Making Sense of Sensitivity: Using Candy and Anthropometric Data to Visually and Manipulatively Illustrate Sensitivity, Positive Predictive Value, and Related Terms

The classic concepts of sensitivity and specificity are commonly taught by definition only, often with discipline-specific jargon and without any tangible relation to their use in the real world. Yet, the COVID pandemic and the spotlight on diagnostic screening tests have brought a need for science and health care students, health professionals, and the general public to have improved understanding of sensitivity and specificity and how they connect to further interpretive values. These understandings are critical for correct communications and explanations to those outside the sciences. Using simple candies or marbles as visuals, in conjunction with real-world scenarios, this activity was designed to help frame these concepts for students. Additionally, this activity provides practice with basic calculations and interpretations to reinforce how data can be used in determining testing values, surrogate testing, data cutoffs, and accuracy predictions. The activity is flexible and can easily be done in 1 to 2 h in a classroom setting, as a laboratory exercise, or as an outreach or online activity.

Kinesthetic and visual scaffolding for understanding oxygen delivery and reading hemoglobin oxygen curves

I describe a kinesthetic activity about oxygen handling by hemoglobin with two specific goals: 1) to help students gain a better understanding of how hemoglobin properties affect oxygen delivery and 2) to improve the ability of the students to actually read the hemoglobin oxygen-binding curve. The activity makes understanding oxygen delivery more intuitive, provides a kinesthetic analog to delivery of oxygen, and provides data to plot for the hemoglobin-oxygen curve.

Encapsulation of FITC to monitor extracellular pH: a step towards the development of red blood cells as circulating blood analyte biosensors

A need exists for a long-term, minimally-invasive system to monitor blood analytes. For certain analytes, such as glucose in the case of diabetics, a continuous system would help reduce complications. Current methods suffer significant drawbacks, such as low patient compliance for the finger stick test or short lifetime (i.e., 3-7 days) and required calibrations for continuous glucose monitors. Red blood cells (RBCs) are potential biocompatible carriers of sensing assays for long-term monitoring. We demonstrate that RBCs can be loaded with an analyte-sensitive fluorescent dye. In the current study, FITC, a pH-sensitive fluorescent dye, is encapsulated within resealed red cell ghosts. Intracellular FITC reports on extracellular pH: fluorescence intensity increases as extracellular pH increases because the RBC rapidly equilibrates to the pH of the external environment through the chloride-bicarbonate exchanger. The resealed ghost sensors exhibit an excellent ability to reversibly track pH over the physiological pH range with a resolution down to 0.014 pH unit. Dye loading efficiency varies from 30% to 80%. Although complete loading is ideal, it is not necessary, as the fluorescence signal is an integration of all resealed ghosts within the excitation volume. The resealed ghosts could serve as a long-term (>1 to 2 months), continuous, circulating biosensor for the management of diseases, such as diabetes.

The reactive nitrogen species peroxynitrite is a potent inhibitor of renal Na-K-ATPase activity

Peroxynitrite is a reactive nitrogen species produced when nitric oxide and superoxide react. In vivo studies suggest that reactive oxygen species and, perhaps, peroxynitrite can influence Na-K-ATPase function. However, the direct effects of peroxynitrite on Na-K-ATPase function remain unknown. We show that a single bolus addition of peroxynitrite inhibited purified renal Na-K-ATPase activity, with IC50 of 107+/-9 microM. To mimic cellular/physiological production of peroxynitrite, a syringe pump was used to slowly release (approximately 0.85 microM/s) peroxynitrite. The inhibition of Na-K-ATPase activity induced by this treatment was similar to that induced by a single bolus addition of equal cumulative concentration. Peroxynitrite produced 3-nitrotyrosine residues on the alpha, beta, and FXYD subunits of the Na pump. Interestingly, the flavonoid epicatechin, which prevented tyrosine nitration, was unable to blunt peroxynitrite-induced ATPase inhibition, suggesting that tyrosine nitration is not required for inhibition. Peroxynitrite led to a decrease in iodoacetamidofluorescein labeling, implying that cysteine modifications were induced. Glutathione was unable to reverse ATPase inhibition. The presence of Na+ and low MgATP during peroxynitrite treatment increased the IC50 to 145+/-10 microM, while the presence of K+ and low MgATP increased the IC50 to 255+/-13 microM. This result suggests that the EPNa conformation of the pump is slightly more sensitive to peroxynitrite than the E(K) conformation. Taken together, these results show that peroxynitrite is a potent inhibitor of Na-K-ATPase activity and that peroxynitrite can induce amino acid modifications to the pump.

Divalent cation interactions with Na,K-ATPase cytoplasmic cation sites: implications for the para-nitrophenyl phosphatase reaction mechanism

The interactions of divalent cations with the adenosine triphosphatase (ATPase) and para-nitrophenyl phosphatase (pNPPase) activity of the purified dog kidney Na pump and the fluorescence of fluorescein isothiocyanate (FITC)-labeled pump were determined. Sr(2+) and Ba(2+) did not compete with K(+) for ATPase (an extracellular K(+) effect). Sr(2+) and Ba(2+) did compete with Na(+) for ATPase (an intracellular Na(+) effect) and with K(+) for pNPPase (an intracellular K(+) effect). These results suggest that Ba(2+) or Sr(2+) can bind to the intracellular transport site, yet neither Ba(2+) nor Sr(2+) was able to activate pNPPase activity; we confirmed that Ca(2+) and Mn(2+) did activate. As another measure of cation binding, we observed that Ca(2+) and Mn(2+), but not Ba(2+), decreased the fluorescence of the FITC-labeled pump; we confirmed that K(+) substantially decreased the fluorescence. Interestingly, Ba(2+) did shift the K(+) dose-response curve. Ethane diamine inhibited Mn(2+) stimulation of pNPPase (as well as K(+) and Mg(2+) stimulation) but did not shift the 50% inhibitory concentration (IC(50)) for the Mn(2+)-induced fluorescence change of FITC, though it did shift the IC(50) for the K(+)-induced change. These results suggest that the Mn(2+)-induced fluorescence change is not due to Mn(2+) binding at the transport site. The drawbacks of models in which Mn(2+) stimulates pNPPase by binding solely to the catalytic site vs. those in which Mn(2+) stimulates by binding to both the catalytic and transport sites are presented. Our results provide new insights into the pNPPase kinetic mechanism as well as how divalent cations interact with the Na pump.

Extracellular terbium and divalent cation effects on the red blood cell Na pump and chrysoidine effects on the renal Na pump

We examined the effect of extracellular terbium (Tb(3+)) and divalent metal cations (Ca(2+), Sr(2+), and Ba(2+)) on (86)Rb(+) influx into rabbit and human red blood cells. We found that Tb(3+) at 15 and 25 microM was a non-competitive inhibitor of (86)Rb(+) influx suggesting that Tb(3+) is not binding to the transport site. This result reduces the usefulness of Tb(3+) as a potential probe for the E(out) conformation (the conformation with the transport site facing extracellularly). Ba(2+), Sr(2+) and Ca(2+), at concentrations >50 mM, had minimal effects on Rb(+) influx into red blood cells (1 mM Rb-out). This suggests that the outside transport site is very specific for monovalent cations over divalent cations, in contrast to the inside transport site. We also found that chrysoidine (4-phenylazo-m-phenylenediamine) competes with Na(+) for ATPase activity and K(+) for pNPPase activity suggesting it is binding to the E(in) conformation. Chrysoidine and similar compounds may be useful as optical probes of the E(in) conformation.

Kinetic characterization of Na,K-ATPase inhibition by Eosin

Eosin is a probe for the Na pump nucleotide site. In contrast to previous studies examining eosin effects on Na only ATPase, we examined Na,K-ATPase- and K-activated pNPPase activity in red blood cell membranes and purified renal Na,K-ATPase. At saturating ATP (3 mM) the eosin IC(50) for Na pump inhibition was 19 microM. Increasing ATP concentrations (0.2-2.5 mM) did not overcome eosin-induced inhibition, thus eosin is a mixed-type inhibitor of ATPase activity. To test if eosin can bind to the high-affinity ATP site, purified Na,K-ATPase was labeled with 20 microM FITC. With increasing eosin concentrations (0.1 microM-10 microM) the incorporation of FITC into the ATP site significantly decreases suggesting that eosin prevents FITC reaction at the high-affinity ATP site. Eosin was a more potent inhibitor of K-activated phosphatase activity than of Na,K-ATPase activity. At 5 mM pNPP the eosin IC(50) for Na pump inhibition was 3.8+/-0.23 microM. Increasing pNPP concentrations (0.45-14.5 mM) did not overcome eosin-induced inhibition, thus eosin is a mixed-type inhibitor of pNPPase activity. These results can be fit by a model in which eosin and ATP bind only to the nucleotide site; in some pump conformations, this site is rigid and the binding is mutually exclusive and in other conformations, the site is flexible and able to accommodate both eosin and ATP (or pNPP). Interestingly, eosin inhibition of pNPPase became competitive after the addition of C(12)E(8) (0.1%) but the inhibition of ATPase remained mixed.

Similarities and differences between organic cation inhibition of the Na,K-ATPase and PMCA

The effects of three classes of organic cations on the inhibition of the plasma membrane Ca pump (PMCA) were determined and compared to inhibition of the Na pump. Quaternary amines (tetramethylammonium, tetraethylammonium, and tetrapropylammonium, TMA, TEA, and TPA, respectively) did not inhibit PMCA. This is not to imply that PMCA is inherently selective against monovalent cations because guanidine and tetramethylguanidine inhibited PMCA by competing with Ca(2+). The divalent organic cation, ethyl diamine, inhibited PMCA but was not competitive with Ca(2+). In contrast, propyl diamine did compete with Ca(2+) and was about 10-fold more potent than butyl diamine in inhibiting PMCA. For the Na pump, both TEA and TPA inhibited, but TMA did not. TEA, guanidine, and tetramethylguanidine inhibition was competitive with Na(+) for ATPase activation and with K(+) for pNPPase activation, both of which are cytoplasmic substrate cation effects. Thus, these findings are consistent with TEA, guanidine, and tetramethylguanidine inhibiting from the cytoplasmic side of the Na pump; in contrast, we have previously shown that TPA did not inhibit from the cytoplasmic side. The divalent alkane diamines ethyl, propyl, and butyl diamine all inhibited the Na pump and all competed at the intracellular surface. The order of potency was ED > PD > BD consistent with an optimal size for binding; similarly, for the quaternary amines TMA is apparently too small to make appropriate contacts, and TPA is too large. Homology models based upon the high-resolution SERCA structure are included to contextualize the kinetic observations.

Na+-inhibitory sites of the Na+/H+exchanger are Li+substrate sites

Amiloride-inhibitable Li+influx in dog red blood cells is mediated by the Na+/H+exchanger, NHE. However, there are substantial differences between the properties of Li+transport and Na+transport through the NHE. Li+influx is activated by cell shrinkage, and Na+influx is not, as we reported previously (Dunham PB, Kelley SJ, and Logue PJ. Am J Physiol Cell Physiol 287: C336–C344, 2004). Li+influx is a sigmoidal function of its concentration, and Na+activation is linear at low Na+concentrations. Li+does not inhibit its own influx; in contrast, Na+inhibits Na+influx. Li+prevents this inhibition by Na+. Na+is a mixed or noncompetitive inhibitor of Li+influx, implying that both a Na+and a Li+can be bound at the same time. In contrast, Li+is a competitive inhibitor of Na+influx, suggesting Li+binding at one class of sites on the transporter. Because the properties of Li+transport and Na+transport are different, a simple explanation is that Na+and Li+are transported by separate sites. The similarities of the properties of Li+transport and the inhibition of Na+transport by Na+suggest that Li+is transported by the Na+-inhibitory sites.

Kinetic characterization of tetrapropylammonium inhibition reveals how ATP and Pi alter access to the Na+-K+-ATPase transport site

Current models of the Na(+)-K(+)-ATPase reaction cycle have ATP binding with low affinity to the K(+)-occluded form and accelerating K(+) deocclusion, presumably by opening the inside gate. Implicit in this situation is that ATP binds after closing the extracellular gate and thus predicts that ATP binding and extracellular cation binding to be mutually exclusive. We tested this hypothesis. Accordingly, we needed a cation that binds outside and not inside, and we determined that tetrapropylammonium (TPA) behaves as such. TPA competed with K(+) (and not Na(+)) for ATPase, TPA was unable to prevent phosphoenzyme (EP) formation even at low Na(+), and TPA decreased the rate of EP hydrolysis in a K(+)-competitive manner. Having established that TPA binding is a measurement of extracellular access, we next determined that TPA and inorganic phosphate (P(i)) were not mutually exclusive inhibitors of para-nitrophenylphosphatase (pNPPase) activity, implying that when P(i) is bound, the transport site has extracellular access. Surprisingly, we found that ATP and TPA also were not mutually exclusive inhibitors of pNPPase activity, implying that when the cation transport site has extracellular access, ATP can still bind. This is consistent with a model in which ATP speeds up the conformational changes that lead to intracellular or extracellular access, but that ATP binding is not, by itself, the trigger that causes opening of the cation site to the cytoplasm.

Bretylium, an organic quaternary amine, inhibits the Na,K-ATPase by binding to the extracellular K-site

The quaternary amine, bretylium, is a class III antiarrhythmic drug used to treat ventricular tachycardia and fibrillation. The primary mode of action for bretylium is thought to be inhibition of voltage-gated K(+) channels. While the Na,K-ATPase has been the pharmacological target of cardiac glycosides for over a century, recent evidence has shown that bretylium may also inhibit the Na pump. Our experimental findings support and extend these previous reports and provide definitive evidence supporting the previous suggestion that bretylium and K compete for the Na pump. We find that bretylium inhibits the Na pump in a dose-dependent manner in both Na,K-ATPase (IC(50) 4.5 mM) and Rb flux experiments (IC(50) 3.5 mM). Furthermore, we show that bretylium and Rb(+) competes for an extracellular site by measuring ouabain-sensitive (86)Rb flux in intact human red blood cells; that is, there is an apparent increase in K(m) for Rb(+) in the presence of 5 mM bretylium, while V(max) remains unchanged. We also determined that unlike K(+), bretylium does not facilitate the hydrolysis of E2-P. However, it stabilizes this conformation by reducing the ability of K(+) to facilitate dephosphorylation. Finally, we show that bretylium, like K(+), reduces [(3)H]ouabain binding to the Na pump. Taken together, these data are consistent with bretylium binding to the extracellular facing cation site within the E2-P state of the enzyme. Moreover, these findings suggest that bretylium may serve as an effective tool for freezing the pump in an extracellularly cation-bound phosphorylated intermediate, which will aid in future structural analyses.

Chloro(2,2':6',2"-terpyridine) platinum inhibition of the renal Na+,K+-ATPase

Chloro(2,2':6',2"-terpyridine) platinum, a bulky, hydrophilic reagent, inhibited the renal sodium pump with a single exponential time course. K(+) increased the rate constant of the reaction by about twofold; the K(+) concentration dependence was monotonic, with a half-maximal effect observed at 1 mM, consistent with K(+) acting at a transport site. Na(+), Mg(2+), eosin, and vanadate did not significantly alter the rate of reaction. The results of proteolysis and mass spectrometer analysis were consistent with terpyridine platinum labeling of Cys452, Cys456, or Cys457. Because phenylarsine oxide reacts with vicinal cysteines and did not prevent terpyridine platinum modification, terpyridine platinum most likely modifies Cys452. This modification prevents ADP binding; interestingly, the analogous residue in sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) is on the exterior of the nucleotide-binding pocket. Thus it appears that the terpyridine platinum residue is more accessible in the presence of K(+) than in its absence and that terpyridine platinum modification prevents nucleotide binding.

Induction of the ZRC1 metal tolerance gene in zinc-limited yeast confers resistance to zinc shock

Zinc is an essential nutrient but toxic to cells with overaccumulation. For this reason, intracellular zinc levels are tightly controlled. In the yeast Saccharomyces cerevisiae, the Zrc1 and Cot1 proteins have been implicated in the storage and detoxification of excess zinc in the vacuole. Surprisingly, transcription of ZRC1 is induced in zinc-limited cells by the zinc-responsive transcription factor Zap1. We show here that this increase in ZRC1 expression is a novel mechanism of zinc homeostasis and stress tolerance. Zinc-limited cells also express high levels of the plasma membrane zinc uptake transporters. As a consequence, when zinc-limited cells are resupplied with small amounts of zinc, large quantities quickly accumulate in the cell, a condition we refer to as "zinc shock." We show here that ZRC1 and its induction in zinc-limited cells are required for resistance to this zinc shock. Experiments using the zinc-responsive fluorophore FuraZin-1 as an indicator of vacuolar zinc levels indicated that Zrc1 is required for the rapid transport of zinc into the vacuole during zinc shock. We also present evidence that cytosolic zinc rises to higher levels in cells unable to sequester this excess zinc. Thus, the increase in ZRC1 expression occurs prior to the zinc shock stress for which this induction is important. We propose that this "proactive" strategy of homeostatic regulation, such as we document here for ZRC1, may represent a common but largely unrecognized phenomenon.

Biochemical properties of vacuolar zinc transport systems of Saccharomyces cerevisiae

The yeast vacuole plays an important role in zinc homeostasis by storing zinc for later use under deficient conditions, sequestering excess zinc for its detoxification, and buffering rapid changes in intracellular zinc levels. The mechanisms involved in vacuolar zinc sequestration are only poorly characterized. Here we describe the properties of zinc transport systems in yeast vacuolar membrane vesicles. The major zinc transport activities in these vesicles were ATP-dependent, requiring a H+ gradient generated by the V-ATPase for function. One system we identified was dependent on the ZRC1 gene, which encodes a member of the cation diffusion facilitator family of metal transporters. These data are consistent with the proposed role of Zrc1 as a vacuolar zinc transporter. Zrc1-independent activity was also observed that was not dependent on the closely related vacuolar Cot1 protein. Both Zrc1-dependent and independent activities showed a high specificity for Zn(2+) over other physiologically relevant substrates such as Ca2+, Fe2+, and Mn2+. Moreover, these systems had high affinities for zinc with apparent K(m) values in the 100-200 nm range. These results provide biochemical insight into the important role of Zrc1 and related proteins in eukaryotic zinc homeostasis.

Extracellular protons regulate the extracellular cation selectivity of the sodium pump

The effects of 0.3-10 nM extracellular protons (pH 9.5-8.0) on ouabain-sensitive rubidium influx were determined in 4,4'-diisocyanostilbene-2, 2'-disulfonate (DIDS)-treated human and rat erythrocytes. This treatment clamps the intracellular H. We found that rubidium binds much better to the protonated pump than the unprotonated pump; 13-fold better in rat and 34-fold better in human erythrocytes. This clearly shows that protons are not competing with rubidium in this proton concentration range. Bretylium and tetrapropylammonium also bind much better to the protonated pump than the unprotonated pump in human erythrocytes and in this sense they are potassium-like ions. In contrast, guanidinium and sodium bind about equally well to protonated and unprotonated pump in human red cells. In rat red cells, protons actually make sodium bind less well (about sevenfold). Thus, protons have substantially different effects on the binding of rubidium and sodium. The effect of protons on ouabain binding in rat red cells was intermediate between the effects of protons on rubidium binding and on sodium binding. Remarkably, all four cationic inhibitors (bretylium, guanidinium, sodium, and tetrapropylammonium) had similar apparent inhibitory constants for the unprotonated pump ( approximately 5-10 mM). The K(d) for proton binding to the human pump, with the empty transport site facing extracellularly is 13 nM, whereas the extracellular transport site loaded with sodium is 9.5 nM, and with rubidium is 0.38 nM. In rat red cells there is also a substantial difference in the K(d) for proton binding to the sodium-loaded pump (14.5 nM) and the rubidium-loaded pump (0.158 nM). These data suggest that important rearrangements occur at the extracellular pump surface as the pump moves between conformations in which the outward facing transport site has sodium bound, is empty, or has rubidium bound and that guanidinium is sodium-like and bretylium and tetrapropylammonium are rubidium-like.

Probing the extracellular release site of the plasma membrane calcium pump

The plasma membrane Ca(2+) pump is known to mediate Ca(2+)/H(+) exchange. Extracellular protons activated (45)Ca(2+) efflux from human red blood cells with a half-maximal inhibition constant of 2 nM when the intracellular pH was fixed. An increase in pH from 7.2 to 8.2 decreased the IC(50) for extracellular Ca(2+) from approximately 33 to approximately 6 mM. Changing the membrane potential by >54 mV had no effect on the IC(50) for extracellular Ca(2+). This argues against Ca(2+) release through a high-field access channel. Extracellular Ni(2+) inhibited Ca(2+) efflux with an IC(50) of 11 mM. Extracellular Cd(2+) inhibited with an IC(50) of 1. 5 mM, >10 times better than Ca(2+). The Cd(2+) IC(50) also decreased when the pH was raised from 7.1 to 8.2, consistent with Ca(2+), Cd(2+), and H(+) competing for the same site. The higher affinity for inhibition by Ni(2+) and Cd(2+) is consistent with a histidine or cysteine as part of the release site. The cysteine reagent 2-(trimethylammonium)ethyl methanethiosulfonate did not inhibit Ca(2+) efflux. Our results are consistent with the notion that the release site contains a histidine.

Identification of critical positive charges in XIP, the Na/Ca exchange inhibitory peptide

The peptides XIP (RRLLFYKYVYKRYRAGKQRG) and C28R2 (LRRGQILWFRGLNRIQTQIRVVKAFRSS) correspond to the autoinhibitory domains of the Na-Ca exchanger and the plasma membrane Ca pump, respectively. An increase of ionic strength reduced the inhibition of exchange activity by XIP and C28R2, consistent with an important role for electrostatic interactions. Sulfosuccinimidyl acetate (SNA)-modified XIP did not inhibit Na-Ca exchange. Because SNA modifies lysines, we conclude that at least one of the positive charges at the XIP lysine positions (7, 11, or 17) is important for inhibition. 2CK-XIP (RRLLFYRYVYRCYCAGRQKG) has cysteines at 12 and 14 and only one lysine (at 19).2CK-XIP inhibited Na-Ca exchange; thus positive charges at 12 and 14 are not essential. SNA-modified 2CK-XIP did not inhibit; thus a positive charge at 19 is important. Iodoacetic acid-modified 2CK-XIP inhibits the Na-Ca exchanger but not the PM Ca pump. These results show that the structural determinants for inhibition of the Na-Ca exchanger and the PM Ca pump are different, that positive charges at 7, 11, or 17 (or some combination) are more important than positive charges at 12 and 14 for inhibition by XIP of the Na-Ca exchanger.

Positive charge modifications alter the ability of XIP to inhibit the plasma membrane calcium pump

Exchange inhibitory peptide (XIP; RRLLFYKYVYKRYRAGKQRG) is the shortest peptide that inhibits the plasma membrane Ca pump at high Ca (A. Enyedi, T. Vorherr, P. James, D. J. McCormick, A. G. Filoteo, E. Carafoli, and J. T. Penniston, J. Biol. Chem. 264: 12313-12321, 1989). Sulfosuccinimidyl acetate (SNA)-modified XIP does not inhibit the Ca pump; SNA neutralizes the positive charge on Lys at positions 7, 11, and 17. Peptide 2CK-XIP (RRLLFYRYVYRCYCAGRQKG) inhibits the pump, but the iodoacetamido-modified peptide does not inhibit. Three peptide analogues, in which 7, 11, and 17 were Ala, Cys, or Lys, inhibited about as well as XIP. SNA modification of these analogues (each with 1 Lys) did not inhibit. SNA modification of 2CK-XIP results in a peptide that does not inhibit; thus position 19 is important. Our results suggest that it is critical that position 19 be positively charged, that positions 7, 11, and 17 are important contact points between XIP and the Ca pump (with at least one positively charged), and that, whereas it is not essential that residues 12 and 14 be positive, they cannot be negative.

Eosin, a potent inhibitor of the plasma membrane Ca pump, does not inhibit the cardiac Na-Ca exchanger

The Na-Ca exchanger and the sarcolemmal/plasma membrane (SL(PM)) Ca pump are the two major pathways for Ca transport to the extracellular space in many cells. In cardiac myocytes, the Na-Ca exchanger appears to be responsible for a greater portion of this Ca flux [Bassani, R. A., et al. (1992) J. Physiol. 453, 591-608]. However, the respective contributions of these two transporters are not as well-defined in all tissues (e.g., smooth muscle). We propose that eosin (tetrabromofluorescein) may be a useful tool for quantitatively determining the proportion of Ca transported by the Na-Ca exchanger vs the SL(PM) Ca pump in various cells. Eosin is the most potent inhibitor known for the SL(PM) Ca pump (IC50 approximately 0.3 microM in red blood cell inside-out vesicles); unlike the Na/K and H/K pumps, eosin does not compete with ATP for the SL(PM) Ca pump [Gatto, C., & Milanick, M. A. (1993) Am. J. Physiol. 264, C1577-C1586]. In the present study, we have shown that eosin was a potent inhibitor of the cardiac SL(PM) Ca pump (IC50 approximately 1 microM); in contrast, eosin (< or = 20 microM) did not inhibit the cardiac Na-Ca exchanger. In experiments where Ca was being transported by both the SL(PM) Ca pump and the Na-Ca exchanger simultaneously, eosin effectively eliminated the Ca pump-mediated transport. In addition, we show that eosin can permeate the human red cell membrane; cell permeability is an attractive feature for using eosin in whole cell studies. We conclude that eosin can be used for determining the role that the SL(PM) Ca pump plays in whole cell Ca homeostasis.

Interaction of cardiac Na-Ca exchanger and exchange inhibitory peptide with membrane phospholipids

We tested the hypothesis that the exchange inhibitory peptide (XIP) domain in the cardiac Na-Ca exchanger is a regulatory site under the control of the membrane lipid environment. We found that 125I-XIP bound to liposomes composed of phosphatidylcholine (PC) and phosphatidylserine (PS) with peak binding at 1:1 PC/PS. No binding was observed in PC liposomes. XIP and pentalysine-inhibitable bovine sarcolemmal (SL) Na-Ca exchange activity was observed in reconstituted proteoliposomes composed of 1:1 PC/PS. Proteolysis of SL membranes resulted in a twofold stimulation of Na-Ca exchange activity, but the half-maximal inhibitory concentration (IC50) for XIP (3 microM) was not significantly changed, suggesting that the XIP binding site remained intact. In contrast, the IC50 for pentalysine was decreased from 500 to 150 microM in proteolyzed membranes. These data are consistent with a model of Na-Ca exchange regulation in which the endogenous XIP domain interacts either with another region of the exchange protein to induce an inactive conformational state or with membrane lipid to produce an active conformation.

Inhibition of the red blood cell calcium pump by eosin and other fluorescein analogues

This paper addresses the mechanism of inhibition of the plasma membrane Ca pump by fluorescein analogues and their isothiocyanate derivatives. Eosin (i.e., tetrabromofluorescein) was found to be one of the most potent reversible inhibitors of the erythrocyte Ca pump [half-maximal inhibitory concentration (IC50) < 0.2 microM]; fluorescein itself was about four orders of magnitude less potent (IC50 approximately 1,000 microM). Eosin decreased the maximum influx and thus did not compete with ATP for the Ca pump. Irreversible inhibition produced by the isothiocyanate analogues of eosin and fluorescein [eosin 5-isothiocyanate (EITC) and fluorescein 5-isothiocyanate (FITC), respectively] was also studied. While EITC bound reversibly at the eosin site, two results suggest that EITC does not react covalently at this site: 1) eosin did not alter the time course of the EITC irreversible reaction, and 2) the concentration dependence for reversible EITC inhibition was different from the concentration dependence for irreversible EITC inhibition. ATP did slow the rate of inactivation of both EITC and FITC consistent with the idea that EITC and FITC bind to the ATP site. Our results are consistent with eosin and ATP binding to separate sites and EITC reacting covalently at the ATP site, but not the eosin site.

Interactions of the exchange inhibitory peptide with Na-Ca exchange in bovine cardiac sarcolemmal vesicles and ferret red cells

The Na-Ca exchange inhibitory peptide (XIP), which corresponds to residues 251-270 of the Na-Ca exchange protein, specifically inhibits exchange activity (Li, Z., Nicoll, D. A, Collins, A., Hilgemann, D. W., Filoteo, A. G., Penniston, J. T., Weiss, J. N., Tomich, J. M., and Philipson, K. D. (1991) J. Biol. Chem. 266, 1014-1020). We have found that XIP decreased Na+i-dependent Ca2+ uptake to 46 and 20% of control in mixed and inside-out bovine sarcolemmal (SL) vesicles, respectively, and to 22% of control in ferret red cell vesicles. XIP inhibited uptake in bovine SL vesicles after proteolytic digestion. XIP also inhibited Na+o-dependent Ca2+ efflux in bovine SL vesicles but did not inhibit Ca2+ uptake in reconstituted proteoliposomes. Extracellular XIP did not inhibit Ca2+ uptake into intact ferret red cells. Inhibition of uptake in bovine SL vesicles was reduced as the ionic strength was increased. 125I-labeled XIP (1 microM) was cross-linked to proteins of bovine SL vesicles, ferret red cell vesicles, and intact ferret red cells. Labeling of bands at approximately 75, 120, and 220 kDa (in bovine SL vesicles) and bands at 55 and 85 kDa (in ferret red cell vesicles) was detected. No cross-linking was detected in intact ferret red cells. We conclude that XIP inhibition is insensitive to proteolytic digestion and is partially dependent on charge association and conformation of the exchanger. XIP binds to and interacts with the intracellular side of the Na-Ca exchanger.

Ferret red cells: Na/Ca exchange and NaKCl cotransport

The primary pathway for K influx in ferret red cells is the Na-K-Cl cotransporter and the primary pathway for Ca influx is the Na/Ca exchanger. This makes ferret red cells favorable models for the study of these two transport systems. The evidence that Na/Ca exchange is of primary importance for steady state cell volume regulation and the Na-K-Cl cotransport has a minor role is presented. The approaches to, and results of, the determination of the stoichiometry, of the mechanism, and of the regulation by ATP and Mg, for Na/Ca exchange is contrasted with that taken for Na-K-Cl cotransport.

Kinetic models of Na-Ca exchange in ferret red blood cells. Interaction of intracellular Na, extracellular Ca, Cd, and Mn

The kinetic equation that best describes the intracellular Na dependence of Ca influx into ferret red cells is sequential; whether this implies that there is a conformation of the protein that has both Na and Ca ions bound remains to be determined. Cd and Mn substitute very well for Ca on the exchanger in ferret red cells; this suggests that the Ca-binding site does not contain an important thiol and that the one of the Na steps may be rate limiting.

Mn and Cd transport by the Na-Ca exchanger of ferret red blood cells

Ca influx via the Na-Ca exchanger into ferret red blood cells is easily measured from a Na-free solution; the intracellular Na concentration is normally approximately 150 mM in ferret red blood cells. We have found that Mn and Cd competitively inhibit Ca influx. Mn influx and Cd influx were a saturable function of the divalent cation concentration, consistent with a carrier mechanism. Indeed, the Km (approximately 10 microM) and the Vmax (usually 1-3 mmol.l packed cells-1.h-1) were similar for Ca, Cd, and Mn. Extracellular Na inhibited divalent cation influx, and intracellular Na stimulated influx. These results are consistent with Na-Cd and Na-Mn influx pathways in ferret red blood cells. Ca (1 mM) almost completely inhibited Mn influx and Cd influx, whereas 1 mM Mg inhibited 5-15%. These results strongly support the notion that Mn and Cd are alternative substrates for Ca on the ferret red cell Na-Ca exchanger. The similarity in the behavior of all three divalent cation places important constraints on kinetic and structural models of the exchanger.

Na-Ca exchange: evidence against a ping-pong mechanism and against a Ca pool in ferret red blood cells

To determine the mechanism of Na-Ca exchange, we estimated the ratio of maximum velocity to Michaelis constant for extra-cellular Ca by measuring the rate of Ca uptake at very low extracellular Ca. In a Ping-Pong mechanism, one set of sites alternatively transports Ca and Na. In a sequential mechanism, Ca and Na sites are both filled during part of the transport cycle. In each set of experiments, two intracellular Na concentrations were studied. The Ca uptake rate (at low Ca) increased as Na increased; this is consistent with a sequential model, as has been found in other cells. We also examined the alternative hypothesis that the exchanger followed Ping-Pong kinetics and that the red blood cells had a submembrane pool for Ca that limited mixing with the cytosol. In these experiments Ca pump activity was monitored by measuring ATP hydrolysis. This model was disproven by experiments that indicated that greater than 80% of the Ca that entered the cell became bound to EGTA and less than 20% resulted in Ca efflux by the Ca pump.

Branched reaction mechanism for the Na/K pump as an alternative explanation for a nonmonotonic current vs. membrane potential response

Nonmonotonic velocity vs. membrane potential curves are often taken as evidence that two steps involve charge movement through the electric field. However, a branched reaction scheme in which only one step involves charge movement per cycle can lead to a nonmonotonic response. A similar case occurs in enzyme kinetics: nonmonotonic velocity vs. substrate curves are often taken as evidence for two different substrate-binding sites. However, a branched reaction scheme in which only one substrate binds per complete cycle can lead to a nonmonotonic response (see Segel, I.H. 1975, Enzyme Kinetics, pp. 657-659. John Wiley & Sons, New York). Some analytical constraints on the relative sizes of the rate constants of a branched reaction mechanism that give rise to nonmonotonic responses are derived. There are two necessary conditions. (i) The rate of at least one step in the branched pathway must be less than the rate of the step after the branch. (ii) The rate of the pathway in which S binds first must be slower than the rate of the other pathway. Analogous cases give rise to nonmonotonic current vs. membrane potential curves. A branched mechanism for the Na/K pump provides an alternative explanation for a nonmonotonic pump current vs. membrane potential relationship.

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.

Na-Ca exchange in ferret red blood cells

Ferrets have high Na (140 mmol/l) red blood cells. To determine whether ferret red cells had a Na-Ca exchange system, Na effluxes via the Na + K + 2Cl cotransrpoter and Ca effluxes via the Ca pump had to be inhibited. This was accomplished by replacing cell chloride with nitrate and by loading the cells with vanadate that inhibits the Ca pump. Under these conditions, extracellular Na (Naout) inhibited Ca influx. Intracellular Na (Nain) was required for the large Ca influx as replacement of Na with Li reduced Ca influx to less than one-tenth of the original rate. Caout stimulated Na efflux by about twofold. The Ca efflux from cells depleted of Na was increased from 0.8 to 3.2 mmol.l packed cells-1.h-1 by the presence of Naout. Cells placed in a Na-free solution accumulated Ca: total intracellular Ca was 20-fold higher than free Caout. Most of this Ca was released on addition of the Ca ionophore, A23187. Because the Na gradient had driven net Ca uphill, the fluxes of Na and Ca are coupled. In a Na-free solution, the K1/2 for Ca influx was usually approximately 10 microM (occasionally approximately 100 microM), and the maximal velocity (Vmax was 1.5-4.4 mmol.l packed cells-1.h-1. At Naout = 150 mM, K1/2 increased 5- to 150-fold. In some cells, Naout decreased Vmax by approximately fourfold, suggesting that Naout does not always compete with Caout.(ABSTRACT TRUNCATED AT 250 WORDS)

Proton inhibition of chloride exchange: asynchrony of band 3 proton and anion transport sites?

The inhibition of chloride exchange at 0 degrees C by protons at the cytoplasmic and the extracellular surface of the band 3 protein of human erythrocytes was measured between pH 4.6 and 7.6. At constant external pH and chloride concentration, internal protons were a mixed inhibitor of chloride flux, with the apparent pK2 = 6.1 for protonation of the inward-facing empty transporter conformation and the apparent pK3 = 5.7 for protonation of the chloride-transporter complex. The activation of chloride exchange by external chloride was inhibited by internal protons, and internal protonation of the externally facing empty conformation had a pK1 = 6.1. External protons were also a mixed inhibitor of chloride exchange with the apparent pK1 = 5.0 for the empty outward-facing transporter conformation. Because of the pHo dependence of self-inhibition, the value of pK3 on the outside for chloride could not be accurately determined, but the apparent pK3 for protonation of the iodide-transporter complex on the extracellular surface was 4.9. The data support a mechanism with a single proton binding site that can alternatively have access to the cytoplasmic and extracellular solutions. It appears that this proton binding and transport site can be coupled to the single anion transport site for cotransport, but the two sites can be on opposite sides of the membrane at the same time and thus can be asynchronously transported by conformational changes of band 3.

Proton-sulfate cotransport: external proton activation of sulfate influx into human red blood cells

Sulfate influx into human red blood cells was measured at 0 and 22 degrees C at several fixed external pH values between 3 and 10. These cells had normal internal pH and chloride concentrations so that sulfate influx was not limited by the efflux half-cycle reactions. The flux was a Michaelis-Menten function of sulfate concentration at each pH with K1/2SO4 = 4-10 mM. External protons activated influx 100-fold at a single site with a pK = 5.9 at 22 degrees C and 5.5 at 0 degrees C. This pK is similar to the value 5.99 +/- 0.3 for external proton binding to the sulfate-loaded transporter at 0 degrees C (J. Gen. Physiol. 79: 87-114, 1982). The flux was stilbene sensitive even in valinomycin-treated cells and was independent of membrane potential. This proton-activated influx appears to be proton-sulfate cotransport. At high pH there was a proton-independent flux that was membrane potential and stilbene sensitive. This proton-insensitive flux appears to be SO4(2-)/Cl- exchange or net sulfate influx. The sulfate influx over the entire pH range may be described in terms of an equation for the sum of the influxes through these two pathways on band 3.

Proton-sulfate co-transport: mechanism of H+ and sulfate addition to the chloride transporter of human red blood cells

Proton and sulfate inhibition of the obligatory chloride-chloride exchange of human erythrocytes was measured at 0 degrees C to determine their mechanism of reaction with the anion transporter. The proton and sulfate that are co-transported by this mechanism at higher temperatures behaved as nontransported inhibitors at 0 degrees C. We analyzed the data in terms of four molecular mechanisms: (1) HSO4- addition to the transporter; (2) ordered addition with the proton first; (3) ordered addition with the sulfate first; (4) random addition to the transporter. The Dixon plots of 1/MCl vs. [SO4] at different proton concentrations were not parallel. Thus protons and sulfate ions were not mutually exclusive inhibitors. The slope of these Dixon plots was independent of pH above 7.0, which indicates that sulfate could bind to the unprotonated carrier and excludes the first two mechanisms. Protons were inhibitors of chloride flux in the absence of sulfate, which indicates that protons could bind to the unloaded carrier and excludes mechanism 3. The KI for sulfate was 4.35 +/0 0.36 mM. The pK for the protonatable group was 5.03 +/- 0.02. The binding of either a proton or sulfate to the carrier decreased the KI of the other by ninefold. The only simple mechanism consistent with the data is a random-ordered mechanism with more transporters loaded with a sulfate than loaded with a proton at the pH and sulfate concentrations of plasma.