Cation regulation of guinea pig cardiac nitrobenzylthioinosine binding sites

The effects of several ions on [3H]NBMPR specific binding were characterized in guinea pig cardiac membranes. Binding of [3H]NBMPR was enhanced by Mg2+ Ca2+, Na+ and K+ but inhibited by Mn2+, Cu2+ and Zn2+. The effects of Cu2+ and Zn2+ were observed in micromolar concentrations while the effects of the other cations were observed between 1 and 300 mM. The anions tested had no significant effect on the binding. Scatchard analyses of the [3H]NBMPR saturation equilibrium data showed that the enhancement by Na+ and Mg2+ was due to an increase in the number of binding sites number with no change in apparent affinity. The inhibition of the binding by Cu2+ and Zn2+ was due to a decrease in both the apparent affinity and the number of binding sites. The regulation by cations imply that [3H]NBMPR binding sites are physiological receptors that may be involved in postreceptor coupling.