glycoprotein was equivalent to an intravesicular cation concentration of 8 mM. Thus, this stoichiometry and transport capacity of P-glycoprotein resemble various ion-translocating ATPases, that handle millimolar substrate concentrations. This constitutes the first demonstration of comparable rates of P-glycoprotein-catalyzed substrate transport and ATP hydrolysis.

Na+ channels in membrane vesicles from intralobular salivary ducts

Electrical potential-driven 22Na+ fluxes were measured in membrane vesicles prepared from male and female rat submandibular intralobular ducts. A relatively temperature-independent (Q10 = 1.45 +/- 0.15), amiloride-inhibitable (mean affinity constant approximately 1 microM), rheogenic Na+ transport pathway was observed. The relative potency of amiloride analogues for inhibition of this pathway was amiloride > ethylisopropyl-amiloride > phenamil, similar to that of the "low-amiloride-affinity" Na+ channel recently observed in a number of other tissues. These results are consistent with the existence of the apical Na+ channel thought to be involved in intralobular ductal salt reabsorption. No significant difference was found in the magnitude or pharmacology of electrogenic Na+ fluxes in vesicles prepared from male and female rat intralobular ducts, suggesting that the sexual dimorphism observed in this tissue is not reflected at the level of the apical membrane Na+ channel. Amiloride-sensitive 22Na+ fluxes in intralobular ductal membranes were of the same magnitude as 22Na+ fluxes measured in similarly prepared and assayed vesicles from the toad bladder, a tissue thought to be a rich source of amiloride-sensitive Na+ channels.

Effects of divalent cations and pH on amiloride-sensitive Na+ fluxes into luminal membrane vesicles from pars recta of rabbit proximal tubule

The effect of Ca2+, Cd2+, Ba2+, Mg2+ and pH on the renal epithelial Na(+)-channel was investigated by measuring the amiloride-sensitive 22Na+ fluxes into luminal membrane vesicles from pars recta of rabbit proximal tubule. It was found that intravesicular Ca2+ as well as extravesicular Ca2+ substantially lowered the channel-mediated flux. Amiloride sensitive Na+ uptake was nearly completely blocked by 10 microM Ca2+ at pH 7.4. The inhibitory effect of Ca2+ was dependent on pH. Thus, 10 microM Ca2+ produced 90% inhibition of 22Na+ uptake at pH 7.4, and only 40% inhibition at pH 7.0. The tracer fluxes measured in the absence of Ca2+ were pH independent over the range from 7.0 to 7.4. All the cations Ca2+, Cd2+, Ba2+ except Mg2+ inhibited the 22Na+ influx drastically when added extravesicularly in millimolar concentrations. The cations Cd2+, Ba2+ and Mg2+ in the same concentrations intravesicularly inhibited the 22Na+ influx only slightly. A millimolar concentration of Ca2+ intravesicularly blocked the amiloride-sensitive 22Na+ flux completely. The data indicate that Ca2+ inhibits Na+ influx specifically by binding to sites composed of one or several deprotonated groups on the channel proteins.

Direct inhibition of epithelial Na+ channels by a pH-dependent interaction with calcium, and by other divalent ions

Direct inhibitory effects of Ca2+ and other ions on the epithelial Na+ channels were investigated by measuring the amiloride-blockable 22Na+ fluxes in toad bladder vesicles containing defined amounts of mono- and divalent ions. In agreement with a previous report (H.S. Chase, Jr., and Q. Al-Awqati, J. Gen. Physiol. 81:643-666, 1983) we found that the presence of micromolar concentrations of Ca2+ in the internal (cytoplasmic) compartment of the vesicles substantially lowered the channel-mediated fluxes. This inhibition, however, was incomplete and at least 30% of the amiloride-sensitive 22Na+ uptake could not be blocked by Ca2+ (up to 1 mM). Inhibition of channels could also be induced by millimolar concentrations of Ba2+, Sr2+, or VO2+, but not by Mg2+. The Ca2+ inhibition constant was a strong function of pH, and varied from 0.04 microM at pH 7.8 to greater than 10 microM at pH 7.0. Strong pH effects were also demonstrated by measuring the pH dependence of 22Na+ uptake in vesicles that contained 0.5 microM Ca2+. This Ca2+ activity produced a maximal inhibition of 22Na+ uptake at pH greater than or equal to 7.4 but had no effect at pH less than or equal to 7.0. The tracer fluxes measured in the absence of Ca2+ were pH independent over this range. The data is compatible with the model that Ca2+ blocks channels by binding to a site composed of several deprotonated groups. The protonation of any one of these groups prevents Ca2+ from binding to this site but does not by itself inhibit transport. The fact that the apical Na+ conductance in vesicles, can effectively be modulated by minor variations of the internal pH near the physiological value, raises the possibility that channels are being regulated by pH changes which alter their apparent affinity to cytoplasmic Ca2+, rather than, or in addition to changes in the cytoplasmic level of free Ca2+.