The Perception of Saltiness is Eliminated by NaCl Adaptation: Implications for Gustatory Transduction and Coding

The tastes of salts to humans are complex. NaCl is the mostpurely salty of all salts, but even this stimulus tastes sweetat low concentrations and somewhat sour at mid-range intensities.Other salts taste significantly sour or bitter in addition tosalty. Previous studies have shown that the saltiness of simplehalide salts is reduced by adaptation to NaCl, suggesting thata single mechanism might be responsible for the salty tasteof these stimuli. In electrophysiological studies in rodents,the response to NaCl is reduced by application to the tongueof the Na⁺- channel blocker amiloride. Organic Na⁺ salts aremore heavily dependent on this amiloride-sensitive transductioncomponent than NaCl, and are generally less salty and more sour.In order to investigate the relationship between NaCl saltinessand that evoked by other salts, we adapted the tongue to distilledH₂O and to 0.1 M NaCl and obtained direct magnitude estimatesof the taste intensity of 15 organic and inorganic Na⁺, Li⁺,K⁺ and Ca²⁺ salts, matched for total intensity. Subjects dividedthese magnitude estimates among the component taste qualities.Adaptation to NaCl abolished the taste of NaCl and LiCl, andeliminated the saltiness of all other salts. The magnitude estimatesof the bitterness and sourness of many salts increased afterNaCI adaptation. Since recent biophysical data suggest thatadaptation in taste receptors may involve whole-cell mechanisms,we propose that saltiness is reduced by NaCl adaptation becauseit originates in the subset of taste receptors responsive toNaCl. This implies that saltiness is coded within the CNS incells whose receptive fields include the NaCl-sensitive receptorcells and that the degree to which any salt tastes salty isdetermined by its ability to drive these receptors. This modelproposes, for example, that KCl has a salty component becauseit stimulates some of the same receptor cells as NaCl, eventhough the transduction mechanisms for KCl are different thanthose engaged by NaCl. Adaptation to NaCl blocks the saltinessof KCl and other salts because they stimulate NaCl-sensitivereceptor cells. Chem. Senses 20: 545–557, 1995.     

Anterior Tongue Stiomulation with Amiloride Suppresses NaCl Saltiness, but not Citric Acid Sourness in Humans

Suppression of the saltiness of NaCI solutions by amiloride,a sodium channel blocker, has previously been reported a numberof times in humans. This suppression was seen with techniquesthat involved stimulation of small areas of the tongue. It wasnot certain, however, whether amiloride would suppress saltinesswith stimulation of a much larger area of the tongue; one publishedstudy, in fact, found negative results with whole mouth stimulation.For this study, eight subjects dipped a large part of the anteriorportion of the tongue into a 10-ml sample of NaCI solution,or a NaCI and amiloride solution, and reported its magnitudeof saltiness intensity. The results show that amiloride suppressedthe saltiness of NaCI when a large area of the anterior tonguewas stimulated. Consistent with previous studies, there wasindividual variability across subjects in this suppressive effectof amiloride. This study also used this method to test the effectsof amiloride on the sourness of citric acid, which was not expectedto be affected. No suppression of sourness was seen with amiloride.Chem. Senses 21: 113–120, 1996.     

Na+ fluxes in Chara under salt stress

The influx and efflux of Na⁺ across the plasma membrane of Characorallina and Chara longifolia were examined under mild saltstress conditions. Na⁺ influx was found to be rapid in bothspecies with the freely exchangeable cytoplasmic Na⁺ cominginto isotopic equilibrium with external ²²Na⁺ within 1 h ofexposure to isotope. Cytoplasmlc Na⁺ concentration and Na⁺ influxwere greater in C. corallina than in C. longifolla under thesame conditions. Na⁺ influx across the tonoplast was much lowerthan the flux across the plasma membrane. Na⁺ efflux was stimulatedat pH 5 relative to pH 7 by 218% in C. coralllna and 320% inC. longifolia. In both species externally applied Li⁺ inhibitedNa⁺ efflux at pH 5 but not at pH 7. Na⁺ etflux was not significantlyinhibited by amiloride. Key words: Na⁺ influx, Na⁺ efflux, Na⁺/H⁺ antiport, Chara     

Ionic Relations of Garden Orache, A triplex hortensis L.: Growth and Ion Distribution at Moderate Salinity and the Function of Bladder Hairs

The growth of garden orache, A triplex hortensis was studiedunder conditions of mild NaCl or Na₂SO₄ salinity. Growth, drymatter production and leaf size were substantially stimulatedat 10 mM and 50 mM Na⁺ salts. Increased growth, however, appearedto be due to a K⁺-sparing effect of Na⁺ rather than to salinityper se. The distribution of K⁺ and Na⁺ in the plant revealeda remarkable preference for K+ in the roots and the hypocotyl.In the shoot the K/Na ratio decreased strongly with leaf age.However, the inverse changes in K⁺ and Na⁺ content with leafage were dependent on the presence of bladder hairs, which removedalmost all of the Na⁺ from the young leaf lamina. Measurementsof net fluxes of K⁺ and Na⁺ into roots and shoots of growingAtriplex plants showed a higher K/Na selectivity of the netion flux to the root compared to the shoot. With increasingsalinity the selectivity ratio S_{K, Na}^* of net ion fluxes tothe roots and to the shoots was increased. The data suggestthat recirculation of K⁺ from leaves to roots is an importantlink in establishing the K/Na selectivity in A. hortensis plants.The importance of K⁺ recirculation and phloem transport forsalt tolerance is discussed. Key words: Atriplex hortensis, Salinity, Potassium, Sodium, K⁺ retranslocation, Bladder hairs, Growth stimulation     

Opposite effects of Ni2+ on Xenopus and rat ENaCs expressed in Xenopus oocytes

Opposite effects of Ni²⁺ on Xenopus and rat ENaCs expressed in Xenopus oocytes. Am J Physiol Cell Physiol 289: C946–C958, 2005. First published June 8, 2005; .—The epithelial Na⁺ channel (ENaC) is modulated by various extracellular factors, including Na⁺, organic or inorganic cations, and serine proteases. To identify the effect of the divalent Ni²⁺ cation on ENaCs, we compared the Na⁺ permeability and amiloride kinetics of Xenopus ENaCs (xENaCs) and rat ENaCs (rENaCs) heterologously expressed in Xenopus oocytes. We found that the channel cloned from the kidney of the clawed toad Xenopus laevis [wild-type (WT) xENaC] was stimulated by external Ni²⁺, whereas the divalent cation inhibited the channel cloned from the rat colon (WT rENaC). The kinetics of amiloride binding were determined using noise analysis of blocker-induced fluctuation in current adapted for the transoocyte voltage-clamp method, and Na⁺ conductance was assessed using the dual electrode voltage-clamp (TEVC) technique. The inhibitory effect of Ni²⁺ on amiloride binding is not species dependent, because Ni²⁺ decreased the affinity (mainly reducing the association rate constant) of the blocker in both species in competition with Na⁺. Importantly, using the TEVC method, we found a prominent difference in channel conductance at hyperpolarizing voltage pulses. In WT xENaCs, the initial ohmic current response was stimulated by Ni²⁺, whereas the secondary voltage-activated current component remained unaffected. In WT rENaCs, only a voltage-dependent block by Ni²⁺ was obtained. To further study the origin of the xENaC stimulation by Ni²⁺, and based on the rationale of the well-known high affinity of Ni²⁺ for histidine residues, we designed -subunit mutants of xENaCs by substituting histidines that were expressed in oocytes, together with WT - and -subunits. Changing His²¹⁵ to Asp in one putative amiloride-binding domain (WYRFHY) in the extracellular loop between Na⁺ channel membrane segments M1 and M2 had no influence on the stimulatory effect of Ni²⁺, and neither did complete deletion of this segment. Next, we mutated His⁴¹⁶ flanked by His⁴¹¹ and Cys⁴¹⁷, a unique site for possible heavy metal ion chelation, and, with this quality, most proximal (100 amino acids upstream of the second putative amiloride binding site at the pore entrance), was found localized at M2. Replacing His⁴¹⁶ with arginine, aspartate, tyrosine, and alanine clearly affected amiloride binding in all cases, as well as Na⁺ conductance, as expressed in the xENaC current-voltage relationship, especially with regard to aspartate and tyrosine. However, similarly to those obtained with the WYRFHY stretch, none of these mutations could either abolish the stimulating effect of Ni²⁺ or reverse it to an inhibitory type. epithelia; divalent cations; amiloride; Na⁺; voltage clamp     

Na+/H+ Antiporter in Tonoplast Vesicles from Rice Roots

The Na⁺/H ⁺ antiporter in vacuolar membranes transports Na⁺from the cytoplasm to vacuoles using a pH gradient generatedby proton pumps; it is considered to be related to salinitytolerance. Rice (Oryza sativa L.) is a salt-sensitive crop whosevacuolar antiporter is unknown. The vacuolar pH of rice roots,determined by ³¹P-nuclear magnetic resonance (NMR), increasedfrom 5.34 to 5.58 in response to 0.1 M NaCl treatment. Transportof protons into the tonoplast vesicles from rice roots was fluorometricallymeasured. Efflux of protons was accelerated by the additionof Na⁺. Furthermore, the influx of ²²Na⁺ into the tonoplastvesicles was accelerated by a pH gradient generated by proton-translocatingadenosine 5'-triphosphatase (H⁺-ATPase) and proton-translocatinginorganic pyro-phosphatase (H⁺-PPase). We concluded that thisNa⁺/H⁺antiporter functioned as a Na⁺ transporter in the vacuolarmembranes. The antiporter had a K_m of 10 mM for Na⁺ and wascompetitively inhibited by amiloride and its analogues. TheKⁱ values for 5-(N-methyl-N-isobutyl)-amiloride (MIA), 5-(N-ethyl-N-isopropyI)-amiloride(EIPA), and 5-(N, N-hexamethylene)-amiloride (HMA) were 2.2,5.9, and 2.9 µ M, respectively. Unlike barley, a salt-tolerantcrop, NaCl treatment did not activate the antiporter in riceroots. The amount of antiporter in the vacuolar membranes maybe one of the most important factors determining salt tolerance. ¹This work was supported by a grant from Bio-Media Project ofthe Japanese Ministry of Agriculture, Forestry and Fisheries(BMP96-III-1).     

Dynamics of Salt Secretion by Sporobolus spicatus(Vahl) Kunth from Sites of Differing Salinity

Secretion of salts by bicellular salt glands and the water relationsof the grass Sporobolus spicatus were investigated at four sitesalong the coast of the Red Sea in Egypt that differed in theextremity of salinity and drought. Salt eliminated by the leaveswas similar in its composition at all sites. Na⁺and Cl^-werethe dominant ions in the soil, and together comprised about93% of the dry weight of secreted salt. The molar ratio of K⁺:Na⁺inthe plant leaves was more than ten-fold that in the interstitialsoil solution and thirteen-times that in the secreted salts,reflecting the high selectivity of the secretion mechanism forNa⁺. The concentration of Na⁺in the solution transported tothe leaves between 0900 and 1500 h was less than 0.1% of thatin the soil solution. Accumulation of salts by the plant shoots,which increased with increasing soil salinity and drought, wasmaximal during the day when the extent of secretion greatlyreduced. The ionic osmotic potential (     

Effect of Amiloride on the Taste of NaCl, Na-gluconate and KCl in Humans: Implications for Na+ Receptor Mechanisms

Sodium-salt transduction in many species may be mediated byboth apical and submucosal ion channels on the taste receptorcell membrane. The apical ion channel is blockable by the diureticamiloride, whereas the submucosal pathway is not. Sodium saltswith small anions, such as NaCl, can stimulate submucosal aswell as apical ion channels; sodium salts with large anions,such as Na-gluconate, activate primarily the apical channels.In humans, reports on the effects of amiloride on the tasteof NaCl are conflicting and no data exist on the effects ofamiloride on organic sodium salts. In the present experiment,subjects gave magnitude estimates of the total intensity andof each of the basic taste qualtities for NaCl, Na-gluconateand KCl. Five concentrations of each of these stimuli were presentedto the anterior tongue following distilled water adaptationand after amiloride treatment. There was a significant decreasein the total taste intensity of NaCl and Na-gluconate afteramiloride, but no effect on KCl. The saltiness of all threesalts was unaffected, but amiloride decreased the preceivedsourness of the sodium salts. KCl sourness was unaffected byamiloride. There was a proportionately larger effect of amilorideon Na-gluconate than on Nacl, which is consistent with a largerrole for the apical ion channel in Na-gluconate transduction.However, an appreciable amiloride-insensitive component is presentfor both NaCl and Na-gluconate, suggesting that an amiloride-insensitivepathway also plays a role in the transduction of both sodiumsalts. These data support the hypothesis that an amiloride-sensitivetransduction component exists in humans, but suggest that itis considerably smaller than in many other species.     

Transepithelial resistance can be regulated by the intestinal brush-border Na(+)/H(+) exchanger NHE3

Initiation of intestinal Na⁺-glucose cotransport results intransient cell swelling and sustained increases in tight junction permeability. Since Na⁺/H⁺ exchange has beenimplicated in volume regulation after physiological cell swelling, wehypothesized that Na⁺/H⁺ exchange might also berequired for Na⁺-glucose cotransport-dependent tightjunction regulation. In Caco-2 monolayers with activeNa⁺-glucose cotransport, inhibition ofNa⁺/H⁺ exchange with 200 µM5-(N,N-dimethyl)- amiloride induced 36 ± 2% increases in transepithelial resistance (TER). Evaluation using multiple Na⁺/H⁺ exchange inhibitors showed thatinhibition of the Na⁺/H⁺ exchanger 3 (NHE3)isoform was most closely related to TER increases. TER increases due toNHE3 inhibition were related to cytoplasmic acidification becausecytoplasmic alkalinization with 5 mM NH₄Cl prevented bothcytoplasmic acidification and TER increases. However, NHE3 inhibitiondid not affect TER when Na⁺-glucose cotransport wasinhibited. Myosin II regulatory light chain (MLC) phosphorylationdecreased up to 43 ± 5% after inhibition ofNa⁺/H⁺ exchange, similar to previous studiesthat associate decreased MLC phosphorylation with increased TER afterinhibition of Na⁺-glucose cotransport. However, NHE3inhibitors did not diminish Na⁺-glucose cotransport. Thesedata demonstrate that inhibition of NHE3 results in decreased MLCphosphorylation and increased TER and suggest that NHE3 may participatein the signaling pathway of Na⁺-glucosecotransport-dependent tight junction regulation.

     

In vivo fluorescence measurement of Na+ concentration in the pericryptal space of mouse descending colon

A methodinvolving surgical exposure of the colonic mucosa, fluorescent dyeaddition, and confocal microscopy has been developed for monitoringcolonic crypt function in vivo in mice. Na⁺ concentrationin the extracellular pericryptal space of descending colon was measuredusing a low-affinity Na⁺-sensitive fluorescent indicatorconsisting of an Na⁺-sensitive chromophore (sodium red) andan Na⁺-insensitive chromophore (Bodipy-fl) immobilized on200-nm-diameter polystyrene beads. The Na⁺ indicator beadsaccumulated in the pericryptal spaces surrounding the colonic cryptsafter a 1-h exposure of the colonic luminal surface to the beadsuspension. Na⁺ concentration ([Na⁺]) in thepericryptal space was 491 ± 62 mM (n = 4). Aftera 70-min exposure to amiloride (0.25 mM), pericryptal[Na⁺] was reduced to 152 ± 21 mM. Blockage of thecrypt lumen with mineral oil droplets reduced pericryptal[Na⁺] to 204 ± 44 mM. Exposure of the colonicmucosa to FITC-dextran (4.5 kDa) led to rapid accumulation of the dyeinto the crypt lumen with a half time of 19.8 ± 1.0 s, whichwas increased to 77.9 ± 6.0 s after amiloride treatment.These results establish an in vivo fluorescence method to measurecolonic crypt function and provide direct evidence for accumulation ofa hypertonic absorbate in the pericryptal space of descending colon.The pericryptal space represents the first example of a hypertonicextracellular compartment in mammals that is not created by acountercurrent amplification mechanism.

     

Effect of Sodium on Phosphate Uptake in Unicellular and Filamentous Cyanobacteria

The effect of Na⁺ on phosphate uptake was studied in four strainsof cyanobacteria: Synechococcus PCC 7942, Gloeothece PCC 6501,Phormidium sp. and Chlorogloeopsis PCC 6912. Phosphate uptakewas stimulated by Na⁺ in all cases. Li⁺ and K⁺ acted as partialanalogues for Na⁺. Half-saturation [K_1/2(Na⁺)] of phosphateuptake was reached with Na⁺ concentrations ranging from 317µM in Chlorogloeopsis to 659 µM in Phormidium. Theconcentration of phosphate required to reach half-saturationof phosphate uptake [K_1/2(P_i)]was not changed by the presenceof Na⁺. (Received April 11, 1994; Accepted July 5, 1994)     

Effect of Divalent Cations on Na+ Permeability of Chara corallina and Freshwater Grown Chara buckellii

The effect of elevated Na⁺ concentration on Na⁺ permeability(P_Na) and Na⁺ influx in the presence of two levels of externaldivalent cations was determined in Chara corallina and freshwater-culturedChara buckellii. When Na⁺ in the medium was increased from 1.0to 70 mol m^–3, Na⁺ influx increased in both species ifCa²⁺ was low (0.1 mol m–3). If Ca²⁺ was increased to 7.0mol m^–3 when Na⁺ was increased, Na⁺ influx remained atthe low control level in C. corallina, and showed only a temporaryincrease in C. buckellii. Mg²⁺ was a better substitute for Ca²⁺in C. buckellii than in C. corallina. Na⁺ permeability data suggest that when the external Ca²⁺ concentrationis low, P_Na does not increase in the presence of elevated NaCl;the increase in Na⁺ influx appears to be due to the increasein external Na⁺ concentration alone. Ca^{2 +} supplementation appearsto decrease P_Na whereas supplemental Mg²⁺ has no effect. Na⁺ effluxes were computed from previously determined net fluxesand the influxes. It was found that for both species, fluxesin both directions were stimulated in response to all experimentaltreatments, but Na⁺ influx always exceeded efflux. This resultedin net Na⁺ accumulation in the vacuoles of both species. The results are discussed with reference to net flux and electrophysiologicaldata obtained previously under identical conditions, as wellas the comparative salinity tolerance of both species and theNa⁺/divalent cation ratio. Key words: Na⁺ influx, Na⁺ tolerance, membrane potential, permeability, Chara     

Changes of action potentials and force at lowered [Na+]o in mouse skeletal muscle: implications for fatigue

We examined 1) whether the effects of lowered trans-sarcolemmal Na⁺ gradient on force differed between nonfatigued fast- and slow-twitch muscles of mice and 2) whether effects on action potentials could explain the decrease of force. The Na⁺ gradient was reduced by lowering the extracellular [Na⁺] ([Na⁺]_o). The peak force-[Na⁺]_o relationships for the twitch and tetanus were the same in nonfatigued extensor digitorum longus and soleus muscles: force was maintained over a large range of [Na⁺]_o and then decreased abruptly over a much smaller range. However, fatigue was significantly exacerbated at a lowered [Na⁺]_o that had little effect in nonfatigued soleus muscle. This finding suggests that substantial differences exist in the Na⁺ effect on force between nonfatigued and fatigued muscle. The reduced contractility in nonfatigued muscles at lowered [Na⁺]_o was largely due to 1) an increased number of inexcitable fibers and threshold for action potentials, 2) a reduction of action potential amplitude, and 3) a reduced capacity to generate action potentials throughout trains. sodium gradient; muscle contraction; action potential train; extensor digitorum longus; soleus     

Effect of Sudden Salt Stress on Ion Fluxes in Intact Wheat Suspension Cells

Although salinity is one of the major problems limiting agriculturalproduction around the world, the underlying mechanisms of highNaCl perception and tolerance are still poorly understood. Theeffects of different bathing solutions and fusicoccin (FC),a known activator of plasma membrane ATPase, on plasma membranepotential (E_m) and net fluxes of Na⁺, K⁺and H⁺were studied inwheat suspension cells (Triticum aestivum) in response to differentNaCl treatments. E_mof cells in Murashige and Skoog (MS) mediumwas less negative than in cells exposed to a medium containing10 mM KCl + 0.1 m M CaCl₂(KSM) and to a basic salt medium (BSM),containing 1 m M KCl and 0.1 m M CaCl₂. Multiphasic Na⁺accumulationin cells was observed, peaking at 13 min after addition of 120m M NaCl to MS medium. This time scale was in good agreementwith net Na⁺flux changes measured non-invasively by moving ion-selectivemicroelectrodes (the MIFE system). When 120 m M NaCl was addedto all media studied, a quick rise of Na⁺influx was reversedwithin the first 20 min. In both 120 and 20 m M NaCl treatmentsin MS medium, net Na⁺efflux was observed, indicating that activeNa⁺transporters function in the plant cell response to saltstress. Lower external K⁺concentrations (KSM and BSM) and FCpre-treatment caused shifts in Na⁺fluxes towards net influxat 120 m M NaCl stress. Copyright 2000 Annals of Botany Company Sodium, potassium, proton, membrane potential, fusicoccin, salt stress, wheat, Triticum aestivum     

Sodium-dependent inactivation of sodium/calcium exchange in transfected Chinese hamster ovary cells

High concentrations of cytosolic Na⁺ ions induce the time-dependent formation of an inactive state of the Na⁺/Ca²⁺ exchanger (NCX), a process known as Na⁺-dependent inactivation. NCX activity was measured as Ca²⁺ uptake in fura 2-loaded Chinese hamster ovary (CHO) cells expressing the wild-type (WT) NCX or mutants that are hypersensitive (F223E) or resistant (K229Q) to Na⁺-dependent inactivation. As expected, 1) Na⁺-dependent inactivation was promoted by high cytosolic Na⁺ concentration, 2) the F223E mutant was more susceptible than the WT exchanger to inactivation, whereas the K229Q mutant was resistant, and 3) inactivation was enhanced by cytosolic acidification. However, in contrast to expectations from excised patch studies, 1) the WT exchanger was resistant to Na⁺-dependent inactivation unless cytosolic pH was reduced, 2) reducing cellular phosphatidylinositol-4,5-bisphosphate levels did not induce Na⁺-dependent inactivation in the WT exchanger, 3) Na⁺-dependent inactivation did not increase the half-maximal cytosolic Ca²⁺ concentration for allosteric Ca²⁺ activation, 4) Na⁺-dependent inactivation was not reversed by high cytosolic Ca²⁺ concentrations, and 5) Na⁺-dependent inactivation was partially, but transiently, reversed by an increase in extracellular Ca²⁺ concentration. Thus Na⁺-dependent inactivation of NCX expressed in CHO cells differs in several respects from the inactivation process measured in excised patches. The refractoriness of the WT exchanger to Na⁺-dependent inactivation suggests that this type of inactivation is unlikely to be a strong regulator of exchange activity under physiological conditions but would probably act to inhibit NCX-mediated Ca²⁺ influx during ischemia. ischemia; cytosolic calcium concentration; cytosolic sodium concentration; cellular phosphatidylinositol-4,5-bisphosphate     

Sodium-Stimulation of Phosphate Uptake in the Cyanobacterium Anabaena PCC 7119

Anabaena PCC 7119 showed higher rates of phosphate uptake whencells were under P-starvation. Phosphate uptake was energy-dependentas indicated the decrease observed when assays were performedin the dark or in the presence of inhibitors of photosyntheticelectron transport, energy transfer and adenosine triphosphataseactivity. Phosphate uptake was stimulated by Na⁺ both in P-sufficientcells and P-starved cells. Li⁺ and K⁺ acted as partial analoguesfor Na⁺. The Na⁺-stimulation of phosphate uptake followed Michaelis-Mentenkinetics, half-saturation (K) of phosphate uptake was reachedwith a Na⁺ concentration of 212 µM. The absence of Na⁺reduced the rates of phosphate uptake at all phosphate concentrationsassayed (1–20 µM). The maximum uptake rates (V_max)decreased from 658 nmol P (mg dry wt)^-1 h^-1 in the presenceof Na⁺ to 149 nmol P (mg dry wt)^-1 h^-1 in the absence of Na⁺.The absence of Na⁺ did not change significantly the concentrationof phosphate required to reach half-saturation (K) (3.01 µMin the presence of Na⁺ vs 3.21 µM in the absence of Na⁺).In the presence of Na⁺ the rate of phosphate uptake was affectedby the pH; optimal rates were observed at pH 8. In the absenceof Na⁺ phosphate uptake was not affected by the pH; low rateswere observed in all cases. Monensin, an ionophore which collapsesNa⁺-gradients, reduced the rate of phosphate uptake in Na⁺-supplementedcells. These results indicated the existence of a Na⁺-dependentphosphate uptake in Anabaena PCC 7119. (Received September 8, 1992; Accepted November 17, 1992)     

Alloxan-induced diabetes reduces sarcolemmal Na+-K+ pump function in rabbit ventricular myocytes

The effect of diabetes on sarcolemmal Na⁺-K⁺ pump function is important for our understanding of heart disease associated with diabetes and design of its treatment. We induced diabetes characterized by hyperglycemia but no other major metabolic disturbances in rabbits. Ventricular myocytes isolated from diabetic rabbits and controls were voltage clamped and internally perfused with the whole cell patch-clamp technique. Electrogenic Na⁺-K⁺ pump current (I_p, arising from the 3:2 Na⁺-to-K⁺ exchange ratio) was identified as the shift in holding current induced by Na⁺-K⁺ pump blockade with 100 µmol/l ouabain in most experiments. There was no effect of diabetes on I_p recorded when myocytes were perfused with pipette solutions containing 80 mmol/l Na⁺ to nearly saturate intracellular Na⁺-K⁺ pump sites. However, diabetes was associated with a significant decrease in I_p measured when pipette solutions contained 10 mmol/l Na⁺. The decrease was independent of membrane voltage but dependent on the intracellular concentration of K⁺. There was no effect of diabetes on the sensitivity of I_p to extracellular K⁺. Pump inhibition was abolished by restoration of euglycemia or by in vivo angiotensin II receptor blockade with losartan. We conclude that diabetes induces sarcolemmal Na⁺-K⁺ pump inhibition that can be reversed with pharmacological intervention. sodium transport; insulin; angiotensin II; cardiomyopathy; hyperglycemia     

Salinity-induced Malate Accumulation in Chara

Ion absorption by Chara corallina from solutions containingpredominantly KC1 or RbCl at up to 100 mol m^–3 resultedin accumulation of salts and turgor regulation. Turgor regulationdid not occur in solutions containing Na⁺ or Li⁺salts. Duringion absorption from various salts of K⁺ and Rb⁺ vacuolar cationconcentration exceeded Cl^– concentration. This differencewas shown to be balanced by the synthesis and accumulation ofmalate. Vacuolar malate concentration reached 48 mol m₃^–,with accumulation occurring at rates of up to 0.45 mol m^–3h^–1. Malate accumulation was inhibited by low externalpH and was dependent upon external HCO₃^– concentration.The synthesis of malic acid and its subsequent dissociationimposed a severe acid load on the cell. Biophysical regulationof cellular pH was achieved by a H⁺efflux at a rate of about40 nmol m^–2 s^–1from the cell. The results presentedargue against cytoplasmic Cl^–, HCO₃^– or pH regulatingmalate accumulation in Chara and it is suggested that malatetransport across the tonoplast may regulate malate accumulation. Key words: Malate, Chara corallina, pH regulation, salinity     

Salivary ions and neural taste responses in the hamster

Saliva is a chemically complex fluid that bathes oral surfacesand may affect early events in mammalian gustation. We measuredchorda tympani responses to taste stimuli in hamsters (Mesocricetusauratus) while their tongues were adapted to either water, artificialsaliva or natural saliva. Artificial saliva on the tongue loweredneural responses to taste stimuli that were present in the artificialsaliva and to those stimuli that cross-adated with salivarycomponents. Changing from a water-adapted tongue to one soakedwith pilocarpine-stimulated saliva from donor hamsters led tosignificantly smaller responses to NaCl. Responses to sucrose,NH₄Cl and quinine were unaffected. Chemical analysis of hamstersaliva revealed ‘normal’ mammalian levels of K⁺,Ca²⁺ and Mg²⁺, but unexpectedly low levels of Na⁺ and Cl^–.     

A Sodium Pump in the Plasma Membrane of the Marine Alga Heterosigma akashiwo

ATP-dependent transport of ²²Na⁺ into liposomes reconstitutedfrom plasma membrane proteins of Heterosigma akashiwo was examined.The apparent K^m values for transport of Na⁺ were 400 µMfor ATP and 7 mM for Na⁺. ATP-dependent transport of ²²Na⁺ wasnot inhibited by a protonophore or a membrane-permeable cationbut was inhibited by an inhibitor of P-type ATPases. (Received October 2, 1995; Accepted February 1, 1996)