CHEMORECEPTOR MECHANISMS 5 



ing effectiveness as follows: Li < Na < Ca < K < NH 4 . The anions are fre- 

 quently arranged: S0 4 > CI > Br > I > HC0 3 > N0 3 . It is probable that 

 both the cations and the anions play a role in stimulating the receptor. The 

 order of efficiency of the cations as given above is also that of the ionic mobilities 

 and the partition coefficients as well. The arrangement of the anions is less clear. 

 The activity of the divalent cations is anomalous in that they do not fit into 

 any of these series. The work of Beidler with the rat has shown that the divalent 

 salts are different from the monovalent salts in several respects. Hodgson (1951) 

 has also noticed this difference in tests with the water beetle, Laccophilus. In 

 man many of the divalent salts are bitter. Moreover, each salt has a different 

 and recognizable taste and so must stimulate different receptors or combina- 

 tions of receptors. 



Hardly any experiments have been undertaken to test the effect of simple 

 organic compounds on the taste receptors of mammals. The lack may stem in 

 part from complications introduced by the concomitant olfactory stimulation 

 effected by most of the compounds in this category. However, data which exist 

 for the homologous aliphatic alcohols and glycols (Figs. 3 and 4) indicate that 

 the thresholds (molar) decrease logarithmically with increasing chain length 

 (Renqvist, 1920; Dethier, 1951b; Gavaudan, 1948, 1951; Poussel, 1951). In 

 the case of organic salts, Beidler (1954) found that there was a decrease in effect 

 with chain length; however, he tested sodium salts only up to butyrate; and 

 data of Dethier (Fig. 5) with longer series of sodium, lithium, and barium salts 

 acting on insects show that beginning with the five carbon compound there is 

 a logarithmic decrease similar in all respects to that observed with non-polar 

 homologues. 



The outstanding feature of all of the data accumulated from tests with sweet 

 and bitter substances has been the remarkable nonconformity of the physiolog- 

 ical effect with any single chemical or physical property of the sort recorded in 

 whole or in part for the non-sweet compounds. On the contrary, there is to be 

 noticed a much more precise dependence upon specific chemical configurations. 

 This dependence is true of the bitter as well as the sweet substances. For ex- 

 ample, or//w-tolylurea is tasteless, we/a-tolylurea is bitter, and ^ara-tolylurea 

 is sweetish (Beidler, 1952). Other equally small changes in molecular structure 

 are reflected as pronounced physiological changes. Stereo isomers also have 

 different tastes. For example, D-asparagine is sweet; L-asparagine is tasteless. 

 In the case of insects, D-arabinose is nearly five times more stimulating than 

 L-arabinose (Hassett, Dethier, and Cans, 1950). 



Insects 



All of the information summarized in the foregoing paragraphs can be multi- 

 plied and elaborated a hundredfold. It is well to remember, however, that in 



