Large enhancement of canine taste responses to sugars by salts

The effects of changed ionic environments on the canine taste responses to sugars were examined by recording the activity of the chorda tympani nerve. a) The responses to various sugars were greatly enhanced by the presence of salts having monovalent cations such as Na+, K+, choline+, or Tris+. The responses to sugars were suppressed by high concentrations of salts. (b) The presence of 100 mM NaCl in fructose solution did not affect the maximal response and changed the Hill constant for the concentration-response relationship from 1.3 to 2.4. (c) CaCl2 greatly enhanced the response to fructose, while MgCl2 exhibited practically no effect. The presence of 20 mM CaCl2 in fructose solution changed the Hill constant from 1.2 to 2.4. (d) CaCl2 suppressed the responses to 0.5 M sugars except for fructose and sucrose and enhanced the responses to all sugars examined at 1 M. In the glucose response, the slope of the concentration-response curve was increased by the presence of CaCl2. Here the curve in the absence of CaCl2 intersected with that in the presence of CaCl2, indicating that CaCl2 suppressed the response to glucose of low concentrations and enhanced that of high concentrations. (e) The enhancement of the sugar responses by salts was not simply explained in terms of ionic permeability at the apical membranes of taste cells. The enhanced and suppressed effects of salts on the sugar responses were interpreted in terms of the cooperativity between receptor molecules for sugars.