Inhibition of taste nerve responses to sugars and amino acids by cupric and zinc ions in mice

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Abstract
Inhibition of chorda tympani nerve responses to sugars and amino acids by CuCl2 and ZnCl2 was studied in ddy mice. Responses to sugars (sucrose, fructose, glucose and maltose) and Na saccharin were markedly depressed after adaptation of the tongue to 0.1 mM CuCl2 or ZnCl2. No significant difference in the amount of inhibition by either salt was found among sugars. The concentration-response curve for sucrose was shifted towards the right along the abscissa by treating the tongue with CuCl2 or ZnCl2 at 0.1 mM. Reciprocal plots of the concentration-response relationship yielded straight lines, which intersected at a point along the ordinate. The results indicate that binding of surcrose as well as other sugars to sweet receptor molecules is competitively inhibited by Cu2+ or Zn2+. Responses to amino acids (Gly, L-Ala, L-Ser, L-Pro, L-Val and D-Trp) were either slightly or scarcely inhibited by 0.1 mM CuCl2 or ZnCl2. Reciprocal plots of the concentration-response relationship for Gly before and after adaptation of the tongue to either metal salt failed to yield straight lines. It is proposed that amino acids would bind to a small proportion of sweet receptor molecules and, in addition, stimulate other receptor mechanisms responsible for initiating impulses in fibers responsive to taste stimuli other than sucrose.

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