Enhancement of canine taste responses to umami substances by salts

The effects of salts on canine taste responses to umami substances were examined by recording the activity of the chorda tympani nerve. 1) The responses to monosodium glutamate (MSG), disodium 5'-guanylate (GMP), and that induced by the synergism between MSG and GMP were enhanced by the presence of various salts. 2) The effective salts were those carrying inorganic cations such as Na, K, and Mg, while CaCl2 had no enhancing effect. Salts carrying organic cations such as tris(hydroxymethyl)aminomethane (Tris), choline, N-methyl-D-glucamine, and 1,3-bis[tris(hydroxymethyl)-methylamino]propane also produced positive results. 3) The dependence of the umami responses on NaCl and MgCl2 concentrations showed a bell-shaped response curve with the maximal enhancing effect being seen at 100 mM NaCl and 3-10 mM MgCl2. 4) The degree of the enhancement depended not only on the species of the cation, but also on that of anion. For example, 100 mM NaCl showed a much larger enhancing effect than Na phosphate, Na2SO4, and Na4Fe(CN)6 at equimolar Na+. 5) The enhancing effects of salts on the responses to the umami substances could not be simply explained in terms of the permeability of cation and anion of salts. It was speculated that the binding of both cations and anions to the receptor membranes leads to changes in the interaction of the umami substances with the receptor proteins.