Divalent cations increase the binding capacity of the [3H]mepyramine binding site, a possible histamine H1 receptor, in rat liver membranes

The [3H]mepyramine binding to rat liver membranes was about 2-fold higher in the presence of 2 mM CaCl2 than in the absence. However, the [3H]-mepyramine binding to rat brain membranes was not affected by the presence of 2 mM CaCl2. Scatchard analysis showed that 2 mM CaCl2 did not change the affinity (KD) of liver membranes to [3H]mepyramine, but increased the binding capacity (Bmax). CaCl2 had a half maximal effect (ED50) at 3 μM and a maximal effect at concentrations of more than 10 μM. Other divalent cations, Mg++, Mn++, Sr++ and Ba++, also increased the [3H]mepyramine binding, whereas monovalent cations, Na+, K+, Li+ and Rb+, had no effect.