Volume and anion dependency of ouabain-resistant K-Rb fluxes in sheep red blood cells

Advances in the development of novel compounds targeting cation-chloride cotransporter physiology

For about half a century, the pharmacology of electroneutral cation-chloride cotransporters has been dominated by a few drugs that are widely used in clinical medicine. Because these diuretic drugs are so good at what they do, there has been little incentive in expanding their pharmacology. The increasing realization that cation-chloride cotransporters are involved in many other key physiological processes and the knowledge that different tissues express homologous proteins with matching transport functions have rekindled interest in drug discovery. This review summarizes the methods available to assess the function of these transporters and describe the multiple efforts that have made to identify new compounds. We describe multiple screens targeting KCC2 function and one screen designed to find compounds that discriminate between NKCC1 and NKCC2. Two of the KCC2 screens identified new inhibitors that are 3-4 orders of magnitude more potent than furosemide. Additional screens identified compounds that purportedly increase cell surface expression of the cotransporter, as well as several FDA-approved drugs that increase KCC2 transcription and expression. The technical details of each screen biased them toward specific processes in the life cycle of the transporter, making these efforts independent and complementary. In addition, each drug discovery effort contributes to our understanding of the biology of the cotransporters.

N-terminal serine dephosphorylation is required for KCC3 cotransporter full activation by cell swelling

The K⁺:Cl⁻ cotransporter (KCC) activity is modulated by phosphorylation/dephosphorylation processes. In isotonic conditions, KCCs are inactive and phosphorylated, whereas hypotonicity promotes their dephosphorylation and activation. Two phosphorylation sites (Thr-991 and Thr-1048) in KCC3 have been found to be critical for its regulation. However, here we show that the double mutant KCC3-T991A/T1048A could be further activated by hypotonicity, suggesting that additional phosphorylation site(s) are involved. We observed that in vitro activated STE20/SPS1-related proline/alanine-rich kinase (SPAK) complexed to its regulatory MO25 subunit phosphorylated KCC3 at Ser-96 and that in Xenopus laevis oocytes Ser-96 of human KCC3 is phosphorylated in isotonic conditions and becomes dephosphorylated during incubation in hypotonicity, leading to a dramatic increase in KCC3 function. Additionally, WNK3, which inhibits the activity of KCC3, promoted phosphorylation of Ser-96 as well as Thr-991 and Thr-1048. These observations were corroborated in HEK293 cells stably transfected with WNK3. Mutation of Ser-96 alone (KCC3-S96A) had no effect on the activity of the cotransporter when compared with wild type KCC3. However, when compared with the double mutant KCC3-T991A/T1048A, the triple mutant KCC3-S96A/T991A/T1048A activity in isotonic conditions was significantly higher, and it was not further increased by hypotonicity or inhibited by WNK3. We conclude that serine residue 96 of human KCC3 is a third site that has to be dephosphorylated for full activation of the cotransporter during hypotonicity.

NH2-terminal heterogeneity in the KCC3 K+-Cl− cotransporter

The SLC12A6 gene encoding the K+-Cl− cotransporter KCC3 is expressed in multiple tissues, including kidney. Here, we report the molecular characterization of several NH2-terminal isoforms of human and mouse KCC3, along with intrarenal localization and functional characterization in Xenopus laevis oocytes. Two major isoforms, KCC3a and KCC3b, are generated by transcriptional initiation 5′ of two distinct first coding exons. Northern blot analysis of mouse tissues indicates that KCC3b expression is particularly robust in the kidney, which also expresses KCC3a. Western blotting of mouse tissue using an exon 3-specific antibody reveals that the kidney is also unique in expressing immunoreactive protein of a lower mass, suggestive evidence that the shorter KCC3b protein predominates in kidney. Immunofluorescence reveals basolateral expression of KCC3 protein along the entire length of the proximal tubule, in both the mouse and rat. Removal of the 15-residue exon 2 by alternative splicing generates the KCC3a-x2M and KCC3b-x2M isoforms; other splicing events at an alternative acceptor site within exon 1a generate the KCC3a-S isoform, which is 60 residues shorter than KCC3a. This variation in sequence of NH2-terminal cytoplasmic domains occurs proximal to a stretch of highly conserved residues and affects the content of putative phosphorylation sites. Kinetic characterization of KCC3a in X. laevis oocytes reveals apparent Kms for Rb+ and Cl− of 10.7 ± 2.5 and 7.3 ± 1.2 mM, respectively, with an anion selectivity of Br− > Cl− > PO4 = I− = SCN− = gluconate. All five NH2-terminal isoforms are activated by cell swelling (hypotonic conditions), with no activity under isotonic conditions. Although the isoforms do not differ in the osmotic set point of swelling activation, this activation is more rapid for the KCC3a-x2M and KCC3a-S proteins. In summary, there is significant NH2-terminal heterogeneity of KCC3, with particularly robust expression of KCC3b in the kidney. Basolateral swelling-activated K+-Cl− cotransport mediated by KCC3 likely functions in cell volume regulation during the transepithelial transport of both salt and solutes by the proximal tubule.

Molecular, functional, and genomic characterization of human KCC2, the neuronal K-Cl cotransporter

The expression level of the neuronal-specific K-Cl cotransporter KCC2 (SLC12A5) is a major determinant of whether neurons will respond to GABA with a depolarizing, excitatory response or a hyperpolarizing, inhibitory response. In view of the potential role in human neuronal excitability we have characterized the hKCC2 cDNA and gene. The 5.9 kb hKCC2 transcript is specific to brain, and is induced during in vitro differentiation of NT2 teratocarcinoma cells into neuronal NT2-N cells. The 24-exon SLC12A5 gene is on human chromosome 20q13, and contains a polymorphic dinucleotide repeat within intron 1 near a potential binding site for neuron-restrictive silencing factor. Expression of hKCC2 cRNA in Xenopus laevis oocytes results in significant Cl(-)-dependent (86)Rb(+) uptake under isotonic conditions; cell swelling under hypotonic conditions causes a 20-fold activation, which is blocked by the protein phosphatase inhibitor calyculin-A. In contrast, oocytes expressing mouse KCC4 do not mediate isotonic K-Cl cotransport but express much higher absolute transport activity than KCC2 oocytes under hypotonic conditions. Initial and steady state kinetics of hKCC2-injected oocytes were performed in both isotonic and hypotonic conditions, revealing K(m)s for K(+) and Cl(-) of 9.3+/-1.8 mM and 6.8+/-0.9 mM, respectively; both affinities are significantly higher than KCC1 and KCC4. The K(m) for Cl(-) is close to the intracellular Cl(-) activity of mature neurons, as befits a neuronal efflux mechanism.

Renal potassium-chloride cotransporters

The electroneutral cotransport of potassium and chloride is mediated by potassium-chloride transporters, which are encoded by members of the gene family of cation-chloride cotransporters. A significant body of evidence argues for swelling-activated, basolateral potassium-chloride transport in the proximal tubule and thick ascending limb, with a potential role in transepithelial salt transport. However, the lack of specific inhibitors has impeded progress in this area. The cloning of the four potassium-chloride cotransporter genes has sparked new interest in this transport pathway, and promises to yield novel insights into their roles in cellular and renal physiology.

Role of polyamine structure in inhibition of K+-Cl- cotransport in human red cell ghosts

1. K+-Cl- cotransport in human red cell ghosts is inhibited by divalent inorganic cations, soluble polycations and amphipathic organic cations. These findings suggest a common mechanism of inhibition, namely, binding of the cations to negative charges at the surface of a hydrophobic structure. 2. We have characterized the inhibitory capacity of a number of polyamines in order to obtain information about the nature of the charges with which they interact. Neomycin inhibited swelling-stimulated cotransport. The diquaternary amines dimethonium and decamethonium were relatively ineffective inhibitors. These compounds are thought to shield negative charges, but not bind to them. 3. Comparison of a homologous series of polyamines indicated that primary amines were better inhibitors than secondary amines, that inhibition increased with the charge of the polyamine, and that inhibition increased as the distance separating the amines increased. 4. The results indicate that the negative charges to which polycations bind are multiple and mobile. Since they must be associated with a hydrophobic environment, it is likely that they are negatively charged phospholipids located in the inner leaflet of the bilayer membrane. 5. Heating red cells or ghosts to 49 C denatures spectrin. Heating markedly increased K+ uptake in swollen ghosts but not in shrunken ghosts. The increase in uptake was reversed when swollen ghosts were shrunk even though denaturation of spectrin was not reversed. Polyamines, which inhibited swelling-activated K+ uptake in control ghosts, similarly inhibited the increased uptake in heated ghosts. 6. We speculate that spectrin, which is closely associated with the inner bilayer leaflet, shields negative charges in a volume-dependent manner and so regulates volume-sensitive K+ transport.

Measurement of homocysteine concentrations and stable isotope tracer enrichments in human plasma

Elevated levels of plasma homocysteine have been established as an independent risk factor for cardiovascular disease. Homocysteine is in low concentration in plasma (5-15 microM) and is bound to other thiols (e.g., cysteine in plasma proteins) via disulfide bonds. Existing methods for measuring homocysteine have difficulty in reducing and maintaining the reduction of homocysteine for measurement. We describe a GC/MS method that first reduces the disulfides in the physiological sample matrix and then immediately alkylates the free thiols with 4-vinylpyridine to prevent the reformation of the disulfide bonds. We use a deuterated internal standard, [3,3,3',3',4,4,4',4'-2H8]homocystine to account for losses associated with the isolation, derivatization, and measurement of the natural homocysteine. The amino acids are separated and derivatized to form the tert-butyldimethylsilyl derivatives. This method requires only 50 microL of plasma to measure homocysteine concentrations to 5 microM. Total homocysteine concentrations in plasma can be measured routinely from 0.5-mL samples with relative intra- and interday precisions of 1.3 and 4.0%, respectively. This method is sensitive enough to determine tracer enrichments of [1-13C]homocysteine with a detection limit of < 0.3 mol% excess and an average tracer precision of 0.6%.

Oxygen-dependent K+ fluxes in sheep red cells

1. This study was designed to investigate the O2 dependence of K+ influx in sheep red cells. Influx was determined using 86Rb+ as a tracer for K+; glass tonometers coupled to a gas mixing pump were used to equilibrate cell samples to the requisite oxygen tension (PO2). 2. Both volume- and H(+)-stimulated K+ influxes in low potassium-containing (LK) sheep red cells were approximately doubled on equilibration with O2 relative to influxes measured in N2.O2-dependent influxes were abolished when Cl- was replaced with NO3-, consistent with mediation by the KCl cotransporter. At pH 7, PO2 required for half-maximal stimulation was 56 +/- 1 mmHg (mean +/- S.E.M., 3 sheep) for the O2-dependent component of K+ influx: thus PO2 values over the physiological range affected K+ influx. 3. K+ influx in fully deoxygenated sheep red cells showed substantial volume and H+ sensitivity. These residual components in N2 were also Cl- dependent, indicating that the KCl cotransporter of LK sheep red cells was active in the absence of O2. 4. Volume-sensitive K+ influxes in high potassium-containing (HK) sheep red cells responded in a similar way to those in cells from LK sheep, although much smaller in magnitude, showing that intracellular [K+] had no significant effect on the O2 dependence of the cotransporter. 5. Intracellular [Mg2+] ([Mg2+]i) was altered by incubating sheep red cells with A23187 (20 microM) and different values of extracellular [Mg2+] ([Mg2+]o). Total [Mg2+]i was determined by atomic absorption spectroscopy and free [Mg2+]i from [Mg2+]o and the Donnan ratio. Total [Mg2+]i was 1.29 +/- 0.08 mM (mean +/- S.E.M., n = 5), similar to that reported in the literature. Estimates of free [Mg2+]i showed an increase from 0.39 +/- 0.05 in oxygenated cells to 0.52 +/- 0.04 mM (mean +/- S.E.M., n = 5; P < 0.05) in deoxygenated ones. 6. Finally, although K+ influxes were altered by pharmacological loading or depletion of cells with Mg2+, the free [Mg2+]i required to affect influxes significantly was outside the physiological range. Results are difficult to reconcile with PO2 modulating KCl cotransport activity directly via changes in free [Mg2+]i or [Mg(2+)-ATP]i.

Human monocyte interleukin-1beta posttranslational processing. Evidence of a volume-regulated response

Interleukin (IL)-1beta produced by monocytes and macrophages is not released via the normal secretory apparatus, and prior to its release, this cytokine must be proteolytically processed to generate a mature biologically active species. Biochemical mechanisms that regulate these posttranslational steps are not well understood. Lipopolysaccharide (LPS) is a poor activator of IL-1 posttranslational processing despite serving as a potent inducer of IL-1 synthesis. For example, freshly isolated human monocytes treated with LPS released <30% of their newly synthesized IL-1beta as the mature 17-kDa cytokine species, and monocytes that were aged overnight in culture prior to LPS treatment released no 17-kDa cytokine. In contrast, addition of extracellular ATP promoted IL-1beta posttranslational processing from both monocyte populations. Previous studies indicated that ATP, acting via surface P2Z-type receptors, promoted major intracellular ionic changes. To explore whether these ionic changes were required for cytokine posttranslational processing, LPS-stimulated human monocytes were maintained in ionically altered media. Hypotonic conditions promoted an efficient and selective release of mature 17-kDa IL-1beta from LPS-activated monocytes in the absence of ATP. In contrast, hypertonic conditions blocked the ATP-induced posttranslational processing reactions. Both hypotonic stress- and ATP-induced processing were blocked when NaI was substituted for NaCl within the medium; substitution with NaSCN or NaNO3 also blocked the ATP response, but these salts were less inhibitory against the hypotonic stimulus. Sodium glucuronate substitution did not inhibit cytokine processing induced by either stimulus. Removal of divalent cations from the medium did not affect the ATP response, but pretreatment of monocytes with the phosphatase inhibitor okadaic acid dose-dependently suppressed ATP-induced IL-1beta posttranslational processing. A volume-induced change to the intracellular ionic environment, therefore, may represent a key element of the mechanism by which IL-1beta posttranslational processing is initiated. The strong dependence of this cytokine release mechanism on chloride anions suggests that selective anion transporters function as important components of this response.

Amoxicillin intestinal absorption reduction by amiloride: possible role of the Na(+) -H+ exchanger

Intestinal absorption of beta-lactam antibiotics has been shown to use the dipeptide carrier system. In vitro experiments have established that the efficiency of uptake by enterocytes depends on an inwardly directed proton gradient--dipeptides and beta-lactam antibiotics being cotransported along with hydrogen ion. This gradient is thought to result from the sodium-hydrogen (Na(+)-H+) exchanger located on the brush-border membrane. The aim of the present study was to assess the in vivo relevance of these data in humans by examining the effect of amiloride, a well-known inhibitor of the Na(+) -H+ exchanger, on the bioavailability of amoxicillin in eight healthy volunteers. The results show that amiloride reduces significantly amoxicillin absorption rate (mean time to maximum concentration increases from 1.0 to 1.6 hours, p < 0.05) and absolute bioavailability (by 27%, p < 0.01) and that amiloride-induced inhibition of the intestinal Na(+) -H+ exchange could be associated with an additional inhibitory effect on (Na/K)-ATPase activity. The present data seem to confirm the role of Na(+) -H+ exchange in the uptake of beta-lactams by the intestine and to support the indirect sodium dependence of this carrier system in vivo.

Inhibition by N-ethylmaleimide of H(+)-ATPase reduces the cellular action of arginine vasopressin in cultured rat renal papillary collecting tubule cells

We examined whether H(+)-ATPase is involved in the control of cellular action of arginine vasopressin (AVP) to produce adenosine 3', 5'-monophosphate (cAMP) and mobilize cellular free calcium ([Ca2+]i) in rat renal papillary collecting tubule cells in culture. N-Ethylmaleimide (NEM), an inhibitor of H(+)-ATPase, reduced the cellular pH (pHi) dose-dependently. AVP increased cellular cAMP production in a dose-dependent manner. 5 x 10(-6) and 1 x 10(-5) M NEM significantly diminished the AVP-induced increase in cAMP production. 1 x 10(-7) M AVP also increased [Ca2+]i from 111.2 to 189.3 nM, which was significantly reduced by NEM in a dose-dependent manner. These results indicate that H(+)-ATPase participates the cellular action of AVP mediated via the pHi control in renal papillary collecting tubule cells.

Trans effects of cellular K and Cl on ouabain-resistant Rb(K) influx in low K sheep red blood cells: further evidence for asymmetry of K-Cl cotransport [corrected]

The electroneutral K-Cl cotransport in low K+ (LK) sheep red blood cells is kinetically asymmetric and thermodynamically outward poised (Delpire and Lauf, 1991a). We have shown previously by trans-inhibition kinetics that Cl- binds prior to K+ to the outside configuration of the carrier. In the present study, we confirm that K+ and Cl- bind randomly to the cytoplasmic aspect of the transporter because K+ in the absence of Cl-, and Cl- in the absence of K+, trans-inhibit ouabain-resistant Rb+(K+) influx in these cells. In contrast to the trans-inhibition pattern observed outside, neither K+ nor Cl- trans-inhibit K+(Rb+) influx in the presence of the cotransported ion, further supporting the asymmetry of the K-Cl cotransporter.

Microfluorimetric imaging study of the mechanism of activation of the Na+/H+ antiport by muscarinic agonist in rat mandibular acinar cells

The mechanism of regulation of intracellular pH (pHi) in dispersed acini from the rat mandibular salivary gland has been studied with a microfluorimetric imaging method and the pH probe 2',7'-bis(2-carboxyethyl)-5(and -6)-carboxyfluorescein. The pHi in the TRIS/HEPES-buffered standard solution was 7.29 +/- 0.01. Addition of 1 mumol/l acetylcholine (ACh) or ionomycin caused a sustained increase in the pHi. These agents decreased pHi in the absence of external Na+ or in the presence of amiloride. The rate of pHi recovery from an acid load after NH+4 prepulse was a linear function of pHi and increased as pHi became more acidic. Addition of ACh shifted the relationship towards a more alkaline pHi range. The increase in pHi induced by ACh or ionomycin was not inhibited by the protein kinase C inhibitors staurosporine (10 nM) and 1-(5-isoquinolinesulfonyl)-1-methylpiperazine (50 mumol/l). Addition of 0.1-1 mumol/l phorbol 12-myristate 13-acetate (TPA) had little effect on pHi within 10 min; however, exposure to TPA for 120 min resulted in a significant rise in pHi. In Ca(2+)-free solution with 50 mumol/l 8-(diethylamino)-octyl-3,4,5-trimethoxybenzoate, the ACh-induced rise in both pHi and cytosolic Ca2+ concentration was suppressed. ACh and ionomycin caused an increment of amiloride-sensitive acid output into the extracellular fluid, while 20 mumol/l 1-oleoyl-2-acetylglycerol had little effect on it. It was concluded that (a) stimulation with ACh activated the Na+/H+ antiport in the plasma membrane, (b) ACh also stimulated the intracellular acid production but acid extrusion by the Na+/H+ antiport prevented the cell from intracellular acidification, and (c) the major route of signal transduction for the ACh-induced activation of the Na+/H+ antiport was independent of protein kinase C but was dependent on the rise in cytosolic Ca2+ concentration. The implication of the cytosolic acidification and cell volume change in pHi regulation is discussed.

Relationship between total magnesium concentration and free intracellular magnesium in sheep red blood cells

The cellular free magnesium concentration of ionophore A23187 permeabilized high potassium sheep erythrocytes was measured by 31P nuclear magnetic resonance spectroscopy, and the total cellular magnesium concentration was determined by atomic absorption spectroscopy. The free versus total cellular magnesium concentrations yield a linear relationship on a log-log scale in the concentration range from 0.3 to 1.92 mmol Mg/liter cells. Thus, free intracellular magnesium concentrations can be calculated from atomic absorption data. The method permits the estimation of physiologically or experimentally induced variations of intracellular free magnesium concentrations between 7 and 405 microM magnesium in cell water. This range encompasses the free magnesium concentration of 335 +/- 60 microM in cell water determined for untreated erythrocytes.

Kinetics of Cl-dependent K fluxes in hyposmotically swollen low K sheep erythrocytes

A detailed kinetic study of K:Cl cotransport in hyposmotically swollen low K sheep red blood cells was carried out to characterize the nature of the outwardly poised carrier. The kinetic parameters were determined from the rate of K efflux and influx under zero-K-trans conditions in red cells with cellular K altered by the nystatin method and with different extracellular K or Rb concentrations. Although apparent affinities for efflux and influx were quite similar, the maximal velocity for K efflux was approximately two times greater than for influx. Furthermore, at thermodynamic equilibrium (i.e., when the ion product of K and Cl within the cell was equal to that outside) a temperature-dependent net K efflux was observed, approaching zero only when the external product reached approximately two times the internal product. The binding order of the ions to the transporter was asymmetric, being ordered outside (Cl binding first, followed by K) and random inside. K efflux but not influx was trans-inhibited by KCl. Trans inhibition of K efflux was used to verify the order of binding outside: trans inhibition by external Cl occurred in the absence of external K, but not vice versa. Thus K:Cl cotransport is kinetically asymmetric in hyposmotically swollen low K sheep red cells.

Volume-sensitive K-Cl cotransport in inside-out vesicles made from erythrocyte membranes from sheep of low-K phenotype

Unidirectional K ion effluxes were measured from inside-out vesicles prepared from erythrocyte membranes from sheep of the low-K phenotype. Total K efflux was 150 nmol per mg of protein per hr in a Cl medium of 295 mosmol/kg (with the Na/K pump inhibited). Cl-dependent K efflux (determined with methanesulfonate replacing Cl) was 54 nmol/(mg.hr). Cl-dependent K efflux (K-Cl cotransport) increased to 77 nmol/(mg.hr) with osmotic swelling of approximately 30% in 230-mosmol/kg medium and decreased to 13 nmol/(mg.hr) after shrinkage of approximately 60% in 430-mosmol/kg medium. Osmotically induced changes in transport and vesicle volume were reversible. K-Cl cotransport was enhanced by ATP. Nonhydrolyzable ATP analogues failed to substitute for ATP, indicating that phosphorylation is involved. However, in the absence of added ATP there was significant K-Cl cotransport, suggesting that phosphorylation is not essential for function. The results provide clues about the nature of the signals detected by the sensor of cell volume changes and demonstrate that inside-out vesicles from sheep erythrocyte membranes provide an advantageous experimental system for investigation of the volume sensor.

Mechanisms of intracellular pH regulation in the hamster inner medullary collecting duct perfused in vitro

To examine the mechanisms of H+ transport in the mid-inner medullary collecting duct of hamsters, we measured the intracellular pH (pHi) in the in vitro perfused tubules by microscopic fluorometry using 2',7'-bis(carboxyethyl)-carboxyfluorescein (BCECF) as a fluorescent probe. In the basal condition, pHi was 6.74 +/- 0.04 (n = 45) in HCO3(-)-free modified Ringer solution. Either elimination of Na+ from the bath or addition of amiloride (1 mM) to the bath produced a reversible fall in pHi. After acid loading with 25 mM NH4Cl, pHi spontaneously recovered with an initial recovery rate of 0.096 +/- 0.012 (n = 23) pH unit/min. In the absence of ambient Na+, after removal of NH+4, the pHi remained low (5.95 +/- 0.10, n = 8) and showed no signs of recovery. Subsequent restoration of Na+ only in the lumen had no effect on pHi. However, when Na+ in the bath was returned to the control level, pHi recovered completely Amiloride (1 mM) in the bath completely inhibited the Na(-)-dependent pHi recovery. Furthermore, elimination of Na+ from the bath, but not from the lumen, decreased pHi from 6.97 +/- 0.07 to 6.44 +/- 0.05 (n = 12) in the HCO3-/Ringer solution or 6.70 +/- 0.03 to 6.02 +/- 0.5 pH unit/min in the presence of CO2/HCO3-, whereas it did not recover in the absence of CO2/HCO3-.(ABSTRACT TRUNCATED AT 250 WORDS)

K-Cl cotransport in LK sheep erythrocytes: kinetics of stimulation by cell swelling

The effects of osmotic cell swelling were studied on the kinetics of Cl-dependent K+ influx, K-Cl cotransport, in erythrocytes from sheep of the low K+ (LK) phenotype. Swelling approximately 25% stimulated transport by increasing maximum velocity (Jmax) approximately 1.5-fold and by increasing apparent affinity for external K (Ko) nearly twofold. Dithiothreitol (DTT) was shown to be a partial, reversible inhibitor of K-Cl cotransport. It inhibited in cells of normal volume by reducing Jmax more than twofold; apparent affinity for Ko was increased by DTT, suggesting that DTT stabilizes the transporter-Ko complex. Cell swelling reduced the extent of inhibition by DTT: Jmax was inhibited by only about one-third in swollen cells, and apparent affinity was only slightly affected. This result suggested that DTT does not act directly on the transporter, but on a hypothetical regulator, an endogenous inhibitor. Swelling relieves inhibition by the regulator, and reduces the effect of DTT. Reducing intracellular Mg2+, Mgc, stimulated cotransport. Swelling of low-Mg2+ cells stimulated transport further, but only by raising apparent affinity for Ko nearly threefold: Jmax was unaffected. Thus effects of swelling on Jmax and apparent affinity are separable processes. The inhibitory effects of Mgc and DTT were shown to be additive, indicating separate modes of action. There appear to be two endogenous inhibitors: the hypothetical regulator, which holds affinity for Ko, low; and Mgc, which affects Jmax, perhaps by holding some transporters in an inactive form. Swelling stimulates transport by relieving both types of inhibition.

Characteristics of the volume- and chloride-dependent K transport in human erythrocytes homozygous for hemoglobin C

In human red cells homozygous for hemoglobin C (CC), cell swelling and acid pH increase K efflux and net K loss in the presence of ouabain (0.1 mM) and bumetanide. We report herein, that K influx is also dependent on cell volume in CC cells: cell swelling induces a marked increase in the maximal rate (from 6 to 18 mmol/liter cell X hr) and in the affinity for external K (from 77 +/- 16 mM to 28 +/- 3 mM) of K influx. When the external K concentration is varied from 0 to 140 mM. K efflux from CC and normal control cells is unaffected. Thus, K/K exchange is not a major component of this K movement. K transport through the pathway of CC cells is dependent on the presence of chloride or bromide; substitution with nitrate, acetate or thiocyanate inhibits the volume- and pH-dependent K efflux. When CC cells are separated according to density, a sizable volume-dependent component of K efflux can be identified in all the fractions and is the most active in the least dense fraction. N-ethylmaleimide (NEM) markedly stimulates K efflux from CC cells in chloride but not in nitrate media, and this effect is present in all the fractions of CC cells separated according to density. The persistence of this transport system in denser CC cells suggests that not only cell age, but also the presence of the positively charged C hemoglobin is an important determinant of the activity of this system. These data also indicate that the K transport pathway of CC cells is not an electrodiffusional process and is coupled to chloride.

Agonist-induced activation of Na+/H+ exchange in rat parotid acinar cells

The present studies were designed to test our previous suggestion that Na+/H+ exchange was activated by muscarinic stimulation of rat parotid acinar cells. Consistent with this hypothesis, we demonstrate here that intact rat parotid acini stimulated with the muscarinic agonist carbachol in HCO3- -free medium show an enhanced recovery from an acute acid load as compared to similarly challenged untreated preparations. Amiloride-sensitive 22Na uptake, due to Na+/H+ exchange, was also studied in plasma membrane vesicles prepared from rat parotid acini pretreated with carbachol. This uptake was stimulated two-fold relative to that observed in vesicles from control (untreated) acini. This stimulation was time dependent, requiring approximately 15 min of acinar incubation with carbachol to reach completion, and was blocked by the presence of the muscarinic antagonist atropine (2 x 10(-5) M) in the pretreatment medium. The effect of carbachol was dose dependent with K0.5 approximately 3 x 10(-6) M. Stimulation of the exchanger was also seen in vesicles prepared from acini pretreated with the alpha-adrenergic agonist epinephrine, but not with the beta-adrenergic agonist isoproterenol, or with substance P. Kinetic analysis indicated that the stimulation induced by carbachol was due to an alkaline shift in the pH responsiveness of the exchanger in addition to an increased apparent transport capacity. Taken together with previous results from this and other laboratories, these results strongly suggest that the Na+/H+ exchanger and its regulation are intimately involved in the fluid-secretory response of the rat parotid.

Kinetic comparison of ouabain-resistant K:Cl fluxes (K:Cl [Co]-transport) stimulated in sheep erythrocytes by membrane thiol oxidation and alkylation

The stimulatory effects of two thiol (SH) group oxidants, methylmethane thiosulfonate (MMTS) and diazene dicarboxylic acid bis [N,N-dimethylamide] (diamide), on the kinetics of ouabain-resistant (OR) K:Cl [co]-transport in low K (LK) sheep red blood cells were compared with the effects of alkylating agents, notably N-ethylmaleimide (NEM). At low concentrations, both MMTS and diamide stimulated K:Cl [co]-transport, and with a latency period, as measured by OR zero-trans K efflux and OR uptake of external Rb, Rbo, as K congener in Cl and NO3 media. At high concentrations the effect of diamide saturated, and that of MMTS disappeared. The stimulatory effect of MMTS was partially reversed by the reducing agent dithiothreitol (DTT) known to fully restore the diamide-activated K flux (Lauf, J. Memb. Biol. 101:179-188, 1988). In diamide preequilibrated LK sheep red cells, the Km of K:Cl [co]-transport for external Cl, Clo, was 84.3 mM, and 18.7 mM for Rbo, with nearly identical Vmax values around 4 mmol Rb/L cells x h for K (Rb) fluxes in Cl and after correction for the small Cl-independent component. Zero net K (Rb) flux existed at Kc (cell K)/Rbo concentration ratios, [K]c/[Rb]c, of 0.8 i.e. when the electrochemical driving forces across the membrane were about equal. The measured K efflux/Rb influx ratios were almost twice those predicted from [K]c/[Rb]o and the Cl equilibrium potential suggesting that the diamide-stimulated K (Rb) flux may occur through non-diffusional, carrier-mediated transport.(ABSTRACT TRUNCATED AT 250 WORDS)