Functional interaction of the K-Cl cotransporter (KCC1) with the Na-K-Cl cotransporter in HEK-293 cells

We have studiedthe regulation of the K-Cl cotransporter KCC1 and its functionalinteraction with the Na-K-Cl cotransporter. K-Cl cotransporter activitywas substantially activated in HEK-293 cells overexpressing KCC1(KCC1-HEK) by hypotonic cell swelling, 50 mM external K, andpretreatment with N-ethylmaleimide(NEM). Bumetanide inhibited ⁸⁶Rbefflux in KCC1-HEK cells after cell swelling [inhibition constant (K_i) ~190µM] and pretreatment with NEM(K_i ~60 µM).Thus regulation of KCC1 is consistent with properties of the red cellK-Cl cotransporter. To investigate functional interactions between K-Cland Na-K-Cl cotransporters, we studied the relationship between Na-K-Clcotransporter activation and intracellular Cl concentration([Cl]_i). Without stimulation, KCC1-HEK cells had greater Na-K-Cl cotransporter activitythan controls. Endogenous Na-K-Cl cotransporter of KCC1-HEK cells wasactivated <2-fold by low-Cl hypotonic prestimulation, compared with10-fold activation in HEK-293 cells and >20-fold activation in cellsoverexpressing the Na-K-Cl cotransporter (NKCC1-HEK). KCC1-HEK cellshad lower resting[Cl]_i than HEK-293cells; cell volume was not different among cell lines. We found a steeprelationship between[Cl]_i and Na-K-Clcotransport activity within the physiological range, supporting aprimary role for [Cl]_iin activation of Na-K-Cl cotransport and in apical-basolateral crosstalk in ion-transporting epithelia.     

Defective coupling of apical PTH receptors to phospholipase C prevents internalization of the Na+-phosphate cotransporter NaPi-IIa in Nherf1-deficient mice

Phosphate reabsorption in the renal proximal tubule occurs mostly via the type IIa Na⁺-phosphate cotransporter (NaP_i-IIa) in the brush border membrane (BBM). The activity and localization of NaP_i-IIa are regulated, among other factors, by parathyroid hormone (PTH). NaP_i-IIa interacts in vitro via its last three COOH-terminal amino acids with the PDZ protein Na⁺/H⁺-exchanger isoform 3 regulatory factor (NHERF)-1 (NHERF1). Renal phosphate reabsorption in Nherf1-deficient mice is altered, and NaP_i-IIa expression in the BBM is reduced. In addition, it has been proposed that NHERF1 and NHERF2 are important for the coupling of PTH receptors (PTHRs) to phospholipase C (PLC) and the activation of the protein kinase C pathway. We tested the role of NHERF1 in the regulation of NaP_i-IIa by PTH in Nherf1-deficient mice. Immunohistochemistry and Western blotting demonstrated that stimulation of apical and basolateral receptors with PTH-(1–34) led to internalization of NaP_i-IIa in wild-type and Nherf1-deficient mice. Stimulation of only apical receptors with PTH-(3–34) failed to induce internalization in Nherf1-deficient mice. Expression and localization of apical PTHRs were similar in wild-type and Nherf1-deficient mice. Activation of the protein kinase C- and A-dependent pathways with 1,2-dioctanoyl-sn-glycerol or 8-bromo-cAMP induced normal internalization of NaP_i-IIa in wild-type, as well as Nherf1-deficient, mice. Stimulation of PLC activity due to apical PTHRs was impaired in Nherf1-deficient mice. These data suggest that NHERF1 in the proximal tubule is important for PTH-induced internalization of NaP_i-IIa and, specifically, couples the apical PTHR to PLC. phosphate cotransporter; PDZ protein; parathyroid hormone; proximal tubule     

Mouse K-Cl cotransporter KCC1: cloning, mapping, pathological expression, and functional regulation

Although K-Cl cotransporter (KCC1) mRNA is expressed in manytissues, K-Cl cotransport activity has been measured in few cell types,and detection of endogenous KCC1 polypeptide has not yet been reported.We have cloned the mouse erythroid KCC1 (mKCC1) cDNA and its flankinggenomic regions and mapped the mKCC1 gene to chromosome 8. Threeanti-peptide antibodies raised against recombinant mKCC1 function asimmunoblot and immunoprecipitation reagents. The tissue distributionsof mKCC1 mRNA and protein are widespread, and mKCC1 RNA isconstitutively expressed during erythroid differentiation of ES cells.KCC1 polypeptide or related antigen is present in erythrocytes ofmultiple species in which K-Cl cotransport activity has beendocumented. Erythroid KCC1 polypeptide abundance is elevated inproportion to reticulocyte counts in density-fractionated cells, inbleeding-induced reticulocytosis, in mouse models of sickle celldisease and thalassemia, and in the corresponding human disorders.mKCC1-mediated uptake of ⁸⁶Rb intoXenopus oocytes requires extracellularCl, is blocked by thediureticR(+)-[2-n-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-indenyl-5-yl-)oxy]acetic acid, and exhibits an erythroid pattern of acute regulation, with activation by hypotonic swelling,N-ethylmaleimide, and staurosporine and inhibition by calyculin and okadaic acid. These reagents and findings will expedite studies of KCC1 structure-function relationships and of the pathobiology of KCC1-mediated K-Cl cotransport.

     

NHE3-dependent cytoplasmic alkalinization is triggered by Na(+)-glucose cotransport in intestinal epithelia

Cytoplasmic pH (pH_i) was evaluated duringNa⁺-glucose cotransport in Caco-2 intestinal epithelialcell monolayers. The pH_i increased by 0.069 ± 0.002 within 150 s after initiation of Na⁺-glucosecotransport. This increase occurred in parallel with glucose uptake andrequired expression of the intestinal Na⁺-glucosecotransporter SGLT1. S-3226, a preferential inhibitor ofNa⁺/H⁺ exchanger (NHE) isoform 3 (NHE3),prevented cytoplasmic alkalinization after initiation ofNa⁺-glucose cotransport with an ED₅₀ of 0.35 µM, consistent with inhibition of NHE3, but not NHE1 or NHE2. Incontrast, HOE-694, a poor NHE3 inhibitor, failed to significantlyinhibit pH_i increases at <500 µM.Na⁺-glucose cotransport was also associated with activationof p38 mitogen-activated protein (MAP) kinase, and the p38 MAP kinase inhibitors PD-169316 and SB-202190 prevented pH_i increasesby 100 ± 0.1 and 86 ± 0.1%, respectively. Conversely,activation of p38 MAP kinase with anisomycin induced NHE3-dependentcytoplasmic alkalinization in the absence of Na⁺-glucosecotransport. These data show that NHE3-dependent cytoplasmic alkalinization occurs after initiation of SGLT1-mediatedNa⁺-glucose cotransport and that the mechanism of this NHE3activation requires p38 MAP kinase activity. This coordinatedregulation of glucose (SGLT1) and Na⁺ (NHE3) absorptiveprocesses may represent a functional activation of absorptiveenterocytes by luminal nutrients.

     

Deciphering PiT transport kinetics and substrate specificity using electrophysiology and flux measurements

Members of the SLC20 family or type III Na⁺-coupled P_i cotransporters (PiT-1, PiT-2) are ubiquitously expressed in mammalian tissue and are thought to perform a housekeeping function for intracellular P_i homeostasis. Previous studies have shown that PiT-1 and PiT-2 mediate electrogenic P_i cotransport when expressed in Xenopus oocytes, but only limited kinetic characterizations were made. To address this shortcoming, we performed a detailed analysis of SLC20 transport function. Three SLC20 clones (Xenopus PiT-1, human PiT-1, and human PiT-2) were expressed in Xenopus oocytes. Each clone gave robust Na⁺-dependent ³²P_i uptake, but only Xenopus PiT-1 showed sufficient activity for complete kinetic characterization by using two-electrode voltage clamp and radionuclide uptake. Transport activity was also documented with Li⁺ substituted for Na⁺. The dependence of the P_i-induced current on P_i concentration was Michaelian, and the dependence on Na⁺ concentration indicated weak cooperativity. The dependence on external pH was unique: the apparent P_i affinity constant showed a minimum in the pH range 6.2–6.8 of 0.05 mM and increased to 0.2 mM at pH 5.0 and pH 8.0. Xenopus PiT-1 stoichiometry was determined by dual ²²Na-³²P_i uptake and suggested a 2:1 Na⁺:P_i stoichiometry. A correlation of ³²P_i uptake and net charge movement indicated one charge translocation per P_i. Changes in oocyte surface pH were consistent with transport of monovalent P_i. On the basis of the kinetics of substrate interdependence, we propose an ordered binding scheme of Na⁺:H₂PO₄^–:Na⁺. Significantly, in contrast to type II Na⁺-P_i cotransporters, the transport inhibitor phosphonoformic acid did not inhibit PiT-1 or PiT-2 activity. Na⁺-P_i cotransport; two-electrode voltage clamp; surface pH electrode; SLC20; retroviral receptor     

Na+ influx and Na+-K+ pump activation during short-term exposure of cardiac myocytes to aldosterone

To examine the effect of aldosterone on sarcolemmalNa⁺ transport, we measuredouabain-sensitive electrogenicNa⁺-K⁺pump current(I_p) involtage-clamped ventricular myocytes and intracellularNa⁺ activity(aⁱ_Na) in right ventricularpapillary muscles. Aldosterone (10 nM) induced an increase in bothI_p and the rateof rise of aⁱ_Na duringNa⁺-K⁺pump blockade with the fast-acting cardiac steroid dihydroouabain. Thealdosterone-induced increase inI_p and rate ofrise of aⁱ_Na was eliminated bybumetanide, suggesting that aldosterone activates Na⁺ influx through theNa⁺-K⁺-2Clcotransporter. To obtain independent support for this, theNa⁺,K⁺, andCl concentrations in thesuperfusate and solution of pipettes used to voltage clamp myocyteswere set at levels designed to abolish the inward electrochemicaldriving force for theNa⁺-K⁺-2Clcotransporter. This eliminated the aldosterone-induced increase inI_p. We concludethat in vitro exposure of cardiac myocytes to aldosterone activates theNa⁺-K⁺-2Clcotransporter to enhance Na⁺influx and stimulate theNa⁺-K⁺pump.

     

Ca2+ sensitivity of BK channels in GH3 cells involves cytosolic phospholipase A2

To test thehypothesis that intracellular Ca²⁺activation of large-conductanceCa²⁺-activatedK⁺ (BK) channels involves thecytosolic form of phospholipase A₂ (cPLA₂), we first inhibited theexpression of cPLA₂ by treating GH₃ cells with antisenseoligonucleotides directed at the two possible translation start siteson cPLA₂. Western blot analysis and a biochemical assay of cPLA₂activity showed marked inhibition of the expression ofcPLA₂ in antisense-treated cells.We then examined the effects of intracellularCa²⁺ concentration([Ca²⁺]_i)on single BK channels from these cells. Open channel probability (P_o) for thecells exposed to cPLA₂ antisenseoligonucleotides in 0.1 µM intracellularCa²⁺ was significantly lower thanin untreated or sense oligonucleotide-treated cells, but the voltagesensitivity did not change (measured as the slope of theP_o-voltagerelationship). In fact, a 1,000-fold increase in[Ca²⁺]_ifrom 0.1 to 100 µM did not significantly increaseP_oin these cells, whereas BK channels from cells in the other treatmentgroups showed a normalP_o-[Ca²⁺]_iresponse. Finally, we examined the effect of exogenous arachidonic acidon theP_oof BK channels from antisense-treated cells. Although arachidonic aciddid significantly increaseP_o,it did so without restoring the[Ca²⁺]_isensitivity observed in untreated cells. We conclude that although [Ca²⁺]_idoes impart some basal activity to BK channels inGH₃ cells, the steepP_o-[Ca²⁺]_irelationship that is characteristic of these channels involves cPLA₂.

     

Protein kinase D inhibits plasma membrane Na+/H+ exchanger activity

The regulation of plasma membraneNa⁺/H⁺exchanger (NHE) activity by protein kinase D (PKD), a novel proteinkinase C- and phorbol ester-regulated kinase, was investigated. Todetermine the effect of PKD on NHE activity in vivo, intracellular pH(pH_i) measurements were made inCOS-7 cells by microepifluorescence using the pH indicator cSNARF-1.Cells were transfected with empty vector (control), wild-type PKD, orits kinase-deficient mutant PKD-K618M, together with green fluorescentprotein (GFP). NHE activity, as reflected by the rate of acid efflux(J_H), wasdetermined in single GFP-positive cells following intracellularacidification. Overexpression of wild-type PKD had no significanteffect on J_H(3.48 ± 0.25 vs. 3.78 ± 0.24 mM/min in control atpH_i 7.0). In contrast,overexpression of PKD-K618M increasedJ_H (5.31 ± 0.57 mM/min at pH_i 7.0;P < 0.05 vs. control). Transfectionwith these constructs produced similar effects also in A-10 cells,indicating that native PKD may have an inhibitory effect on NHE in bothcell types, which is relieved by a dominant-negative action ofPKD-K618M. Exposure of COS-7 cells to phorbol ester significantlyincreased J_H in control cells but failed to do so in cells overexpressing either wild-type PKD (due to inhibition by the overexpressed PKD) or PKD-K618M(because basal J_Hwas already near maximal). A fusion protein containing the cytosolicregulatory domain (amino acids 637-815) of NHE1 (the ubiquitousNHE isoform) was phosphorylated in vitro by wild-type PKD, but with lowstoichiometry. These data suggest that PKD inhibits NHE activity,probably through an indirect mechanism, and represents a novel pathwayin the regulation of the exchanger.

     

Differing temporal roles of Ca2+ and cAMP in nicotine-elicited elevation of tyrosine hydroxylase mRNA

The involvement of cAMP- andCa²⁺-mediated pathways in theactivation of tyrosine hydroxylase (TH) gene expression by nicotine wasexamined in PC-12 cells. ExtracellularCa²⁺ and elevations inintracellular Ca²⁺ concentration([Ca²⁺]_i)were required for nicotine to increase TH mRNA. The nicotine-elicited rapid rise in[Ca²⁺]_iwas inhibited by blockers of either L-type or N-type, and to a lesserextent P/Q-, but not T-type, voltage-gatedCa²⁺ channels. With continualnicotine treatment,[Ca²⁺]_ireturned to basal levels within 3-4 min. After a lag of~5-10 min, there was a smaller elevation in[Ca²⁺]_ithat persisted for 6 h and displayed different responsiveness toCa²⁺ channel blockers. This secondphase of elevated[Ca²⁺]_iwas blocked by an inhibitor of store-operatedCa²⁺ channels, consistent with theobserved generation of inositol trisphosphate.1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM (BAPTA-AM), when added before or 2 h after nicotine,prevented elevation of TH mRNA. Nicotine treatment significantly raised cAMP levels. Addition of the adenylyl cyclase inhibitor2',5'-dideoxyadenosine (DDA) prevented thenicotine-elicited phosphorylation of cAMP response element bindingprotein. DDA also blocked the elevation of TH mRNA only when addedafter the initial transient rise in [Ca²⁺]_iand not after 1 h. This study reveals that several temporal phases areinvolved in the induction of TH gene expression by nicotine, each ofthem with differing requirements forCa²⁺ and cAMP.

     

Regulation of the epithelial Na+ channel by intracellular Na+

The hypothesis that the intracellularNa⁺ concentration([Na⁺]_i)is a regulator of the epithelialNa⁺ channel (ENaC) was tested withthe Xenopus oocyte expression systemby utilizing a dual-electrode voltage clamp.[Na⁺]_iaveraged 48.1 ± 2.2 meq (n = 27)and was estimated from the amiloride-sensitive reversal potential.[Na⁺]_iwas increased by direct injection of 27.6 nl of 0.25 or 0.5 MNa₂SO₄.Within minutes of injection,[Na⁺]_istabilized and remained elevated at 97.8 ± 6.5 meq(n = 9) and 64.9 ± 4.4 (n = 5) meq 30 min after theinitial injection of 0.5 and 0.25 MNa₂SO₄,respectively. This increase of[Na⁺]_icaused a biphasic inhibition of ENaC currents. In oocytes injected with0.5 MNa₂SO₄(n = 9), a rapid decrease of inwardamiloride-sensitive slope conductance(g_Na) to 0.681 ± 0.030 of control within the first 3 min and a secondary, slowerdecrease to 0.304 ± 0.043 of control at 30 min were observed.Similar but smaller inhibitions were also observed with the injectionof 0.25 MNa₂SO₄.Injection of isotonicK₂SO₄(70 mM) or isotonicK₂SO₄made hypertonic with sucrose (70 mMK₂SO₄-1.2M sucrose) was without effect. Injection of a 0.5 M concentration ofeitherK₂SO₄,N-methyl-D-glucamine (NMDG) sulfate, or 0.75 M NMDG gluconate resulted in a much smaller initial inhibition (<14%) and little or no secondary decrease. Thusincreases of[Na⁺]_ihave multiple specific inhibitory effects on ENaC that can betemporally separated into a rapid phase that was complete within 2-3 min and a delayed slow phase that was observed between 5 and 30 min.

     

Phosphate transport by the human renal cotransporter NaPi-3 expressed in HEK-293 cells

The human renal Na-PO4cotransporter gene NaPi-3 was expressed in human embryonic kidneyHEK-293 cells, and the transport characteristics were measured in cellstransfected with a vector containing NaPi-3 or with the vector alone(sham transfected). The initial rate of32PO4influx had saturation kinetics for external Na andPO4 with K Na1/2 of 128 mM(PO4 = 0.1 mM) andK PO41/2of 0.084 mM (extracellular Na = 143 mM) in sham- and NaPi-3-transfectedcells expressing the transporter. Transfection had no effect on theNa-independent 32PO4influx, but transfection increased Na-dependent32PO4influxes 2.5- to 5-fold. Of the alkali cations, only Na significantly supported PO4 influx. Arsenateinhibited flux with an inhibition constant of 0.4 mM. The phosphatetransport in sham- and NaPi-3-transfected cells has nearly the sametemperature dependence in the absence and presence of extracellularNa. The Na-dependent phosphate flux decreased with pH insham-transfected cells but was pH independent in transfected cells. TheNa-dependent32PO4influx was inhibited byp-chloromercuriphenylsulfonate,phosphonoformate, phloretin, vanadate, and5-(N-methyl-N-isobutyl)-amiloridebut not by amiloride or other amiloride analogs. These functional characteristics are in general agreement with the known behavior ofNaPi-3 homologues in the renal tubule of other species and, thus,demonstrate the fidelity of this transfection system for the study ofthis protein. Commensurate with the increased functional expression,there was an increase in the amount of NaPi-3 protein by Westernanalysis.

     

Cloning and characterization of a type III Na-dependent phosphate cotransporter from mouse intestine

Intestinal and renalabsorption of inorganic phosphate (P_i) is critical forphosphate homeostasis in mammals. We have isolated a cDNA that encodesa type III Na-dependent phosphate cotransporter from mouse smallintestine (mPit-2). The nucleotide sequence of mPit-2 predicts aprotein of 653 amino acids with at least 10 putative transmembranedomains. Kinetic studies, carried out in Xenopus oocytes,showed that mPit-2 cRNA induces significant Na-dependent P_iuptake with an apparent Michaelis constant (K_m)for phosphate of 38 µM. The transport of phosphate by mPit-2 isinhibited at high pH. Northern blot analysis demonstrated the presenceof mPit-2 mRNA in various tissues, including intestine, kidney, heart,liver, brain, testis, and skin. The highest expression of mPit-2 in the intestine was found in the jejunum. In situ hybridization revealed thatmPit-2 mRNA is expressed throughout the vertical crypt-villus axis ofthe intestinal epithelium. The presence of mPit-2 in the mouseintestine and its unique transport characteristics suggest thatmultiple Na-dependent cotransporters may contribute to phosphate absorption in the mammalian small intestine.

     

L-type voltage-dependent Ca2+ channels in cerebral microvascular endothelial cells and ET-1 biosynthesis

We investigatedthe role of intracellular calcium concentration([Ca²⁺]_i) in endothelin-1 (ET-1) production,the effects of potential vasospastic agents on[Ca²⁺]_i, and the presence of L-typevoltage-dependent Ca²⁺ channels in cerebral microvascularendothelial cells. Primary cultures of endothelial cells isolated frompiglet cerebral microvessels were used. Confluent cells were exposed toeither the thromboxane receptor agonist U-46619 (1 µM),5-hydroxytryptamine (5-HT; 0.1 mM), or lysophosphatidic acid (LPA; 1 µM) alone or after pretreatment with the Ca²⁺-chelatingagent EDTA (100 mM), the L-type Ca²⁺ channel blockerverapamil (10 µM), or the antagonist of receptor-operated Ca²⁺ channel SKF-96365 HCl (10 µM) for 15 min. ET-1production increased from 1.2 (control) to 8.2 (U-46619), 4.9 (5-HT),or 3.9 (LPA) fmol/µg protein, respectively. Such elevated ET-1biosynthesis was attenuated by verapamil, EDTA, or SKF-96365 HCl. Toinvestigate the presence of L-type voltage-dependent Ca²⁺channels in endothelial cells, the [Ca²⁺]_isignal was determined fluorometrically by using fura 2-AM. Superfusionof confluent endothelial cells with U-46619, 5-HT, or LPA significantlyincreased [Ca²⁺]_i. Pretreatment ofendothelial cells with high K⁺ (60 mM) or nifedipine (4 µM) diminished increases in [Ca²⁺]_i inducedby the vasoactive agents. These results indicate that 1)elevated [Ca²⁺]_i signals are involved in ET-1biosynthesis induced by specific spasmogenic agents, 2) theincreases in [Ca²⁺]_i induced by thevasoactive agents tested involve receptor as well as L-typevoltage-dependent Ca²⁺ channels, and 3) primarycultures of cerebral microvascular endothelial cells express L-typevoltage-dependent Ca²⁺ channels.

     

Mutagenesis of the N-glycosylation site of hNaSi-1 reduces transport activity

The human Na⁺-sulfate cotransporter (hNaSi-1) belongs to the SLC13 gene family, which also includes the high-affinity Na⁺-sulfate cotransporter (hSUT-1) and the Na⁺-dicarboxylate cotransporters (NaDC). In this study, the location and functional role of the N-glycosylation site of hNaSi-1 were studied using antifusion protein antibodies. Polyclonal antibodies against a glutathione S-transferase fusion protein containing a 65-amino acid peptide of hNaSi-1 (GST-Si65) were raised in rabbits, purified, and then used in Western blotting and immunofluorescence experiments. The antibodies recognized native NaSi-1 proteins in pig and rat brush-border membrane vesicles as well as the recombinant proteins expressed in Xenopus oocytes. Wild-type hNaSi-1 and two N-glycosylation site mutant proteins, N591Y and N591A, were functionally expressed and studied in Xenopus oocytes. The apparent mass of N591Y was not affected by treatment with peptide-N-glycosylase F, in contrast to the mass of wild-type hNaSi-1, which was reduced by up to 15 kDa, indicating that Asn⁵⁹¹ is the N-glycosylation site. Although the cell surface abundance of the two glycosylation site mutants, N591Y and N591A, was greater than that of wild-type hNaSi-1, both mutants had greatly reduced V_max, with no change in K_m. These results suggest that Asn⁵⁹¹ and/or N-glycosylation is critical for transport activity in NaSi-1. antifusion protein antibodies; Xenopus oocytes; sulfate; immunofluorescence     

Contribution of Na(+)-K(+)-Cl(-) cotransporter to high-[K(+)](o)- induced swelling and EAA release in astrocytes

We hypothesized that highextracellular K⁺ concentration([K⁺]_o)-mediated stimulation ofNa⁺-K⁺-Cl cotransporter isoform 1 (NKCC1) may result in a net gain of K⁺ and Cland thus lead to high-[K⁺]_o-induced swellingand glutamate release. In the current study, relative cell volumechanges were determined in astrocytes. Under 75 mM[K⁺]_o, astrocytes swelled by 20.2 ± 4.9%. This high-[K⁺]_o-mediated swelling wasabolished by the NKCC1 inhibitor bumetanide (10 µM, 1.0 ± 3.1%; P < 0.05). Intracellular³⁶Cl accumulation was increased from acontrol value of 0.39 ± 0.06 to 0.68 ± 0.05 µmol/mgprotein in response to 75 mM [K⁺]_o. Thisincrease was significantly reduced by bumetanide (P < 0.05). Basal intracellular Na⁺ concentration([Na⁺]_i) was reduced from 19.1 ± 0.8 to16.8 ± 1.9 mM by bumetanide (P < 0.05).[Na⁺]_i decreased to 8.4 ± 1.0 mM under75 mM [K⁺]_o and was further reduced to5.2 ± 1.7 mM by bumetanide. In addition, the recovery rate of[Na⁺]_i on return to 5.8 mM[K⁺]_o was decreased by 40% in the presenceof bumetanide (P < 0.05). Bumetanide inhibitedhigh-[K⁺]_o-induced ¹⁴C-labeledD-aspartate release by ~50% (P < 0.05).These results suggest that NKCC1 contributes tohigh-[K⁺]_o-induced astrocyte swelling andglutamate release.

     

Characterization of transport mechanisms and determinants critical for Na+-dependent Pi symport of the PiT family paralogs human PiT1 and PiT2

The general phosphate need in mammalian cells is accommodated by members of the P_i transport (PiT) family (SLC20), which use either Na⁺ or H⁺ to mediate inorganic phosphate (P_i) symport. The mammalian PiT paralogs PiT1 and PiT2 are Na⁺-dependent P_i (NaP_i) transporters and are exploited by a group of retroviruses for cell entry. Human PiT1 and PiT2 were characterized by expression in Xenopus laevis oocytes with ³²P_i as a traceable P_i source. For PiT1, the Michaelis-Menten constant for P_i was determined as 322.5 ± 124.5 µM. PiT2 was analyzed for the first time and showed positive cooperativity in P_i uptake with a half-maximal activity constant for P_i of 163.5 ± 39.8 µM. PiT1- and PiT2-mediated Na⁺-dependent P_i uptake functions were not significantly affected by acidic and alkaline pH and displayed similar Na⁺ dependency patterns. However, only PiT2 was capable of Na⁺-independent P_i transport at acidic pH. Study of the impact of divalent cations Ca²⁺ and Mg²⁺ revealed that Ca²⁺ was important, but not critical, for NaP_i transport function of PiT proteins. To gain insight into the NaP_i cotransport function, we analyzed PiT2 and a PiT2 P_i transport knockout mutant using ²²Na⁺ as a traceable Na⁺ source. Na⁺ was transported by PiT2 even without P_i in the uptake medium and also when P_i transport function was knocked out. This is the first time decoupling of P_i from Na⁺ transport has been demonstrated for a PiT family member. Moreover, the results imply that putative transmembrane amino acids E⁵⁵ and E⁵⁷⁵ are responsible for linking P_i import to Na⁺ transport in PiT2. inorganic phosphate transport; retroviral receptor; SLC20     

E-C coupling failure in mouse EDL muscle after in vivo eccentric contractions

The objectives of this research were to determine thecontribution of excitation-contraction (E-C) coupling failure to the decrement in maximal isometric tetanic force(P_o) in mouse extensor digitorumlongus (EDL) muscles after eccentric contractions and to elucidatepossible mechanisms. The left anterior crural muscles of femaleICR mice (n = 164) wereinjured in vivo with 150 eccentric contractions.P_o, caffeine-,4-chloro-m-cresol-, andK⁺-induced contracture forces,sarcoplasmic reticulum (SR) Ca²⁺release and uptake rates, and intracellularCa²⁺ concentration([Ca²⁺]_i)were then measured in vitro in injured and contralateral control EDLmuscles at various times after injury up to 14 days. On the basis ofthe disproportional reduction inP_o (~51%) compared with caffeine-induced force (~11-21%), we estimate that E-C coupling failure can explain 57-75% of theP_o decrement from 0 to 5 days postinjury. Comparable reductions inP_o andK⁺-induced force (51%), and minorreductions (0-6%) in the maximal SRCa²⁺ release rate, suggest thatthe E-C coupling defect site is located at the t tubule-SR interfaceimmediately after injury. Confocal laser scanning microscopy indicatedthat resting[Ca²⁺]_iwas elevated and peak tetanic[Ca²⁺]_iwas reduced, whereas peak4-chloro-m-cresol-induced[Ca²⁺]_iwas unchanged immediately after injury. By 3 days postinjury, 4-chloro-m-cresol-induced[Ca²⁺]_ibecame depressed, probably because of decreased SRCa²⁺ release and uptake rates(17-31%). These data indicate that the decrease inP_o during the first several daysafter injury primarily stems from a failure in the E-C couplingprocess.

     

VCAM-1-induced inwardly rectifying K(+) current enhances Ca(2+) entry in human THP-1 monocytes

Hyperpolarization in human leukemia THP-1 monocytes adherent tovascular cell adhesion molecule (VCAM)-1 is due to an induction ofinwardly rectifying K⁺ currents(I_ir) (Colden-Stanfield M and Gallin EK,Am J Physiol Cell Physiol 275: C267-C277, 1998).We determined whether the VCAM-1-induced hyperpolarization issufficient to augment the increase in intracellular free calcium([Ca²⁺]_i) produced by Ca²⁺ storedepletion with thapsigargin (TG) and readdition of external CaCl₂ in fura 2-loaded THP-1 monocytes. Whereas there was a2.1-fold increase in [Ca²⁺]_i in monocytesbound to glass for 5 h in response to TG and CaCl₂ addition, adherence to VCAM-1 produced a 5-fold increase in[Ca²⁺]_i. Depolarization of monocytes adherentto VCAM-1 by I_ir blockade or exposure to high[K⁺] abolished the enhancement of the peak[Ca²⁺]_i response. In monocytes bound toglass, hyperpolarization of the membrane potential with valinomycin, aK⁺ ionophore, to the level of hyperpolarization seen incells adherent to VCAM-1 produced similar changes in peak[Ca²⁺]_i. Adherence of monocytes to E-selectinproduced a similar peak [Ca²⁺]_i to cellsbound to glass. Thus monocyte adherence to the physiological substrateVCAM-1 produces a hyperpolarization that is sufficient to enhanceCa²⁺ entry and may impact Ca²⁺-dependentmonocyte function.

     

Two-photon molecular excitation imaging of Ca2+ transients in Langendorff-perfused mouse hearts

The ability to image calciumsignals at subcellular levels within the intact depolarizing heartcould provide valuable information toward a more integratedunderstanding of cardiac function. Accordingly, a system combiningtwo-photon excitation with laser-scanning microscopy was developed tomonitor electrically evoked [Ca²⁺]_itransients in individual cardiomyocytes within noncontracting Langendorff-perfused mouse hearts. [Ca²⁺]_itransients were recorded at depths 100 µm from the epicardial surface with the fluorescent indicators rhod-2 or fura-2 in the presence of the excitation-contraction uncoupler cytochalasin D. Evoked[Ca²⁺]_i transients were highly synchronizedamong neighboring cardiomyocytes. At 1 Hz, the times from 90 to 50%(t_90-50%) and from 50 to 10%(t_50-10%) of the peak[Ca²⁺]_i were (means ± SE) 73 ± 4 and 126 ± 10 ms, respectively, and at 2 Hz, 62 ± 3 and94 ± 6 ms (n = 19, P < 0.05 vs.1 Hz) in rhod-2-loaded cardiomyocytes.[Ca²⁺]_i decay was markedly slower infura-2-loaded hearts (t_90-50% at 1 Hz,128 ± 9 ms and at 2 Hz, 88 ± 5 ms;t_50-10% at 1 Hz, 214 ± 18 ms and at2 Hz, 163 ± 7 ms; n = 19, P < 0.05 vs. rhod-2). Fura-2-induced deceleration of[Ca²⁺]_i decline resulted from increasedcytosolic Ca²⁺ buffering, because the kinetics of rhod-2decay resembled those obtained with fura-2 after incorporation of theCa²⁺ chelator BAPTA. Propagating calcium waves and[Ca²⁺]_i amplitude alternans were readilydetected in paced hearts. This approach should be of general utility tomonitor the consequences of genetic and/or functional heterogeneity incellular calcium signaling within whole mouse hearts at tissue depthsthat have been inaccessible to single-photon imaging.

     

Regulation of cardiac AMP-specific 5'-nucleotidase during ischemia mediates ATP resynthesis on reflow

The ability toresynthesize ATP during recovery from ischemia is limited tothe size of endogenous pool of adenine nucleotides. CytosolicAMP-specific 5'-nucleotidase (5'-NT) plays a key role inATP degradation and hence the capacity for ATP resynthesis. We havesuggested (J. Clin. Invest. 93:40-49, 1994) that intracellular acidosis [intracellular pH(pH_i)] is a potentinhibitor of 5'-NT under in vivo conditions. To test thishypothesis further, we used the hyperthyroid rat heart because we couldalter pH_i during ischemiaand determine the consequences of lowerpH_i on AMPaccumulation (by chemical assay) and ATP resynthesis (by³¹P nuclear magnetic resonancespectroscopy) during reperfusion. Global no-flow ischemiacaused pH_i to decrease from 7.1 under well-oxygenated control perfusion to 6.7. We found thatdecreasing pH_i further from pH 6.7 to 6.4 leads to increased accumulation (30%) of AMP duringischemia and to a 2.5-fold increase in ATP resynthesis duringreperfusion. Analysis of all known substrates, products, activators,and inhibitors of the 5'-NT suggests that 5'-NT isactivated primarily by Mg²⁺ andADP and is inhibited by H⁺. Thusthese observations provide evidence for a salutary effect ofintracellular acidosis on preserving the AMP pool due to inhibition of5'-NT and suggest a novel role ofH⁺ in protecting ischemic tissue.