Competitive inhibition of the nickel-induced response to choline by calcium ions in single water fibers of the frog glossopharyngeal nerve.

NiCl2 induces a response to choline Cl and enhances the response to CaCl2 in water-sensitive fibers (water fibers) of the frog glossopharyngeal nerve. The Ni(2+)-induced choline+ response was inhibited by Ca2+ ions and, conversely, the enhanced Ca2+ response by Ni2+ ions was inhibited by choline+ ions. Hence, there exists a mutual antagonism between Ca2+ and choline+ ions. In the present study, the inhibition of the Ni(2+)-induced choline+ response by Ca2+ ions was investigated quantitatively. The assumption was made that receptors for choline (XCh) exist and that binding of a choline+ ion to XCh brings about a neural response. It was further assumed that the magnitude of the neural response is proportional to the amount of choline-XCh complex minus some constant (the threshold concentration of the choline-XCh complex). The results from analysis of double-reciprocal plot were consistent with the hypothesis that Ca2+ ions compete with choline+ ions for XCh. The dissociation constants for the choline-XCh complex and the CaXCh complex were obtained to be 0.6 M and 7.4 x 10(-5) M, respectively. This result indicates that the affinities of XCh for choline+ and Ca2+ ions are very different. Furthermore, Mg2+ ions did not affect the Ni(2+)-induced choline+ response, an indication that the affinity of XCh is not charge-specific, but is chemically specific. The identification of a competitive inhibitor of the choline+ response provides evidence for existence of a choline-specific receptor at the surface of taste cells that are innervated by the water fibers of the frog glossopharyngeal nerve. Differences between the features of the response to choline Cl in the chorda tympani nerve of the rat and those in the frog glossopharyngeal nerve are discussed.