Teaching critical thinking skills in physiology.
This is a report of a workshop presented at Experimental Biology '99 on April 18, 1999, in Washington, DC.
Humans↗
Biomedical subjects
S Mierson
Publications and source records attributed to S Mierson.
Skits and games to enhance students' learning of physiology.
Explore the source record for details and available documents.
Games, Experimental↗
A basolateral chloride conductance in rat lingual epithelium.
We used Ussing chamber measurements and whole-cell recordings to characterize a chloride conductance in rat lingual epithelium. Niflumic acid (NFA) and flufenamic acid (FFA), nonsteroidal anti-inflammatory aromatic compounds known to inhibit Cl- conductances in other tissues, reduced transepithelial short-circuit current (Isc) in the intact dorsal anterior rat tongue epithelium when added from the serosal side, and reduced whole-cell currents in rat fungiform taste cells. In both Ussing chamber and patch-clamp experiments, the effect of NFA was mimicked by replacement of bath Cl- with methanesulfonate or gluconate. In low Cl- bath solution, the effect of NFA on whole-cell current was reduced. Replacement of bath Ca2+ with Ba2+ reduced the whole-cell Cl- current. We conclude that a Ca2+-activated Cl- conductance is likely present in the basolateral membrane of the rat lingual epithelium, and is present in the taste receptor cells from fungiform papillae. Further experiments will be required to identify the role of this conductance in taste transduction.
4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
Basolateral amiloride-sensitive Na+ transport pathway in rat tongue epithelium.
1. Experiments were conducted to test for the presence of basolateral Na+ channels in the rat lingual epithelium. Researchers have proposed a model in which some lingual taste cells have Na+ channels in the basolateral membrane. That model is designed to account for the portion of the neural taste response and the portion of the transepithelial short-circuit current (Isc) in vitro that are insensitive to mucosal amiloride; some Na+ would diffuse across the tight junction into the cell via this lateral pathway, and would be transported out of the cell by Na+ pumps in the basal membrane. The model could also account for the differential effect of mucosal amiloride on Na+ salts of various anions, in which the neural taste responses to Na+ salts with anions larger than Cl- are more sensitive to mucosal amiloride than is the taste response to NaCl. 2. Voltage-clamp data were obtained from an in vitro preparation of the anterior-dorsal rat tongue epithelium in which the connective tissue was removed by enzyme digestion. Isc in a modified Ussing chamber was reduced by amiloride in the submucosal solution. 3. The pattern of sensitivity to submucosal amiloride differed in several respects from the pattern for mucosal amiloride. The inhibition constant (Ki) was 52 microM amiloride concentration, higher than for the apical amiloride-sensitive Na+ channel. The selectivity for Na+ over K+ was much less than for the response to mucosal amiloride; with 0.5 M NaCl or KCl on the mucosal side, the ratio of inhibition for the NaCl response to inhibition for the KCl response varied between 1 and 3. 4. As the concentration of NaCl in the mucosal solution was varied, submucosal amiloride caused little inhibition of Isc for mucosal NaCl below isosmotic concentration, with the percent inhibition increasing as mucosal salt concentration increased. With 0.5 M sodium gluconate in the mucosal solution, there was very little inhibition due to submucosal amiloride. 5. The results support the presence of amiloride-sensitive Na+ channels in the basolateral membranes of the dorsal tongue epithelium in rat, and are consistent with the proposed model in which these channels are present in taste cells.
Amiloride↗
A checklist to help students analyze published articles in basic medical sciences.
Because critical analysis of published information is an essential component of scientific life, it is important that students be trained in its practice. Undergraduate students who are more accustomed to reading textbooks and taking lecture notes find it difficult to appreciate primary publications. To help such students, we have developed a checklist that helps them analyze different components of a research article in basic biomedical sciences. Students used the checklist to analyze critically a published article. The students were assigned an article and asked to write a paper (maximum 2 pages of single-spaced type) assessing it. This assignment has been found useful to both undergraduate and graduate students in pharmacology and physiology. Student responses to a questionnaire were highly favorable; students thought the exercise provided them with some of the essential skills for life-long learning.
Education, Medical, Undergraduate↗
Ion transport in rat tongue epithelium in vitro: a developmental study.
The responsiveness of the rat gustatory system to monochloride salts changes during development. Neurophysiological recordings in the chorda tympani indicate that a) the taste responses to NaCl and KCl in early postnatal rats are small relative to NH4Cl, b) both salts become more potent stimuli as the animal matures, and c) the developmental increase is accompanied by an increase in sensitivity of the NaCl response to the sodium transport blocker amiloride. We measured ion transport properties of in vitro tongue epithelia from Wistar rats. When the tissue is mounted in an Ussing chamber, the short-circuit current responses to NaCl and KCl are small in the neonatal rat and increase during development in postweaning and adult animals. Amiloride sensitivity of the NaCl response also increases with age. This study confirms that increased sensitivity of the rat gustatory system to NaCl with age reflects changes in the peripheral membranes. The results support hypothesis that the increased sensitivity is due to amiloride-sensitive membrane components being added or becoming functional.
Aging↗
Network thermodynamic model of rat lingual epithelium: effects of hyperosmotic NaCl.
A network thermodynamic model was developed to describe steady-state and transient ion flows (Na+, K+, and Cl-) and related electrical events in the rat lingual epithelium. The model, which uses only standard assumptions about topology and membrane ion transport processes from other tight epithelia, successfully simulates steady-state transepithelial electrical measurements seen in the in vitro rat dorsal lingual epithelium for a wide range (50-2,000 mM) of mucosal NaCl concentrations. It also simulates the intracellular depolarization and subsequent repolarization observed in situ in rat taste cells in response to a mucosal hyperosmotic NaCl stimulus. Because the intracellular potential has been identified with the receptor potential for gustation, the depolarization and repolarization may explain both the phasic and tonic components of taste nerve excitation in response to a salt stimulus. This study suggests that a better understanding of taste transduction may be gained by focusing more attention on the transport properties and mechanisms of lingual epithelia.
Animals↗
Sugar-activated ion transport in canine lingual epithelium. Implications for sugar taste transduction.
There is good evidence indicating that ion-transport pathways in the apical regions of lingual epithelial cells, including taste bud cells, may play a role in salt taste reception. In this article, we present evidence that, in the case of the dog, there also exists a sugar-activated ion-transport pathway that is linked to sugar taste transduction. Evidence was drawn from two parallel lines of experiments: (a) ion-transport studies on the isolated canine lingual epithelium, and (b) recordings from the canine chorda tympani. The results in vitro showed that both mono- and disaccharides in the mucosal bath stimulate a dose-dependent increase in the short-circuit current over the concentration range coincident with mammalian sugar taste responses. Transepithelial current evoked by glucose, fructose, or sucrose in either 30 mM NaCl or in Krebs-Henseleit buffer (K-H) was partially blocked by amiloride. Among current carriers activated by saccharides, the current response was greater with Na than with K. Ion flux measurements in K-H during stimulation with 3-O-methylglucose showed that the sugar-evoked current was due to an increase in the Na influx. Ouabain or amiloride reduced the sugar-evoked Na influx without effect on sugar transport as measured with tritiated 3-O-methylglucose. Amiloride inhibited the canine chorda tympani response to 0.5 M NaCl by 70-80% and the response to 0.5 M KCl by approximately 40%. This agreed with the percent inhibition by amiloride of the short-circuit current supported in vitro by NaCl and KCl. Amiloride also partially inhibited the chorda tympani responses to sucrose and to fructose. The results indicate that in the dog: (a) the ion transporter subserving Na taste also subserves part of the response to K, and (b) a sugar-activated, Na-preferring ion-transport system is one mechanism mediating sugar taste transduction. Results in the literature indicate a similar sweet taste mechanism for humans.
Amiloride↗
Lingual epithelium of spontaneously hypertensive rats has decreased short-circuit current in response to NaCl.
Alterations in ion transport associated with hypertension have been found in a variety of organs. We used a modified Ussing chamber to compare the NaCl dependence of the short-circuit current across the dorsal lingual epithelium in vitro from spontaneously hypertensive rats (SHR) with that from Wistar-Kyoto rats (WKY). The short-circuit current in response to mucosal NaCl was less in SHR than in WKY at hyperosmotic concentrations (above 0.15 M and up to 2.0 M). Since ion transport in the lingual epithelium has been found to play a role in early events of salt taste transduction, the attenuation in the short-circuit current in hypertensive animals may be a factor in the enhanced salt preference of SHR compared with WKY.
Analysis of Variance↗
The identity of the current carriers in canine lingual epithelium in vitro.
Ion transport across the lingual epithelium has been implicated as an early event in gustatory transduction. The fluxes of isotopically labelled Na+ and Cl- were measured across isolated canine dorsal lingual epithelium under short-circuit conditions. The epithelium actively absorbs Na+ and to a lesser extent actively secretes Cl-. Under symmetrical conditions with Krebs-Henseleit buffer on both sides, (1) Na+ absorption accounts for 46% of the short-circuit current (Isc); (2) there are two transcellular Na+ pathways, one amiloride-sensitive and one amiloride-insensitive; (3) ouabain, added to the serosal solution, inhibits both Isc and active Na+ absorption. When hyperosmotic (0.25 M) NaCl is placed in the mucosal bath, both Isc and Na+ absorption increase; net Na+ absorption is at least as much as Isc. Ion substitution studies indicate that the tissue may transport a variety of larger ions, though not as effectively as Na+ and Cl-. Thus we have shown that the lingual epithelium, like other epithelia of the gastrointestinal tract, actively transports ions. However, it is unusual both in its response to hyperosmotic solutions and in the variety of ions that support a transepithelial short-circuit current. Since sodium ion transport under hyperosmotic conditions has been shown to correlate well with the gustatory neural response, the variety of ions transported may likewise indicate a wider role for transport in taste transduction.
Amiloride↗
Salt taste transduction occurs through an amiloride-sensitive sodium transport pathway.
An important early event in mammalian gustatory transduction with respect to sodium chloride has been found to be the passage of sodium ions through specific transport pathways in the apical region of the taste bud. The inward current caused by sodium chloride placed on the mucosal surface of an in vitro preparation of rat dorsal lingual epithelium can be substantially reduced by the blocker of sodium ion transport, amiloride. The data show (i) that amiloride is a specific blocker of the chorda tympani response to sodium chloride, but not to potassium chloride, (ii) that the sodium and potassium gustatory systems are largely independent at the peripheral level, and (iii) that the classical ion taste "receptor" is actually a specific transport pathway permitting the cation to enter the taste-bud cell and thereby to spread depolarizing current.
Amiloride↗
The active ion transport properties of canine lingual epithelia in vitro. Implications for gustatory transduction.
The electrophysiological properties of the dorsal and ventral canine lingual epithelium are studied in vitro. The dorsal epithelium contains a special ion transport system activated by mucosal solutions hyperosmotic in NaCl or LiCl. Hyperosmotic KCl is significantly less effective as an activator of this system. The lingual frenulum does not contain the transport system. In the dorsal surface it is characterized by a rapid increase in inward current and can be quantitated as a second component in the time course of either the open-circuit potential or short-circuit current when the mucosal solution is hyperosmotic in NaCl or LiCl. The increased inward current (hyperosmotic response) can be eliminated by amiloride (10(-4) M). The specific location of this transport system in the dorsal surface and the fact that it operates over the concentration range characteristic of mammalian salt taste suggests a possible link to gustatory transduction. This possibility is tested by recording neural responses in the rat to NaCl and KCl over a concentration range including the hyperosmotic. We demonstrate that amiloride specifically blocks the response to NaCl over the hyperosmotic range while affecting the KCl response significantly less. The results suggest that gustatory transduction for NaCl is mediated by Na entry into the taste cells via the same amiloride-sensitive pathway responsible for the hyperosmotic response in vitro. Further studies of the in vitro system give evidence for paracellular as well as transcellular current paths. The transmural current-voltage relations are linear under both symmetrical and asymmetrical conditions. After ouabain treatment under symmetrical conditions, the short-circuit current decays to zero. The increase in resistance, though significant, is small, which suggests a sizeable shunt pathway for current. Flux measurements show that sodium is absorbed under symmetrical conditions. Mucosal solutions hyperosmotic in various sugars also induce an amiloride-sensitive inward current. In summary, this work provides evidence that the sodium taste receptor is most probably a sodium transport system, specifically adapted to the dorsal surface of the tongue. The transport paradigm of gustation also suggests a simple model for electric taste and possible mechanisms for sweet taste.
Amiloride↗
Transport in mammalian gastric mucosa.
Experiments in mammalian gastric mucosae lead to the following conclusions: 1. Sodium, potassium, and chloride are all required in the serosal solution but not on the mucosal side for acid secretion to occur. The only exception to this is in the dog, for which Cl- is required on both sides. 2. Most mammalian gastric mucosa actively absorb Na+. This 2. Most mammalian gastric mucosa actively absorb Na+. This absorption can be blocked by amiloride in the mucosal solution or by ouabain on the serosal side. The Na+ absorption probably occurs in some cell type other than parital cells. 3. Sodium and chloride move through separate transport pathways across the serosal border. 4. Hydrogen and chloride secretion are closely linked; however the secretion does not appear to be in the neutral HCl form. 5. There is a ouabain-sensitive Na+-K+ pump at the serosal membrane. 6. An increase in intracellular cAMP can initiate acid secretion. In addition cAMP appears to play a role physiologically at least in the initiation of histamine-stimulated acid secretion.
Acetylcholine↗