152 THE PHYSIOLOGY OF INSECT SENSES 



Beidler assumed that the stimulus reacts with some receptor sub- 

 stance and that the reaction obeys the mass action law. His second 

 assumption was that the magnitude of response is directly related to the 

 number of ions or molecules that have reacted with the receptors. He 

 derived an equation which related magnitude of response to the con- 

 centration of the applied chemical stimulus. This is clR = (clRm)-\- 

 iXjKRm), where c = concentration, R — magnitude of response, 

 Rm = magnitude of maximum response, and K = the equilibrium 

 constant. The vahdity of the equation can be tested by measuring c 

 and R. Beidler found the agreement to be excellent. Now, knowing the 

 equilibrium constant, he calculated the free energies of reaction from 

 the expression AF = RT\n K, where AF = change of free energy, 

 R = the gas constant, and T = the absolute temperature. The low 

 value which he found for AF(from —1-22 to —1-37 Cal./mole) for a 

 series of sodium salts was taken to indicate that physical rather than 

 chemical forces are involved in the interaction between the chemical 

 stimulus and the receptors. This conclusion raised the question of what 

 the receptor substance may be. 



Beidler argued that the small temperature dependence and the 

 low values for AF suggest a reaction similar to those that occur with 

 ion binding by proteins and natural polyelectrolytes. Some properties 

 of the reacting groups of the receptor substance can be determined 

 from a study of the effect of changing pH. Since no effect could be 

 demonstrated over the range 3-0-1 1-0, one might conclude that the 

 molecules of the receptor substance are strong acidic radicals. The 

 relatively weak carboxyl radical of a protein, for example, cannot be 

 considered as the reacting group. The phosphate and sulphate radicals 

 of such natural polyelectrolytes as nucleic acids and certain poly- 

 saccharides are able to bind cations in a manner consistent with the 

 properties of the receptors described by Beidler. 



The many complexities of the mammalian taste organs, as, for 

 example, the receptor cell being non-neural and synaptically con- 

 nected to a neuron, the multiple innervation of a receptor cell, and the 

 innervation of more than one cell by a single neuron, obstructed 

 further analyses of the nature of the receptor sites and the process of 

 stimulation. None of these handicaps are associated with the salt 

 receptor of the insect. A quantitative study of the repetitive response 

 of the labellar hair receptor to NaCl has shown that Beidler's theory 

 applies equally well to this receptor (Evans and Mellon, 1962 b). The 

 calculated relative free energy change of the reaction between salt and 

 receptor site in this case is the range to — 1 kcal./mole. This value 



