138 LLOYD M. BEIDLER 



change in the configuration of a macromolecule on the receptor surface 

 and forming a hole which allows potassium ions to leak out of the receptor. 

 Since thermodynamics show that ^F = — RT In K, the change in free 

 energy, ^F, which is a measure of the binding force, can be readily 

 obtained. The magnitude of these forces, several kilocalories per mole, 

 is in agreement with the concept that the chemical stimulus is adsorbed 

 to the receptor and that enzymatic reactions are not involved in the initial 

 response-triggering process, although, of course, they are important in 

 metabolic reactions that produce the energy necessary for the electrical 

 taste cell response. 



Many different types of ions and molecules can interact with taste 

 receptors and stereoselectivity has been shown (Steinhardt, 1962). This 

 impHes different types of receptor sites, some showing complementariness. 

 Van de Waals forces, hydrogen bonding, dipole-dipole interactions, ionic 

 forces, etc., are important in the stimulus-receptor binding and no single 

 parameter can be expected to exist which would be a direct measure of 

 stimulating efficiency of a taste substance. Not only must the reactive 

 groups of the taste substance be considered, but also their spatial arrange- 

 ment and degree of hydration. 



One type of receptor site may be filled by several kinds of ions or 

 molecules with both different binding energies and different efficiencies 

 in response elicitation (intrinsic activities). Their competition for the 

 same site can be described mathematically (Beidler, 1961a). Several 

 different sites that react to two or more independent kinds of stimuli may 

 exist on one receptor surface. One such example is the existence of sites 

 for Na+ on the same receptor surface containing sites for fructose or 

 other sugars. 



The types of receptor sites and the number of each vary from receptor 

 to receptor and from one species of animal to another. It is possible for 

 a given chemical stimulus to react with two different types of receptor 

 sites. In this case two equilibrium constants are necessary to describe 

 the experimental data. One example is NH4CI (Beidler, 1961a). Mixed 

 tastes are often associated with such stimuli. 



The membrane of the taste receptor is electrically charged. A micro- 

 pipette thrust into a taste receptor shows that such a charge separation 

 exists across the membrane and that it decreases with chemical stimulation 

 of the receptors (Kimura and Beidler, 1961). It is likely that the potential 

 difference is due to an ionic gradient similar to that which exists in other 

 sensory cells. However, it must be remembered that the microvilli do 

 not live in a medium of fixed chemical ingredients. Saliva normally 

 surrounds the microvilli but is often replaced by the many taste substances 

 taken into the oral cavity. It is also known that water can be continually 

 flushed over the surface of the tongue for 24 hr with little effect on the 



