Divalent Cations Increase the Binding Capacity of the [³H]Mepyramine Binding Site, a Possible Histamine H₁ Receptor, in Rat Liver Membranes

Abstract

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 (K_D) of liver membranes to [³H]mepyramine, but increased the binding capacity (B_max). 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.