Suppression of Bitterness by Sodium: Variation Among Bitter Taste Stimuli

Taste interactions between salts (NaCl, LiCl, KCl, L-arginine:L-asparticacid, Na-acetate and Na-gluconate) and bittertasting compounds(urea, quinine HCI, magnesium sulphate, KCI, amiloride HCI andcaffeine) were investigated. In each study binary combinationsof three or four concentrations of one bitter compound withfour concentrations (0, 0.1, 0.3 and 0.5 M) of one salt wererated for bitterness and saltiness using the method of magnitudeestimation. In most cases, perceived bitterness was suppressedby salts, although the degree of suppression varied. In general,bitterness suppression was not accompanied by an equivalentreciprocal suppression of saltiness. Only MgSO₄ and amiloridehad suppressing effects on the saltiness of NaCl at the intermediateconcentrations and no bitter compound affected the saltinessat the high concentrations of NaCl. Since salt suppressed thebitterness of urea effectively, a detailed analysis of suppressionof the bitterness of urea by different salts was conducted.Those studies indicated that the key component in this effectwas the sodium or lithium ion for two reasons: first, all threesodium salts and the lithium salt had a suppressive effect onbitterness, whereas KCl did not; secondly, the effect of a salton suppression of the bitterness of urea was independent ofits perceived saltiness; that is, NaCl, Na-acetate (which isperceived as less salty than NaCl), and Na-gluconate (whichis perceived as less salty than Na-acetate) reduced bitternesscomparably. These results suggest that there is a major peripheralcomponent to the suppression of the bitterness of urea, andperhaps other bitter tasting compounds, by sodium. Chem. Senses20: 609–623, 1995.     

Generalization of conditioned taste aversions in hamsters: evidence for multiple bitter receptor sites

Multiple bitter receptor sites appear to exist within the hamstergustatory system supporting the data of other investigatorson humans, rats and frogs. The sodium salts of four anions,m-nitrobenzene sulfonate (NBSA), picrate, m-nitrobenzoate (NBA)and cholate, were tested in two-bottle preference tests andfor generalization to a variety of stimuli in a conditionedtaste aversion (CTA) paradigm. All four of these anions arebitter to humans. One, NBSA, generalized to sucrose suggestinga sweet taste, while the remaining three appear to be bitterwith varying degrees of saltiness. The bitterness of these threeanions to hamsters appears to be perceptually different froma quinine-type bitterness. Separate bitter receptor sites areindicated for quinine and urea, plus a third site acceptingNBA, picrate and cholate. More bitter sites are plausible. Separatesites for quinine and urea appear to occur across species. Itwas also concluded that quinine does not serve as a prototypicbitter stimulus for all bitters in the CTA test.     

Some Basic Psychophysics of Calcium Salt Solutions

Detection thresholds and the taste qualities of suprathresholdconcentrations of calcium salt solutions were assessed. Averagetaste detection thresholds for calcium chloride (CaCl₂), lactate(CaLa), hydroxide, phosphate and gluconate ranged between 8and 50 mM, with no reliable differences among the various salts.Between-subject variability ranged over four orders of magnitudeand reliability coefficients for repeated detection thresholdtests of CaCl₂ averaged r = 0.52. In an odor detection test,subjects could reliably discriminate 100 but not 1 mM CaCl₂and CaLa from water. The taste of suprathreshold concentrations(1–100 mM) of CaCl₂ and CaLa was considered unpleasant.At 1 mM, CaCl₂ solution was rated as 35% bitter, 32% sour, 29%sweet and 4% salty. At higher concentrations the sweet componentdiminished and the salty component increased, so that 100 mMCaCl₂ was rated as 44% bitter, 20% sour, 1% sweet and 35% salty.CaLa solutions were considered to be significantly less bitterand marginally more sour than equimolar CaCl₂ solutions. Thus,the taste of calcium varied with both the form and concentrationof salt tested, but included both sour and bitter components.Saltiness was identified only in high (     

A psychophysical investigation of binary bitter-compound interactions

The aim of this study was to determine if taste interactions occur when bitter stimuli are mixed. Eight bitter stimuli were employed: denatonium benzoate (DB), quinine-HCl (QHCl), sucrose octaacetate (SOA), urea, L-tryptophan (L-trp), L-phenylalanine (L-phe), ranitidine-HCl, and Tetralone. The first experiment constructed individual psychophysical curves for each subject (n = 19) for each compound to account for individual differences in sensitivities when presenting bitter compounds in experiment 2. Correlation analysis revealed two groupings of bitter compounds at low intensity (1, L-trp, L-phe, and ranitidine; 2, SOA and QHCl), but the correlations within each group decreased as the perceived intensity increased. In experiment 2, intensity ratings and two-alternative forced-choice discrimination tasks showed that bitter compounds generally combine additively in mixture and do not show interactions with a few specific exceptions. The methods employed detected synergy among sweeteners, but could not detect synergy among these eight bitter compounds. In general, the perceived bitterness of these binary bitter-compound mixtures was an additive function of the total bitter-inducing stimuli in the mouth.     

Effects of the bitter additives naringin and sucrose octa-acetate on sweet persistence and sweet quality of neohesperidin dihydrochalcone

The maximal perceived sweet intensity (Ip_max) and the sweetpersistence constant (T) of neohesperidin dihydrochalcone (NHD),were significantly reduced in a mixture containing naringin(NAR), a bitter flavone analog of NHD Sucrose octa-acetate (SOA),another bitter stimulus, reduced the Ip_max of NHD in mixtures,but no appreciable decrease in T values was found. Linear regressionanalyses performed on the IP_max data of either NHD + NAR orNHD + SOA (logIp_max versus log concentration) produced slopevalues lower than those of NHD alone. Moreover, taste similarityexperiments revealed that the mixture of NHD + NAR was locatedfurther than NHD from the sugar area in the multi-dimensionalscaling (MDS) map. It is concluded that the reduction in Tvaluesof NHD by NAR was apparently related to the reduced Ip_max levelsand that such a mixture produces a sweet quality inferior tothat of NHD.     

Cross-adaptation and bitterness inhibition of L-tryptophan, L-phenylalanine and urea: further support for shared peripheral physiology

A previous study investigating individuals' bitterness sensitivities found a close association among three compounds: L-tryptophan (L-trp), L-phenylalanine (L-phe) and urea (Delwiche et al., 2001, Percept. Psychophys. 63, 761-776). In the present experiment, psychophysical cross-adaptation and bitterness inhibition experiments were performed on these three compounds to determine whether the bitterness could be differentially affected by either technique. If the two experimental approaches failed to differentiate L-trp, L-phe and urea's bitterness, then we may infer they share peripheral physiological mechanisms involved in bitter taste. All compounds were intensity matched in each of 13 subjects, so the judgments of adaptation or bitterness inhibition would be based on equal initial magnitudes and, therefore, directly comparable. In the first experiment, cross-adaptation of bitterness between the amino acids was high (>80%) and reciprocal. Urea and quinine-HCl (control) did not cross-adapt with the amino acids symmetrically. In a second experiment, the sodium salts, NaCl and Na gluconate, did not differentially inhibit the bitterness of L-trp, L-phe and urea, but the control compound, MgSO(4), was differentially affected. The bitter inhibition experiment supports the hypothesis that L-trp, L-phe and urea share peripheral bitter taste mechanisms, while the adaptation experiment revealed subtle differences between urea and the amino acids indicating that urea and the amino acids activate only partially overlapping bitter taste mechanisms.     

STUDIES ON THE PATHWAY OF SULFIDE PRODUCTION IN A COPPER-ADAPTED YEAST

Metabolism of some sulfur-containing substances was studiedin a copper-resistant strain of yeast (R), its parent strain(P) and respiratory-deficient(RD) mutants from them. The resultsobtained are as follows:

1. Using sulfate, sulfite and thiosulfate as sulfur sources, Rproducedmore H₂S than P, and both of these had the activityhigher than their RD mutants. All of them produced a large amountof H₂S from cysteine, but only little from methionine, cysteinesulfinic acid and S-sulfocysteine.
2. From sulfite and thiosulfate,P and R produced more H₂S inaerobicthan in anaerobic condition.With sulfate and cysteine, however,H₂S production did not differunder those conditions.
3. In both P and R, the sulfate-to-sulfiteand sulfite-to-sulfidereactions were remarkably lowered byiron and zinc deficiencies.But the cysteine-to-sulfide reactionwas not affected by themetal-deficiencies.
4. H₂S productionfrom sulfate was remarkably depressed by highconcentrationsof pantothenate.
5. Rates of reaction steps on a plausible pathway from sulfatetosulfide and to organic sulfur compounds areestimated forthe strainsused. R is characterized by its largecapacity ofthe reaction step from sulfate to sulfite, and excessivesulfitethus formed is liberatedas sulfide not by the way ofcysteine.

¹Present address: Research Reactor Institute, Kyoto University,Kumatori-cho, Sennan-gun, Osaka     

Phytoplankton uptake of ammonium and urea during growth on oxidized forms of nitrogen

Uptake capabilities for ammonium (NH₄⁺) and urea by diatoms(Thalassiosira pseudonana and Skeletonema costatum) growingon oxidized forms of nitrogen were studied in short-term uptakeexperiments. Even when nutrient-saturated, an enhanced uptakecapability not coupled with the growth rate was present forNH₄⁺ and urea. No such enhanced uptake ability was seen forNO₂^– or NO₃^– under either nutrient-saturated ornutrient-depleted conditions. The presence of NH₄⁺ decreasedthe enhanced ability to take up urea, but the urea uptake ratein 5 min incubations remained greater than the growth rate evenwhen NH₄⁺ was present.     

Characterization of Sulfate Transport in the Green Alga Chlorella ellipsoidea

A rapid induction of sulfate transport was observed in the greenalga Chlorella ellipsoidea during sulfur-limited growth. Bothaffinity and V_max increased about five-fold within 6 h of transferringcells from Bold's basal medium with 350 µM MgSO₄ to sulfur-deficientBold's medium. High affinity sulfate transport was induced within15 min and reached maximum rate within 3 h of transferring cellsto sulfur-deficient condition, indicating that a new, high-affinity-sulfatetransport system is induced by sulfur starvation in C. ellipsoidea.Eadie-Hofstee plots of initial rates of sulfate uptake indicatedthat the K of sulfur-starved cells was about 17 µM. Bothsulfur-starved and unstarved cells grown in air had a V_max of1.5 times higher than that of high-CO₂ grown cells. Sulfatetransport was completely inhibited by 30 µM CCCP or 800µMKCN both in the light and the dark but transport in the lightwas not inhibited by 20 µM DCMU. Treatment with 50 µMor 500 µM vanadate caused 50% inhibition of uptake. Therate of sulfate uptake in the dark was twice that in the lightand was stimulated by low pH. These results suggest that thesulfate transport system in C. ellipsoidea is operated by protonsymport across the plasmamembrane which is partially mediatedby P-type ATPase and that these systems depend exclusively onenergy derived from oxidative phosphorylation in the mitochondria. (Received June 28, 1995; Accepted August 8, 1995)     

The Transport and Metabolism of Urea in Chara australis: I. PASSIVE DIFFUSION, SPECIFIC TRANSPORT AND METABOLISM OF UREA, THIOUREA AND METHYLUREA

Most of the urea entering Chara australis cells is rapidly metabolizedto produce CO₂, which diffuses out of the cells into the surroundingmedium. A simple and convenient apparatus to measure both the¹⁴C-urea retained by cells and the ¹⁴CO₂ released into the mediumwas developed and used in a study of urea transport in Chara.The permeability coefficient for urea in the Chara plasmalemmawas estimated from the slope of an uptake versus concentrationfunction as 85 nm s^-1. Computer modelling of urea uptake andmetabolism suggests that this could be a 20% underestimate ofthe true value.The corresponding permeability coefficients forthiourea and N-methyl-urea were estimated in the same way as34 and 35 nm s^-1, respectively. These permeabilities are muchgreater than expected on the basis of either/water partitioncoefficients for the solutes and are consistent with the diffusionof urea and its similarly-sized analogues through aqueous poresin the plasmalemma.At external concentrations of urea less than20 mmol m^-3, the bulk of the uptake is effected by a specifictransport mechanism with an apparent K_m for urea of less than1.0 mmol m^-3. This transport system operates most rapidly withexternal pH in the range 6.5–7.5 and is influenced bythe nitrogen status of the cell.Evidence is produced here suggestingthat the specific transport of urea may be an active process. Key words: Chara, urea uptake, metabolism, diffusion, specific transport     

Iontophoretic application of bitter taste stimuli in hamsters

Recent electrophysiological studies on the iontophoretic applicationof taste stimuli by weak electric currents using rodents andfrogs have produced stimuli which appear to mimic the actionof salty, sour and sweet solutions. However, there has beenno report of an ionic stimulus which might serve as a bitteriontophoretic probe. Many common bitter stimuli are either uncharged(e.g. quinine, urea) or have mixed quality sensations (e.g.the bitter salts KCl, MgCl₂) and therefore are unsuitable. Thisreport investigates the use of four organic anions, all of whichare bitter to humans, which may serve as potential bitter stimulifor iontophoretic application to the tongue of the hamster whilerecording electrophysiologically from its chorda tympani nerve.These anions are m-nitrobenzene sulfonate (NBSA), picrate, cholateand m-nitrobenzoate (NBA). The electrophysiological responsesto cathodal polarization via these four anions plus saccharin,an effective cathodal stimulus in the hamster, form the sameefficacy series as chemical (i.e. normal sapid) presentationsof sodium salts of these anions, i.e. saccharin > NBSA >picrate > NBA > cholate. Behavioral evidence suggeststhat NBSA is sweet to hamsters while the latter three anions,picrate, NBA and cholate, are bitter. Electrophyiological observations,based on magnitude of response, appear to support these behavioralfindings. It was concluded that picrate, NBA and cholate mayserve as useful bitter stimulus probes for ionto-phoretic applicationin the hamster.     

Synthesis of Citrulline and Arginine in Chlorella pyrenoidosa

The distribution of radioactivity in Chlorella during dark ¹⁴CO₂fixation was investigated either (a) in normal cells with andwithout added ammonium chloride, or (b) in nitrogen-starvedcells supplied with intermediates of the Krebs-Henseleit ureacycle. In the control experiments almost all the activity was presentin compounds of or associated with, the tricarboxylic acid cycle. The amino-acids citrulline and arginine became radioactive onlyin the presence of ammonia or ornithine where initially theycomprised 40–60 per cent. of the total activity, reactionsof the Krebs–Henseleit urea cycle being implicated intheir formation. No evidence could be found for a complete ureacycle. Unidentified compounds deriving their radioactivity fromthe C₄ carbon of citrulline and/or arginine were detected andformed up to 40 per cent. of the total ¹⁴CO₂ incorporated after25 min.     

Influence of Ni Supply on Growth and Nitrogen Metabolism ofBrassica napusL. Grown with NH4NO3or Urea as N Source

The significance of nickel (Ni), which is essential for ureaseactivity, for growth and nitrogen (N) metabolism ofBrassicanapusgrown in nutrient solution with either NH₄NO₃or urea assole N source was investigated. Although Ni contents were below25 µg kg^-1d. wt, growth of plants relying on NH₄NO₃wasnot affected by the Ni status. However, supplementing the growthmedium with 0.04 µMNi enhanced dry matter production ofurea-grown plants significantly. Urease activity was significantlyreduced in leaves and roots of plants grown without supplementaryNi irrespective of N source. Plants grown with urea withoutadditional Ni accumulated large amounts of urea and had loweramino acid contents indicating impaired usage of the N supplied,while those grown with NH₄NO₃under Ni-deprived conditions accumulatedendogenous urea in their older leaves. It is suggested thatNi may not be strictly essential for plants receiving mineralN, or that the critical level is well below 25 µg kg^-1d.wt. These results confirm that Ni is required for urease activityand thus for growth of plants on urea-based media, as well asfor recycling endogenous urea.Copyright 1999 Annals of BotanyCompany. Brassica napusvar.annua, amino acids, N nutrition, nickel, spring rape, urea, urease activity.     

Tolerance of bitter stimuli and attenuation/accumulation of their bitterness in humans

Some components of bitterness make key flavor contributions to promote the palatability of foods, whereas other components are recognized as aversive signals to avoid consuming harmful substances. These contradictory behaviors suggest that humans tolerate tastes of bitterants based on certain criteria. Here, we investigated human taste tolerance and sensory cues leading to diverse taste tolerance of bitter compounds. Tolerance of eight bitter compounds, which are typically contained in foods, was evaluated by measuring detection and rejection thresholds. The results revealed that the level of tolerance of each compound was variable, and some compounds showed an acceptable concentration regarding the suprathreshold intensity. Tolerance did not depend on the nutritive value or attenuation and accumulation characteristics of bitterness and bitter taste receptors. These results suggest that the criteria controlling tolerance of bitter compounds may be derived from a complex relationship between the taste quality and cognitive process.     

The Transport and Metabolism of Urea in Chara australis: II. UREASE AND THE DISTRIBUTION OF TRANSPORTED UREA

The ureolytic enzyme in Chara was investigated. This enzymewas shown to be a urease with an unusually high affinity forurea(K_m = 158 mmol m^-3). Little inhibition of urease activitywas found when intact Chara cells were exposed to the ureaseinhibitors hydroxyurea, acetohydroxamic acid and N-ethylmaleimide,although there was some inhibition of urea uptake. The distribution of radioactivity amongst the amino acid, organicacid and sugar/neutral fractions, determined by ion-exchangechromatography, was very similar whether the Chara internodeswere exposed to ¹⁴C-urea or to H¹⁴CO₃. This suggests that thefraction of the urea-carbon liberated by the urease as CO₂ andretained by the cell is used in photosynthetic carbon-fixation.During the initial 15 min of ¹⁴C-urea uptake, label appearsin the vacuole only in the form of unmetabolized urea. Afterthis time a variety of labelled compounds appear in the vacuole,presumably reflecting the gradual movement of carbon-fixationproducts from the chloroplasts to the cytoplasm and thence intothe vacuole. Key words: Urea transport, metabolism, Chara, urease     

EFFECTS OF TEMPERATURE ON CHRONIC HYPOXIA TOLERANCE IN THE NON-INDIGENOUS BROWN MUSSEL, PERNA PERNA (BIVALVIA: MYTILIDAE) FROM THE TEXAS GULF OF MEXICO

Effects of temperature (15°, 20° and 25°C), O₂ partialpressure (P_O2=0, 1, 2, 4, and 6 kPa), and individual size(12–79 mm shell length; SL) on survivorship of specimensof the non-indigenous, marine, brown mussel, Perna perna, fromTexas were investigated to assess its potential distributionin North America. Its hypoxia tolerance was temperature-dependent,survivorship being significantly extended at lower temperaturesunder all tested lethal P_O2. Incipient tolerated P_O2 was 4 and6 kPa at 15 and 20°C, respectively, with >50% mortalityoccurring at 25°C at all tested levels of hypoxia. P_O2 hadless of an effect on survival of hypoxia than temperature. At25°C, survivorship was not different over a P_O2 range of0–2 kPa and increased only at 4 and 6 kPa. Survivorshipwas size-dependent. Median survival times increased with increasingSL in anoxia and P_O2=1 kPa, but at 2, 4 and 6 kPa,smaller individuals survived longer than larger individuals.With tolerance levels similar to other estuarine bivalve species,P. perna should withstand hypoxia encountered in estuarine environments.Thus, its restriction to intertidal rocky shores may be dueto other parameters, particularly its relatively low temperaturetolerance. (Received 26 January 2004; accepted 31 March 2005)     

Growth of Phaseolus vulgaris on Various Nitrogen Sources: The Importance of Urease

Rhizobium-inoculatcd plants of Phaseolus vulgaris L. were grownwith different N-sources (nitrate, ammonium, urea) and differentconcentrations of urea. The distribution of growth between plantparts varied with N-sources. Nitrate and ammonium were moreinhibitory to nodulation than urea, which at 40 mol m^–3N had no effect. Urease activity varied in amount and locationover a range of urea concentrations. At higher concentrations,more urea was transported to and increased urease activity wasfound in the shoot Lower levels of activity in plants relianton N₂-fixation were consistent with a ureide-degradation pathwaynot involving urea. Moderate doses of urea could be assimilatedconcomitantly with N₂-fixation. At higher levels of appliedurea, nodulation and ureide transport to the shoots were reduced,although increased growth could not be maintained at concentrationsof applied urea greater than 6.0 mol m^–3 urea N. Key words: Phaseolus vulgaris, growth, nitrogen source, urease     

Shortening velocity and myosin heavy- and light-chain isoform mRNA in rabbit arterial smooth muscle cells

In smooth muscle cells (SMCs)isolated from rabbit carotid, femoral, and saphenous arteries, relativemyosin isoform mRNA levels were measured in RT-PCR to test forcorrelations between myosin isoform expression and unloaded shorteningvelocity. Unloaded shortening velocity and percent smooth muscle myosinheavy chain 2 (SM2) and myosin light chain 17b(MLC_17b) mRNA levels were not significantly different insingle SMCs isolated from the luminal and adluminal regions of thecarotid media. Saphenous artery SMCs shortened significantly faster(P < 0.05) than femoral SMCs and had more SM2 mRNA(P < 0.05) than carotid SMCs and lessMLC_17b mRNA (P < 0.001) and higher tissuelevels of SMB mRNA (P < 0.05) than carotid and femoralSMCs. No correlations were found between percent SM2 and percentMLC_17b mRNA levels and unloaded shortening velocity in SMCsfrom these arteries. We have previously shown that myosin heavy chain(MHC) SM1/SM2 and SMA/SMB and MLC_17a/MLC_17b isoform mRNA levels correlate with protein expression for these isoforms in rabbit smooth muscle tissues. Thus we interpret these results to suggest that 1) SMC myosin isoform expression andunloaded shortening velocity do not vary with distance from the lumenof the carotid artery but do vary in arteries located longitudinally within the arterial tree, 2) MHC SM1/SM2 and/orMLC_17a/MLC_17b isoform expression does notcorrelate with unloaded shortening velocity, and 3)intracellular expression of the MHC SM1/SM2 and MLC_17a/MLC_17b isoforms is not coregulated.

     

Polygenic determination of quinine aversion among mice

There are substantial differences among inbred mouse strainsin avoidance of quinine solutions in two-bottle preference tests.x A Mendelian cross-breeding experiment was conducted to testthe hypothesis that a single locus Qui has a major influenceon quinine aversion. Inbred strains C57B1/6J (B6, avoider) andC3HeB/FeJ (C3, indifferent) were progenitors of two segregatinggenerations. Phenotypic ratios for 100 µM and 30 µMquinine sulfate (QSO4) in these generations were not consistentwith ratios expected for a single gene. Frequency distributionsfor individual preference ratios were more characteristic ofa polygenic trait. An outbred strain (CFW/Crl) which displayssegregation for the Soa locus was tested for both QSO4 and sucroseocta-acetate (SOA) avoidance. Correlated avoidance patternsfor the two bitter compounds were found in these mice. A Soaeffect might not have been seen in the C3.B6 cross because bothstrains are relatively poor SOA avoiders. A second Mendeliancross was made between strains C3 and SWR/J (SW, SOA and QSO4avoider). One segregating generation was tested with both compounds.In these mice, as in the CFW population, QSO4 aversion was correlatedwith SOA aversion. These results suggest that quinine aversionis polygenic, that there is a relationship between SOA sensitivityand quinine sensitivity, and that this association may be theresult of variation at the Soa locus.     

The influence of sodium salts on binary mixtures of bitter-tasting compounds

In order to study potential mixture interactions among bitter compounds, selected sodium salts were added to five compounds presented either alone or as binary bitter-compound mixtures. Each compound was tested at a concentration that elicited 'weak' perceived bitterness. The bitter compounds were mixed at these concentrations to form a subset of possible binary mixtures. For comparison, the concentration of each solitary compound was doubled to measure bitterness inhibition at the higher intensity level elicited by the mixtures. The following sodium salts were tested for bitterness inhibition: 100 mM sodium chloride (salty), 100 mM sodium gluconate (salty), 100 and 20 mM monosodium glutamate (umami), and 50 mM adenosine monophosphate disodium salt (umami). Sucrose (sweet) was also employed as a bitterness suppressor. The sodium salts differentially suppressed the bitterness of compounds and their binary combinations. Although most bitter compounds were suppressed, the bitterness of tetralone was not suppressed, nor was the bitterness of the binary mixtures that contained it. In general, the percent suppression of binary mixtures of compounds was predicted by the average percent suppression of its two components. Within the constraints of the present study, the bitterness of mixtures was suppressed by sodium salts and sucrose independently, with few bitter interactions. This is consistent with observations that the bitter taste system integrates the bitterness of multi-compound solutions linearly.