Depression of gustatory receptor potential in frog taste cell by parasympathetic nerve-induced slow hyperpolarizing potential

Parasympathetic nerve (PSN) innervates taste cells of the frog taste disk, and electrical stimulation of PSN elicited a slow hyperpolarizing potential (HP) in taste cells. Here we report that gustatory receptor potentials in frog taste cells are depressed by PSN-induced slow HPs. When PSN was stimulated at 30 Hz during generation of taste cell responses, the large amplitude of depolarizing receptor potential for 1 M NaCl and 1 mM acetic acid was depressed by approximately 40% by slow HPs, but the small amplitude of the depolarizing receptor potential for 10 mM quinine-HCl (Q-HCl) and 1 M sucrose was completely depressed by slow HPs and furthermore changed to the hyperpolarizing direction. The duration of the depolarizing receptor potentials depressed by slow HPs prolonged with increasing period of PSN stimulation. As tastant-induced depolarizing receptor potentials were increased, the amplitude of PSN-induced slow HPs inhibiting the receptor potentials gradually decreased. The mean reversal potentials of the slow HPs were approximately -1 mV under NaCl and acetic acid stimulations, but approximately -14 mV under Q-HCl and sucrose stimulations. This implies that when a slow HP was evoked on the same amplitude of depolarizing receptor potentials, the depression of the NaCl and acetic acid responses in taste cells was larger than that of Q-HCl and sucrose responses. It is concluded that slow HP-induced depression of gustatory depolarizing receptor potentials derives from the interaction between gustatory receptor current and slow hyperpolarizing current in frog taste cells and that the interaction is stronger for NaCl and acetic acid stimulations than for Q-HCl and sucrose stimulations.     

The effects of amiloride on the labellar taste receptor cells of the fleshfly Boettcherisca peregrina

Amiloride is known to inhibit the taste response of vertebrates to salt by blocking the amiloride-sensitive sodium channel. In this study, we investigated electrophysiologically the effect of amiloride on the taste response of the fleshfly Boettcherisca peregrina. When 0.5 mM amiloride was included in taste solutions, the response of the salt receptor cell (salt response) to sodium chloride (NaCl) was not depressed but those of the sugar receptor cell (sugar responses) to sucrose, glucose, fructose, l-valine (l-Val) and l-phenylalanine (l-Phe) were strongly depressed. An inhibitory effect of amiloride on the concentration-response relationship for both sucrose and l-Phe was clearly revealed, but not at high concentrations of sucrose. After pretreatment of a chemosensory seta with 0.15 mM amiloride for 10 min, the salt response to NaCl was not affected. On the other hand, the sugar responses to sucrose, fructose, l-Val and l-Phe were depressed just after amiloride pretreatment. The sugar response to adenosine 5’-diphosphate (ADP) mixed with 0.5 mM amiloride was not depressed, but the response to ADP alone was depressed after amiloride pretreatment. It was therefore observed that amiloride depressed the responses to all stimulants that react with each of the receptor sites of the sugar receptor cell.     

低分子量有机酸对红壤无机态磷转化及酸度的影响

以鄂南、赣北两红壤样品为材料,加入不同有机酸并经室温培养后,测定不同P组分、pH及活性Al含量的变化。结果表明,供试有机酸均使土壤Ca2－P含量增高,增幅大小依次为柠檬酸＞苹果酸＞琥珀酸＞乙酸;2种土壤的Ca8－P和Ca10－P含量无明显变化规律,Fe-P、Al-P和O－P含量有所下降,除乙酸处理的土壤pH值无显著变化外,其它有机酸的加入使pH下降0．65－1．96;有机酸引起活性Al量增多,除乙酸处理的变化较小外,其它有机酸或混合物的加入使土壤中0．02mol.L^-1CaCl2提取Al增加4．7－50．3倍,1mol.L^-1提取Al增加4．0－67．3倍。可见,有机酸具有双重作用,既增加P的有效性,又增加土壤酸度和Al毒。     

Effect of Maillard reacted peptides on human salt taste and the amiloride-insensitive salt taste receptor (TRPV1t)

Maillard reacted peptides (MRPs) were synthesized by conjugating a peptide fraction (1000-5000 Da) purified from soy protein hydrolyzate with galacturonic acid, glucosamine, xylose, fructose, or glucose. The effect of MRPs was investigated on human salt taste and on the chorda tympani (CT) taste nerve responses to NaCl in Sprague-Dawley rats, wild-type, and transient receptor potential vanilloid 1 (TRPV1) knockout mice. MRPs produced a biphasic effect on human salt taste perception and on the CT responses in rats and wild-type mice in the presence of NaCl + benzamil (Bz, a blocker of epithelial Na+ channels), enhancing the NaCl response at low concentrations and suppressing it at high concentrations. The effectiveness of MRPs as salt taste enhancers varied with the conjugated sugar moiety: galacturonic acid = glucosamine > xylose > fructose > glucose. The concentrations at which MRPs enhanced human salt taste were significantly lower than the concentrations of MRPs that produced increase in the NaCl CT response. Elevated temperature, resiniferatoxin, capsaicin, and ethanol produced additive effects on the NaCl CT responses in the presence of MRPs. Elevated temperature and ethanol also enhanced human salt taste perception. N-(3-methoxyphenyl)-4-chlorocinnamid (a blocker of TRPV1t) inhibited the Bz-insensitive NaCl CT responses in the absence and presence of MRPs. TRPV1 knockout mice demonstrated no Bz-insensitive NaCl CT response in the absence or presence of MRPs. The results suggest that MRPs modulate human salt taste and the NaCl + Bz CT responses by interacting with TRPV1t.     

棉铃虫幼虫对人类呈味物质的取食反应

利用叶碟法在室内测定了棉铃虫对人类酸、甜、苦、咸4种基本呈味物质和麻、辣味2种植物提取物的取食反应。正交试验结果表明,棉铃虫幼虫对用甜味、苦味和辣味物质(蔗糖、奎宁和辣椒提取物)处理过的烟叶取食选择率较高,对这3种呈味物质表现出有较好的适应性;而幼虫对咸味、酸味和麻味物质(氯化钠、柠檬酸和花椒提取物)处理过的烟叶取食量较少,这3种呈味物质表现出较强的拒食活性。在选择性条件下,幼虫的取食量与花椒提取物剂量显著相关;而在非选择性条件下,幼虫的取食量与氯化钠剂量显著相关。     

Amiloride-sensitive and amiloride-insensitive responses to NaCl + acid mixtures in hamster chorda tympani nerve

Component signaling in taste mixtures containing both beneficial and dangerous chemicals depends on peripheral processing. Unidirectional mixture suppression of chorda tympani (CT) nerve responses to sucrose by quinine and acid is documented for golden hamsters (Mesocricetus auratus). To investigate mixtures of NaCl and acids, we recorded multifiber responses to 50 mM NaCl, 1 and 3 mM citric acid and acetic acid, 250 μM citric acid, 20 mM acetic acid, and all binary combinations of each acid with NaCl (with and without 30 μM amiloride added). By blocking epithelial Na(+) channels, amiloride treatment separated amiloride-sensitive NaCl-specific responses from amiloride-insensitive electrolyte-generalist responses, which encompass all of the CT response to the acids as well as responses to NaCl. Like CT sucrose responses, the amiloride-sensitive NaCl responses were suppressed by as much as 50% by citric acid (P = 0.001). The amiloride-insensitive electrolyte-generalist responses to NaCl + acid mixtures approximated the sum of NaCl and acid component responses. Thus, although NaCl-specific responses to NaCl were weakened in NaCl-acid mixtures, electrolyte-generalist responses to acid and NaCl, which tastes KCl-like, were transmitted undiminished in intensity to the central nervous system. The 2 distinct CT pathways are consistent with known rodent behavioral discriminations.     

Taste confusions following chlorhexidine treatment.

Chlorhexidine, a bitter bis-biguanide antiseptic, is the only known blocker of the human salty taste. In order to characterize the effects of chlorhexidine on stimulus identification, taste confusion matrix (TCM) performance was measured for subjects treated with 1.34 mM chlorhexidine gluconate (n = 9) and water controls (n = 9). Ten stimuli [water, 0.1 M NaCl, 0.1 M KCl, 0.1 mM quinine-HCl (QHCl), 0.1 M monosodium glutamate (MSG), 3 mM citric acid, 0.3 M sucrose and mixtures of NaCl, QHCl and citric acid with sucrose] were presented in 10 replicates for identification from a list of 10 stimulus names. T(10), a measure of performance consistency from information theory, was lower for chlorhexidine-treated subjects (2.02 +/- 0.11 bits) than controls (2.73 +/- 0.11 bits) (P < 0.0001). T(2), an indirect measure of pairwise stimulus discrimination, approached chance levels (0.40 bit) in chlorhexidine-treated subjects for all possible pairs of NaCl, KCl, QHCl and water, as well as pairs composed of sucrose and the NaCl-sucrose and quinine-sucrose mixtures. In controls T(2) values approached perfect scores (1.00 bit) for all stimulus pairs except NaCl-KCl and NaCl-MSG. The results demonstrate a decreased ability to identify taste stimuli that is consistent with alterations in the ability of stimuli to elicit salty and bitter taste perceptions. As a selective, effective, persistent and reversible blocker of taste perceptions, chlorhexidine should prove useful in defining taste mechanisms in humans.     

Enhanced consumption of an aversively conditioned taste following the presentation of a “medicine” taste

Rats given presentations of a citric acid solution while recovering from LiCl-induced illness (i.e., a “medicine effect” treatment) subsequently drank more of an aversively conditioned NaCl solution at test, when the NaCl presentation was immediately preceded by citric acid. That is, citric acid passed a summation test of conditioned inhibition. Such an effect was not observed in a group given explicitly unpaired presentations of LiCl and citric acid. It is proposed that enhanced consumption of an aversive taste due to the previous presentation of a “medicine” taste can provide an animal model of human maladaptive behavior in regards to food consumption.     

Effects of linoleic acid on sweet, sour, salty, and bitter taste thresholds and intensity ratings of adults

Evidence supporting a taste component for dietary fat has prompted study of plausible transduction mechanisms. One hypothesizes that long-chain, unsaturated fatty acids block selected delayed-rectifying potassium channels, resulting in a sensitization of taste receptor cells to stimulation by other taste compounds. This was tested in 17 male and 17 female adult (mean +/- SE age = 23.4 +/- 0.7 yr) propylthiouracil tasters with normal resting triglyceride concentrations (87.3 +/- 5.6 mg/day) and body mass index (23.3 +/- 0.4 kg/m(2)). Participants were tested during two approximately 30-min test sessions per week for 8 wk. Eight stimuli were assessed in duplicate via an ascending, three-alternative, forced-choice procedure. Qualities were randomized over weeks. Stimuli were presented as room-temperature, 5-ml portions. They included 1% solutions of linoleic acid with added sodium chloride (salty), sucrose (sweet), citric acid (sour), and caffeine (bitter) as well as solutions of these taste compounds alone. Participants also rated the intensity of the five strongest concentrations using the general labeled magnitude scale. The suprathreshold samples were presented in random order with a rinse between each. Subjects made the ratings self-paced while wearing nose clips. It was hypothesized that taste thresholds would be lower and absolute intensity ratings or slopes of intensity functions would be higher for the stimuli mixed with the linoleic acid. Thresholds were compared by paired t-tests and intensity ratings by repeated measures analysis of variance. Thresholds were significantly higher (i.e., lower sensitivity) for the sodium chloride, citric acid, and caffeine solutions with added fatty acid. Sweet, sour, and salty intensity ratings were lower or unchanged by the addition of a fatty acid. The two highest concentrations of caffeine were rated as weaker in the presence of linoleic acid. These data do not support a mechanism for detecting dietary fats whereby fatty acids sensitize taste receptor cells to stimulation by taste compounds.     

Proton currents through amiloride-sensitive Na channels in hamster taste cells. Role in acid transduction.

The activity of taste cells maintained in the intact hamster tongue was monitored in response to acid stimulation by recording action currents from taste receptor cells with an extracellular "macro" patch pipette: a glass pipette was pressed over the taste pore of fungiform papillae and perfused with citric acid, hydrochloric acid, or NaCl. Because this technique restricted stimulus application to the small surface area of the apical membranes of the taste cells, many nonspecific, and potentially detrimental, effects of acid stimulation could be avoided. Acid stimulation reliably elicited fast transient currents (action currents of average amplitude, 9 pA) which were consistently smaller than those elicited by NaCl (29 pA). The frequency of action currents elicited by acid stimuli increased in a dose-dependent manner with decreasing pH from a threshold of about pH 5.0. Acid-elicited responses were independent of K+, Na+, Cl-, or Ca2+ at physiological (salivary) concentrations, and were unaffected by anthracene-9-carboxylic acid, tetraethylammonium bromide, diisothiocyanate-stilbene-2,2'-disulfonic acid, vanadate, or Cd2+. In contrast, amiloride (< or = 30 microM) fully and reversibly suppressed acid-evoked action currents. At submaximal amiloride concentrations, the frequency and amplitude of the action currents were reduced, indicating a reduction of the taste cell apical conductance concomitant with a decrease in cell excitation. Exposure to low pH elicited, in addition to transient currents, an amiloride-sensitive sustained d.c. current. This current is apparently carried by protons instead of Na+ through amiloride-sensitive channels. When citric acid was applied while the taste bud was stimulated by NaCl, the action currents became smaller and the response resembled that produced by acid alone. Because of the strong interdependence of the acid and salt (NaCl) responses when both stimuli are applied simultaneously, and because of the similarity in the concentration dependence of amiloride block, we conclude that amiloride-sensitive Na+ channels on hamster taste receptor cells are permeable to protons and may play a role in acid (sour) taste.     

Sensory irritation and taste produced by NaCl and citric acid: effects of capsaicin desensitization

Three experiments were conducted to measure the sensory irritationproduced by two prototypical gustatory stimuli: citric acidand NaCl. The stimuli were applied to the tip of the tongueon filter paper disks. The first experiment revealed that solutionsof NaCl and citric acid that produced approximately equal tastesensations also produced similar amounts of irritation; thatthe psychophysical functions for irritation were approximatelytwice as steep as the functions for taste; and that irritationgrew over time for NaCl but not for citric acid. When viewedas a percentage of the taste sensation at 25 s, NaCl irritationaveraged 23% at the lowest concentration and 70% at the highestconcentration; citric acid irritation averaged 44% at the lowestconcentration and 98% at the highest concentration. The secondexperiment investigated whether the irritation produced by thesetwo stimuli was mediated via capsaicin-sensitive (CS) fibers.The experiment included a pre-test, an irritation treatmentwith either capsaicin (a desensitizing agent) or zingerone (anon-desensitizing agent), a 15 min rest period and a post-test.Reductions in irritation and taste occurred following treatmentwith both capsaicin and zingerone. A third experiment demonstratedthat the majority of the effect of zingerone on taste and irritationwas due to a perceptual context effect. After the context effectwas taken into account, capsaicin desensitization remained significantfor both salt taste and salt irritation at the highest concentration.A similar pattern of results for citric acid suggests that bothcitric acid and NaCl produce irritation in part via CS fibers.The results are discussed in terms of the ability of subjectsto discriminate the gustatory and chemesthetic components oforal sensations and the role of salt and acid irritation inflavor perception.     

Exposure to salt and organic acids increases the ability of Listeria monocytogenes to invade Caco-2 cells but decreases its ability to survive gastric stress

The effects of environmental stress exposure on Listeria monocytogenes growth and virulence-associated characteristics were investigated. Specifically, we measured the effects of temperature (7 or 37 degrees C), pH (5.5 or 7.4), the presence of salt and organic acids (375 mM NaCl, 8.45 mM sodium diacetate [SD], 275 mM sodium lactate [SL], or a combination of NaCl, SD, and SL), and deletion of sigB, which encodes a key stress response regulator, on the ability of L. monocytogenes to grow, invade Caco-2 cells, and survive exposure to synthetic gastric fluid (pH 2.5 or 4.5). Our results indicate that (i) L. monocytogenes log-phase generation times and maximum cell numbers are not dependent on the alternative sigma factor sigmaB in the presence of NaCl and organic acids at concentrations typically found in foods; (ii) growth inhibition of L. monocytogenes through the addition of organic acids is pH dependent; (iii) the ability of L. monocytogenes to invade Caco-2 cells is affected by growth phase, temperature, and the presence of salt and organic acids, with the highest relative invasion capabilities observed for cells grown with SL or NaCl at 37 degrees C and pH 7.4; (iv) growth of L. monocytogenes in the presence of NaCl, SD, or SL reduces its ability to survive exposure to gastric fluid; and (v) exposure of L. monocytogenes to gastric fluid reduces the enhanced invasiveness caused by growth in the presence of NaCl or SL. These findings suggest that virulence-associated characteristics that determine the L. monocytogenes infectious dose are likely to be affected by food-specific properties (e.g., pH or the presence of salt or organic acid).     

Enhancement of retronasal odors by taste

Psychophysical studies of interactions between retronasal olfaction and taste have focused most often on the enhancement of tastes by odors, which has been attributed primarily to a response bias (i.e., halo dumping). Based upon preliminary evidence that retronasal odors could also be enhanced by taste, the present study measured both forms of enhancement using appropriate response categories. In the first experiment, subjects rated taste ("sweet," "sour," "salty," and "bitter") and odor ("other") intensity for aqueous samples of 3 tastants (sucrose, NaCl, and citric acid) and 3 odorants (vanillin, citral, and furaneol), both alone and in taste-odor mixtures. The results showed that sucrose, but not the other taste stimuli, significantly increased the perceived intensity of all 3 odors. Enhancement of tastes by odors was inconsistent and generally weaker than enhancement of odors by sucrose. A second experiment used a flavored beverage and a custard dessert to test whether the findings from the first experiment would hold for the perception of actual foods. Adding sucrose significantly enhanced the intensity of "cherry" and "vanilla" flavors, whereas adding vanillin did not significantly enhance the intensity of sweetness. It is proposed that enhancement of retronasal odors by a sweet stimulus results from an adaptive sensory mechanism that serves to increase the salience of the flavor of nutritive foods.     

Acetic acid activates PKD1L3-PKD2L1 channel—A candidate sour taste receptor

The polycystic kidney disease (PKD) 1L3-PKD2L1 channel is a candidate sour taste receptor expressed in mammalian taste receptor cells. Various acids are reported to activate PKD channels after the removal of the acid stimuli, but little information is available on the activation of these channels by acetic acid. It was difficult to analyze the PKD channel activation by acetic acid using Ca²⁺ imaging experiments because this acid induces a transient and nonspecific response in cultured cells. Here, we developed a novel method to evaluate PKD channel activation by acetic acid. Nonspecific responses were observed only over a short period after the application of acetic acid. In contrast, PKD channel activation evoked by acetic acid as well as citric acid was detected even at a later time point. This method revealed that PKD1L3-PKD2L1 channel activation by acetic acid was pH-dependent and occurred when the ambient pH was <3.1.     

Hormonal Regulation of Organic and Phosphoric Acid Release by Barley Aleurone Layers and Scutella

The release of acid from the aleurone layer and scutellum of barley (Hordeum vulgare L. cv Himalaya) was investigated. Aleurone layers isolated from mature barley grains acidify the external medium by releasing organic and phosphoric acids. Gibberellic acid and abscisic acid stimulate acid release 2-fold over control tissue incubated in 10 mM CACl2. Gibberellic acid causes medium acidification by stimulating the release of phosphoric and citric acids, whereas abscisic acid stimulates the release of malic acid. The accumulation of these acids in the incubation medium buffers the medium against changes in pH, particularly between pH 4 and 5. The amounts of amino acids that accumulate in the medium are low (2-12 nmol/layer) compared to other organic and phosphoric acids (100-500 nmol/layer). The scutellum does not play a major role in medium acidification but participates in the uptake of organic acids. The organic acid composition of the starchy endosperm changes after 3 d of imbibition; malic, succinic, and lactic acids decrease, whereas citric and phosphoric acids remain unchanged or increase. These results indicate that during postgerminative growth, the acidity of the starchy endosperm is maintained by acid production by the aleurone layer.     

Influence of arbuscular mycorrhiza on organic solutes in maize leaves under salt stress

A pot experiment was conducted to examine the effect of the arbuscular mycorrhizal (AM) fungus, Glomus mosseae, on plant biomass and organic solute accumulation in maize leaves. Maize plants were grown in sand and soil mixture with three NaCl levels (0, 0.5, and 1.0 g kg⁻¹ dry substrate) for 55 days, after 15 days of establishment under non-saline conditions. At all salinity levels, mycorrhizal plants had higher biomass and higher accumulation of organic solutes in leaves, which were dominated by soluble sugars, reducing sugars, soluble protein, and organic acids in both mycorrhizal and non-mycorrhizal plants. The relative abundance of free amino acids and proline in total organic solutes was lower in mycorrhizal than in non-mycorrhizal plants, while that of reducing sugars was higher. In addition, the AM symbiosis raised the concentrations of soluble sugars, reducing sugars, soluble protein, total organic acids, oxalic acid, fumaric acid, acetic acid, malic acid, and citric acid and decreased the concentrations of total free amino acids, proline, formic acid, and succinic acid in maize leaves. In mycorrhizal plants, the dominant organic acid was oxalic acid, while in non-mycorrhizal plants, the dominant organic acid was succinic acid. All the results presented here indicate that the accumulation of organic solutes in leaves is a specific physiological response of maize plants to the AM symbiosis, which could mitigate the negative impact of soil salinity on plant productivity.     

(+)-(S)-alapyridaine--a general taste enhancer?

N-(1-Carboxyethyl)-6-hydroxymethyl-pyridinium-3-ol inner salt (alapyridaine), recently identified in heated sugar/amino acid mixtures as well as in beef bouillon, has been shown to exhibit general taste-enhancing activities, although tasteless on its own. Differing from other taste enhancers reported so far, racemic (R/S)-alapyridaine and, to an even greater extent (+)-(S)-alapyridaine, the physiologically active enantiomer, are able to enhance more than one basic taste quality. The threshold concentrations for the sweet taste of glucose and sucrose, for the umami taste of monosodium L-glutamate (MSG) and guanosine-5'-monophosphate (GMP), as well as the salty taste of NaCl, were significantly decreased when alapyridaine was present. In contrast, perception of the bitter tastes of caffeine and L-phenylalanine, as well as of sour-tasting citric acid, was unaffected. Furthermore, alapyridaine was shown to intensify known taste synergies such as, for example, the enhancing effect of L-arginine on the salty taste of NaCl, as well as that of GMP on the umami taste of MSG. The activity of (+)-(S)-alapyridaine could be observed not only in solutions of single taste compounds, but also in more complex tastant mixtures; for example, the umami, sweet and salty taste of a solution containing MSG, sucrose, NaCl and caffeine was significantly modulated, thus indicating that alapyridaine is a general taste enhancer.     

The taste of polycose in hamsters

Hamsters show a preference for Polycose, a mixture of starch-derived glucose polymers, that is as strong as their preference for sucrose. However, in the hamster, taste aversions to Polycose may be less easily acquired than taste aversions to sucrose and the qualitative aspects of Polycose are unknown in this species. In order to examine the taste of Polycose in the hamster, we utilized a taste-aversion protocol with two conditioning trials. Animals were trained to avoid one of three different conditioning stimuli: 50 mM sucrose, 100 mM Polycose and a mixture of 50 mM sucrose with 100 mM Polycose. Control animals were conditioned with deionized water. After the second conditioning trial, generalization testing began for the three conditioning stimuli plus 3 mM citric acid, 300 mM KCI and 30 mM NaCl. The results showed that aversions to Polycose, sucrose or the Polycose/sucrose mixture cross- generalized, demonstrating that Polycose and sucrose share a common taste percept in the hamster. None of the aversions generalized to NaCl, citric acid or KCI. In addition, comparisons among the patterns of taste generalizations indicated that the tastes of Polycose and sucrose also had distinct qualitative components. Finally, although the taste of 100 mM Polycose was more salient than the taste of 50 mM sucrose, the taste of sucrose could still be detected in a mixture with Polycose.      

Non-synaptic transformation of gustatory receptor potential by stimulation of the parasympathetic fiber of the frog glossopharyngeal nerve

When the glossopharyngeal nerve (GP) in the frog was strongly stimulated electrically, slow potentials were elicited from the tongue surface and taste cells in the fungiform papillae. Injection of atropine completely blocked these slow potentials. The present and previous data indicate that the slow potentials induced in the tongue surface and taste cells are due to a liquid junction potential between saliva secreted from the lingual glands due to parasympathetic fiber activity and an adapting solution on the tongue surface. Intracellularly recorded depolarizing receptor potentials in taste cells induced by 0.5 M NaCl and 3 mM acetic acid were enhanced by depolarizing slow potentials induced by GP nerve stimulation, but were depressed by the hyperpolarizing slow potentials. On average, the receptor potential of taste cells for 0.5 M NaCl was increased by 25% by the GP nerve-induced slow potential, but the receptor potential of taste cells for 3 mM acetic acid was decreased by 1% by the slow potential. These transformations of receptor potentials in frog taste cells were not due to a synaptic event initiated between taste cells and the efferent nerve fiber, but due to a non-synaptic event, a lingual junction potential generated in the dorsal lingual epithelium by GP nerve stimulation.     

The physiological basis of seed dormancy in Avena fatua. VII. Action of organic acids and pH

Citric, succinic, fumaric, malic, pyruvic and lactic acids induced germination in two genetically pure dormant lines of Avena fatua L. The sensitivity to these acids was low immediately after harvest and increased markedly after a period of dry after-ripening. Because the acids could only overcome dormancy in partly after-ripened caryopses, the mode of their action in these caryopses differed from that of another germination promotor, ethanol, and was similar to that of the germination promoter, sodium nitrate. The mode of action of the organic acids on the partly after-ripened caryopses through lowering pH was indicated by the observation that other non-metabolic weak acids could also break dormancy while neutral pH value salt solutions of some of the tested acids were inactive. The dose-response curves of citric acid for the stimulation of germination and for oxygen uptake were similar, indicating that this organic acid may stimulate germination by promoting oxygen uptake. A time sequence study showed that citric acid stimulated oxygen uptake before the first visible signs of germination. Stimulation of germination and oxygen uptake over a range of pH values showed that those values of pH which stimulated germination also stimulated oxygen uptake, indicating that the ability to stimulate oxygen uptake was not confined to organic acids. The stimulation of both germination and oxygen uptake by citric acid was not inhibited by salicylhydroxamic acid, an inhibitor of alternative respiration, therefore stimulation of both germination and oxygen uptake by citric acid does not require the operation of the alternative pathway of respiration. The function of weak acids as promoters of oxygen uptake is discussed with reference to the breakage of dormancy in partly after-ripened caryopses and the involvement of various respiratory pathways is indicated.