| Abstract: | In the frog glossopharyngeal nerve, single water fibers respondto low CaCl2 (1–2 mM) and relatively high MgCl2 (100 mM).In the present study, it was found that stimulation by a mixtureof low CaCl2 and relatively high MgCl2 led to a small response.This suggests that the Ca+ response is inhibited by the presenceof Mg2+ and the Mg2+ response is inhibited by the presence ofCa2+. Hence, it is suggested that there are different receptorsites for divalent cations in single water fibers of the frogglossopharyngeal nerve, a calcium receptor site (XCa) responsiblefor the Ca2+ response and a magnesium receptor site (XMg) responsiblefor the Mg2+ response. It has been reported that Mg2+ inhibitsthe Ca2+ response by competing with Ca2+ for XCa (Kitada andShimada, 1980). In the present study, the inhibition of theMg2+ response by Ca2+ was examined quantitatively under theassumption that the magnitude of the neural response is proportionalto the amount of MgXMg complex minus a constant (the thresholdconcentration of the MgXMg complex). The results obtained indicatethat Ca2+ competes with Mg2+ for XMg. The apparent dissociationconstants for MgXMg complex and CaXMg complex, which were obtainedfrom the present study, were 8.0 x 10–2 M and 7.2 x 10–4M, respectively. Thus, competition between Ca+ and Mg2+ forthe distinct receptor sites involved in taste reception wasdemonstrated by the results described in this paper. Since thedivalent cations do not always bring about activation of tastereceptors, the responses to salts in the frog glossopharyngealnerve cannot be explained in terms of changes in the surfacepotential outside the taste cells. The present results suggestthat there exist multiple specific receptor sites for cationsinvolved in salt taste responses, and only the binding of eachseparate cation to its appropriate receptor sites leads to activationof the receptor and the initiation of impulses in sensory nerveendings. |
