Amiloride effects on taste quality: comparison of single and multiple response category procedures.

Although there is compelling evidence that amiloride reduces the intensity of Na+ and Li+ salts in humans, its effects on saltiness are conflicting. Many salts elicit not only a salty taste but also one or more side tastes (sweetness, sourness or bitterness). Some studies have shown a suppression of saltiness by amiloride; others show no effect on saltiness but a significant reduction in sourness. In the experiments demonstrating a reduction of saltiness, subjects estimated only saltiness; in those showing an amiloride effect on sourness and not saltiness, subjects estimated all qualities on each trial. The present study examines the role of the psychophysical method in these conflicting results. We have investigated the effects of amiloride on taste quality by modifying only the instructions to the subjects, keeping all other variables constant. One group of subjects (intensity-only) gave magnitude estimates of the overall intensity of a LiCl concentration series. A second group (salty-only) was instructed to estimate the saltiness of the stimuli, and a third group (sour-only) estimated their sourness. Finally, a fourth group (profile) rated all of the taste qualities on each stimulus presentation, using a modified taste profile method. The ratings of all groups were made comparable by the use of 0.1 mM quinine-HCl as a modulus. When subjects used only one response category, amiloride reduced their estimates (of intensity, saltiness or sourness), but if subjects attended to all four qualities, amiloride specifically reduced the sourness of LiCl and had no significant effect on its saltiness. Comparison of the saltiness estimates of the salty-only group to the sum of the salty and sour estimates of the profile group demonstrated that subjects combined these sensations when presented with only one response alternative. To reveal the effect of amiloride on a specific quality of a salt, the psychophysical method must allow subjects to attend to all qualities on each trial. These data and previous results suggest that apical Na+ channels on the taste receptor cell membrane mediate the sourness but not the saltiness of Na+ and Li+ salts.

Amiloride suppresses the sourness of NaCl and LiCl.

The transduction of Na+ salts in many species is mediated by both apical and submucosal ion channels on the taste receptor-cell membrane. The apical ion channel is blockable by the diuretic amiloride, whereas the submucosal pathway is not. Previous human psychophysical studies have shown a decrease in NaCl taste intensity caused by amiloride that is smaller than the reduction of the electrophysiological response produced by amiloride in other species. Many salts, including NaCl, elicit not only a salty taste to humans, but also sweet, sour, or bitter side tastes. Amiloride has been shown to reduce the sourness, but not the saltiness, of NaCl and Na gluconate and to have no effect on the taste of KCl. The present experiment further evaluated the hypothesis that the sour taste of Na+ and Li+ salts is mediated by the amiloride-sensitive transduction mechanism, by examining the effect of amiloride on the taste of LiCl, which is considerably more sour than NaCl. Four concentrations of NaCl, LiCl, and KCl were presented to the anterior tongue following adaptation to water or after 10 microM amiloride treatment. Subjects estimated the intensity of the taste of each stimulus and divided this estimate among the appropriate taste qualities. There was a significant decrease in the total taste intensity of NaCl and LiCl after amiloride, but no effect on KCl; LiCl was more greatly suppressed than NaCl. These data show no effect on the saltiness of LiCl or NaCl, except for a small reduction in the saltiness of 0.1 M NaCl. On the contrary, there was a significant effect on the sourness of both NaCl and LiCl. Citric acid (3.2 mM) was also used as a stimulus, but amiloride treatment had no effect on its sourness. These data indicate that the amiloride-sensitive channel plays a key role in the perception of the sour taste of NaCl and LiCl (but not citric acid) and little role in the perception of saltiness. The salty taste of these salts may arise from other transduction pathways.

Amiloride suppression of the taste intensity of sodium chloride: evidence from direct magnitude scaling.

The transduction of Na+ salts has been shown in many species to be mediated in part by an epithelial ion channel on the apical surface of the taste receptor cell membrane, which is blockable by the diuretic amiloride. In addition to this apical ion channel, Na+ is also transduced via a paracellular pathway, which is not sensitive to amiloride. There are significant species differences in the contribution of the amiloride-sensitive pathway to Na+ transduction. Previous human psychophysical studies have reported conflicting results on the effects of amiloride in suppressing the intensity of NaCl. In general, these studies used amiloride doses that were much higher than those showing clear suppressive effects in electrophysiological studies in other species. In the present experiment, we used direct magnitude scaling of the intensities of five NaCl concentrations flowed over the anterior portion of the tongue to determine the effects of amiloride treatment at lower doses. NaCl was presented after adaptation of the tongue to water or mixed with and presented after adaptation to 10, 50, or 100 microM amiloride-HCl. Subjects estimated the intensity of NaCl and of these concentrations of amiloride in each treatment condition using magnitude estimation with a 0.1 M NaCl modulus presented following a water rinse prior to each session. Results showed that amiloride had a significant suppressive effect on the perceived intensity of NaCl, with a similar effect seen at all three amiloride doses. The psychophysical function after amiloride showed a parallel shift to the right. The average suppression over all NaCl concentrations was 21%.(ABSTRACT TRUNCATED AT 250 WORDS)

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 by both apical and submucosal ion channels on the taste receptor cell membrane. The apical ion channel is blockable by the diuretic amiloride, whereas the submucosal pathway is not. Sodium salts with small anions, such as NaCl, can stimulate submucosal as well 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 taste of NaCl are conflicting and no data exist on the effects of amiloride on organic sodium salts. In the present experiment, subjects gave magnitude estimates of the total intensity and of each of the basic taste qualities for NaCl, Na-gluconate and KCl. Five concentrations of each of these stimuli were presented to the anterior tongue following distilled water adaptation and after amiloride treatment. There was a significant decrease in the total taste intensity of NaCl and Na-gluconate after amiloride, but no effect on KCl. The saltiness of all three salts was unaffected, but amiloride decreased the perceived sourness of the sodium salts. KCl sourness was unaffected by amiloride. There was a proportionately larger effect of amiloride on Na-gluconate than on NaCl, which is consistent with a larger role for the apical ion channel in Na-gluconate transduction. However, an appreciable amiloride-insensitive component is present for both NaCl and Na-gluconate, suggesting that an amiloride-insensitive pathway also plays a role in the transduction of both sodium salts. These data support the hypothesis that an amiloride-sensitive transduction component exists in humans, but suggest that it is considerably smaller than in many other species.