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.
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 opencircuit 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' 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 shortcircuit 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 .Address reprint requests to Dr.
In small-group problem-based learning (PBL), students work cooperatively to solve complex, real-world problems. The problems lead the students to learn basic concepts rather than being presented as applications of concepts they have already learned. The goals are for students to learn and be able to apply the disciplinary content, develop critical thinking abilities, and acquire skills of life-long learning, communication, and team building. PBL has been widely used in recent years in medical and related areas of professional education. In those settings each small group typically has its own faculty facilitator. PBL can be successfully adapted for teaching undergraduate and graduate basic science students, in part by having multiple groups meet in one room with a roving facilitator. This report describes a two-semester PBL sequence in organ-systems physiology. To keep the interest of a diverse group of seniors and graduate students, several types of problems were used: clinical, laboratory research-based, real-life scenarios, and published research articles. The majority of students have responded enthusiastically.
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 NaC1 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-methylgiucose 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 NaC1 by 70-80% and the response to 0.5 M KCI by ~40%. This agreed with the percent inhibition by amiloride of the short-circuit current supported in vitro by NaCI and KC1. 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.
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