A Quantitative Study of the Inhibitory Effect of Na+ and Mg2+ on the Ca2+ Response of Water Fibers in the Frog Tongue

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Abstract
Impulse dicharges from a single water fiber elicited by stimulation of the frog tongue with Ca2+ were recorded. The inhibitory mechanism of Na+ and Mg2+ on the Ca2+ response was quantitatively studied. A dose-response curve for Ca2+ was obtained. The impulse frequency increased with increasing Ca2+ concentrations and was saturated at 1-5 mM. The results were explained by assuming that Ca2+ interacts reversibly with a receptor site, X, at the surface of the receptor membrane and the reaction obeys the mass action law, and that the magnitude of the response is directly proportional to the number of CaX. The apparent dissociation constant for CaX was 5.9 .times. 10-5 M. Na+ antagonized the effect of Ca2+. Addition of Na+ to Ca2+ stimulating solution shifted the log dose-response curve for Ca2+ to the right in a graded and parallel manner, as the concentration of Na+ was increased. A model was proposed in which Na+ competes with Ca2+ for X by forming inactive NaX and Na2X. Mg2+ as well as Na+ antagonized the effect of Ca2+. The inhibitory effect of Mg2+ was greater than that of Na+. Addition of Mg2+ to Ca2+ stimulating solution shifted the log dose-response curve for Ca2+ to the right without altering the slope. The results were consistent with the hypothesis that there is competition between Mg2+ and Ca2+, and that Mg2+ can combine with X to form MgX which is inactive. The apparent dissociation constant for MgX was 5.2 .times. 10-5 M. This value for MgX was very similar to that for CaX.

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